The Sterling Power Pro Batt Ultra DC to DC Charger

In our continuing series on CMD’s or Charge Management Devices, this article examines and looks at the benefits of the Sterling Power DC to DC chargers and examines the installations where you may find them useful.

*This article includes the latest Sterling Power Pro Batt Ultra series of battery to battery chargers. The latest model is identified by the green stripe across the top of the face plate label.

Definitions Used In This Article

• Battery to Battery Charger – A DC to DC charge source used for charging one bank of batteries from another bank of batteries
• B2B – Short form for battery to battery charger
• DC to DC Charger – A  battery charger that operates from one DC battery bank to another
• CMD – Charge Management Device/s – Devices used to charge from battery to battery or alternator to battery
  
• Target Bank – The bank the B2B is feeding its output current to
• Source Bank – The bank the B2B is getting its input current from
• LiFePO4 – A lithium-ion battery chemistry – Lithium Iron Phosphate
• LFP – Short hand for LiFePo4
• BMS – A Battery Management System is the protection device for an LiFePo4 battery.

The Sterling Pro Batt Ultra B2B Charger

The above photo illustrates how the Pro Batt Ultra comes out of the box. It includes, as standard equipment, a battery temp sensor for the target bank/battery. The 12V to 12V BB1260 is shown and it’s quite small, much smaller than a 60A shore based charger. Size cab be scaled by comparing it to the battery temp sensor on the left. The small size means it can fit many places a standard AC to DC shore charger may not.

“Why Would I Want or Need a Pro Batt Ultra Battery to Battery Charger?”

The Pro Batt Ultra is the most feature filled, fully programmable DC to DC charge management device in existence today. On top of the cram-packed feature set they are actually priced quite reasonably. Compare what the Pro Batt Ultra is capable of, then compare it to other CMD’s that can’t do half of what the Pro Batt Ultra can do, and you see how good a value this charge management device is. Feature wise, the Pro Batt Ultra actually beats the very good Sterling Pro Charge Ultra shore chargers, and these are very good shore power chargers. The Pro Batt Ultra allows an owner to customize the absorption duration, a very useful feature when dealing with LiFePO4 batteries or even some lead acid and it also has dedicated voltage sensing. These are features not found in many AC chargers even at 3-4 times the price. While we can’t really compare a DC to DC charger to an AC to DC charger, output side charging features count no matter what the charge device is.

When You Would Use a DC to DC Charger vs. Other CMD’s

• Differing On-Board Battery Types/Chemistries – eg: GEL House > AGM Start or AGM Start > LiFePO4 “Drop-In” etc.
• Differing On-Board Bank Voltages – eg: 12V House > 24V Windlass or Thruster Bank or 24V House to 12V Navigation Electronics Bank

What is a Pro Batt Ultra?

Very simply put the Pro Batt Ultra is an extremely full featured battery charger, more so than most AC powered shore chargers you’ll find. That’s it. The only difference between a shore charger and the Pro Batt Ultra is that the Pro Batt Ultra can be powered by another battery bank so that it can be used with all your charge sources to charge the target battery.

Pro Batt Ultra = Buck or Boost

The Pro Batt Ultra can not only “buck” voltage (buck means reduce beyond input) but, unlike most DC to DC chargers, it can also “boost” voltage (boost means increase beyond input). CMD’s such as an ACR/VSR/Combiner, Echo Charger or Digital Duo Charger can only reduce voltage to the target bank and can not increase it. None of the aforementioned CMD’s can float a target bank independently from the source bank but the Pro Batt Ultra can float independently.

For example the Pro Batt Ultra 12V to 12V models can take an input of 14.1V from a source bank and output a higher voltage to charge the target bank that may require 14.8V. Another major plus is the Pro Batt Ultra is available for both like voltage and for mixed voltage vessels where both 12V and 24V banks are installed.

Output Current vs. Input Current: Please be aware that unlike a shore based charger, the Sterling B2B chargers are rated based on max input current. In other words a BB1230 is not a 30A output but rather a 30A input. You will get less than 30A on the output side. This is done so you know the maximum the unit can pull from the “source bank” which is a critical measurement to know. The average efficiency of the Sterling B2B chargers is about 86%, but depends on the input voltage.

Pro Batt Ultra Models Include:

BB1230 – 12V 30A Input to 12V Output

BB1260 – 12V 60A Input 12V Output

BB122470 – 12V 70A Input to 24V 35A Output

BB242435 – 24V 35A Input to 24V Output

BB241235 – 24V 35A Input to 12V 70A Output

BB123670 – 12V 70A Input to 36V 23A Output

BBURC – Remote Display for Pro Batt Ultra

What’s the difference between a DC to DC charger and a shore charger?

An AC powered shore charger needs AC power as the input in order to operate. When at sea this means you’d need an AC generator to run your shore charger. The Pro Batt Ultra B2B does not require AC power and instead requires only a DC input.

The Pro Batt Ultra can be used with any bank that’s being charged by another source such as alternator, *solar or a shore charger.

*If large enough to satisfy the input requirement

What Can it Do Differently Than an ACR/Combiner/VSR, Echo Charger or Digital Duo Charger?

• Can independently float the target bank even when the source bank is in bulk or absorption
• Provides a true multi-stage smart charging algorithm completely independent of source bank
• Has a built in voltage sense circuit
• Has a battery temp sensor as standard equipment
• Can be fully custom programmed including absorption, float, absorption duration on/off points etc.
• Can be activated via ignition excite or automatically by voltage
• Can accept a BMS trigger signal from a LiFePO4 battery BMS
• Can withstand a load dump from a an opened battery switch or a Lithium-Ion BMS Load Dump
• Can reduce or increase the charging voltage to the target bank eg: 14.1V GEL House and 14.7V TPPL AGM Bow Bank
• Can charge a 24V bow bank from a 12V House bank
• Multiple models: 12V input & 12V output, 12V input & 24V output, 24V input & 24V output, 24V input & 24V output
• Is an excellent CMD for use with mixed on board chemistries including lead-acid and LiFePO4
• Is an excellent CMD for charging a 24V bow bank from a 12V house bank.
• Is an excellent CMD for charging a 12V electronics bank from a 24V house bank.
• It is not just a simple “voltage follower” like an ACR, Echo Charger or Digital Duo Charger, it is an actual DC to DC battery charger

Mixed On-Board Bank Voltages:

12V to 24V Charging – BB122470:

The scenario below is quite common on vessels using large DC bow thrusters or larger windlasses or winches. Until now charging a 24V bow bank from a 12V source was a bit of a kludge work around often requiring running a genset and an AC shore charger. The BB122470 is an ideal tool for this type of bank layout where the house bank is 12v and a bow, start, winch or thruster bank is 24V. The Pro Batt Ultra BB122470 makes easy work of this and gives a true multi-stage charge algorithm to the target bank.

In a 12V to 24V situation the BB122470 can be placed closer to house bank, where the input wire needs to be large and handle 70A, and then a smaller gauge 24V output wire can be run to the target bank along with a voltage sense wire and the temp sensor.

24V to 12V Charging – BB241235:

On larger boats it’s not uncommon to have a 24V house bank and a 12V bank to supply 12V marine navigation electronics. In this scenario a BB241235 is being used to charge a navigation electronics bank from a 24V bank of house batteries.

The flexibility of the Pro Batt Ultra is really quite amazing compared to what we’ve had in the past regarding mixed bank voltages.

Typical Installation Consideration for the Sterling Pro Batt Ultra

Wiring

In this image we have a BB1230 being used to charge a start battery from the house. Because the Pro Batt Ultra’s use *pressure plate terminal blocks, meaning bare wire is inserted and the screw tightened, you really want to ensure the wires are properly secured as close to the unit as is feasible. The BB1230 is capable of accepting 6 AWG marine wire and here we’re using 6GA wire.  Each 6 GA wire is secured with wire tie mounts & wire ties to secure each wire within a few inches of the unit. The wiring is also labeled to identify it including the voltage sense wire and the temperature sensor.

*The use of pressure plate terminal blocks should ideally mean they are ABYC compliant and that a “screw” does not impinge upon the bare wire strands. The Pro Batt Ultra’s use internal “plates” that compress the wire and no direct screw is twisting on the wires during torquing making them ABYC compliant.

PRO TIP: With pressure plate terminal blocks, and finely stranded marine wire, it is best to snug the wire, then wait a bit, and snug it once more. The finely stranded wire can slowly compress into shape and the original clamping pressure can decrease.

Because the terminals accept bare wire it is recommended to use tinned wire. Tinned wire is not an ABYC requirement, but it certainly corrodes much less rapidly than bare copper.

Wiring of a Pro Batt Ultra should comply with ABYC E-11 standards

• Fusing input & output positive wires within 7″ of each battery bank (Min fuse size should be 125% of input rating)
• Proper strain relief & wire support
• Chafe protection where necessary
• Wire bundling considerations
• Placement of unit in relation to batteries or moist areas
• Wire labeling
• Use of proper crimp tooling & terminals
• Temp Sensor Placement (negative battery terminal or battery case only)
• Over-Current Protection for Voltage Sense Wire within 7″ of battery positive
• Proper gauge wiring for the amperage (ideally not to exceed 3% voltage drop)

In the wiring example below we have a typical cruising boat installation with all charging feeding the house bank. It is set up for automatic voltage activation. Once the house bank hits 13.2V the B2B will boot up and begin charging the target bank.

As can be seen above the typical marine installation is just 5 total wires or 6, if you purchase the optional remote display:

• Temp Sensor
• Volt Sense
• Output Positive
• Input Positive
• Negative
• Remote Display (optional)

You may be wondering why we took so many man-hours to create all these diagrams? The answer is actually quite simple, the Sterling Power manuals are rather difficult for most owners to make sense of. Even we had to send Charlie Sterling a rather long list to get clarification on some of the wording. Please understand that Charlie is an electrical engineer, and writing manuals for the non-electrical engineer boat owner can be very difficult, to convey clearly, to a DIY. Sterling is not alone in this regard and it is common in the industry. We know the manuals can be difficult because we get the support calls & emails before our customers ever reach out to Sterling Power. Almost always it is just a misunderstanding of the instruction manual, not an actual problem with the product.

Sterling Powers main market for the Pro Batt Ultra is for “caravans“, or RV’s & Trailers to those of us in the USA. In order to cater to this market the product needs to be in compliance with the Euro 6 Emission Standards. In Euro 6 installs the B2B’s are connected to the vehicles starter battery and feed the trailer or RV’s hotel/house battery bank. Due to Euro 6 regulations you can’t really tinker with the alternator and many of these are now controlled by the vehicles CPU to work in concert with regenerative braking etc..

The Pro Batt Ultra allows for regenerative braking / Euro 6 compliance. In short, the stock alternators do a horrible job of charging deeply cycled house banks in the trailer or RV, and you can’t really modify them. The good news for Euro 6 installations is the auto and RV recreational industry typically over-size most alternators, and this results in a very low warranty rate. It also means there is room left over, in terms of amperage, for the B2B charger to pull from.

Bottom Line on the Owner Manual?

For a marine application please ignore the vast majority of the Pro Batt Ultra manual that primarily deals with Euro 6 compliance. We have asked Sterling Power for a Pro Batt Ultra marine only manual but so far no luck.

Input Bank Charge Source Sizing:

Lead Acid Alternator Sizing – Sterling Power recommends that your alternator be sized (based on its SAE output rating) to be at least 30% larger than the B2B unit you choose. In our testing we found 30% to be a bit low for many “stock” alternators and find that double the B2B size means a much cooler running alternator. The reason for this is actually quite simple, you don’t want to tax your small alternator to death. Alternator output also varies based on RPM and winding temp. We see small alternators taxed to death quite regularly on boats.

Unlike automobiles, marine alternators are typically grossly undersized for the work they are expected to do. On the flip side, automotive alternators are typically grossly oversized for the work they are expected to do. As an example the stock alternator in my truck is a 150A Denso hairpin wound unit that’s charging a single G24 starting battery. With every device on max the most I have measured for alternator loads is about 28A. That same alternator, charging a 600Ah house bank, would be at maximum output for close to two hours straight. Most owners of marine engines would kill for a 150A rated alternator instead of the grossly undersized 35A-80A alternator many marine diesels are shipped with.

LiFePO4 Alternator Sizing – If you intend to feed a LiFePO4 battery bank with a ProBatt Ultra, and many do, then Sterling’s suggestion of 30% over-sized, for the alternator, we find to be rather inadequate. We recommend at least double the alternator rating, or larger for the B2B charger. So, if you want to use a 30A B2B on LiFePO4 then you would need an alternator rated at a bare minimum of 60A but preferably one sized for 80A would be much better. Each engine bay, and its heat characteristics, will be different, so predicting how much larger is impossible to really say. In our experiments with stock alternators we find double the B2B input rating for the alternator is a bare minimum.

The best scenario with LiFePO4 is to upgrade your alternator before using a Sterling B2B to feed your LiFePO4 bank.

Shore Charger Sizing – Like the alternator, consideration needs to be given to the demand placed on it by the Pro Batt Ultra. Can the charge source handle it? If the charger can run at 100% of its output rating, and do so continuously, and not all chargers can do this, there is little need to over-size by more than 30%. If however, like some AC chargers, it will limit output if it gets too hot you may want to consider upping it to 40% – 50% larger than the B2B input rating. Again, if feeding LiFePO4, this will need even more examination as demand on the Pro Batt Ultra will be near 100% during the vast majority of the LiFePO4 charge cycle.

Alternative Energy Charging – This will be entirely up to you the owner as to how you choose to use the Pro Batt Ultra. With smaller arrays we generally advise ignition excite, it charges start or bow bank when the engine runs, but if you have a large array, voltage excitation can certainly work.

Just try to ensure your charge sources can exceed the demand placed on the B2B by the target bank. In situations where the B2B is charging only a start battery, this is not going to be a big deal, start batteries require very little charging hence little demand on the source bank charger, but as the target bank requires more current, such as a bow-thruster bank or LiFePO4 it can cause B2B on/off cycling so charge source sizing becomes more critical.

Ignition Excitation

For some situations, such as LiFePo4, or a low current PV system, an owner may want the B2B to run when the engine is turned on, even if the input voltage is below the “automatic” turn on point of 13.2V. This is called “ignition excitation” or “key-on excite“. If ignition excite is used the unit can boot up and start charging a target bank with input voltages as low as 10V. The image below shows where the +12V ignition feed would be wired to. To use “Ignition Excitation” all that is required is one wire from the run position of the engine switch.

In this image we can see the ignition terminal of the B2B connected to the “run” position of the engines key switch. When the engine is fired up, there is a brief delay, and then the Pro Batt Ultra will boot up and begin charging the target battery.

Voltage Excitation / Activation

The Pro Batt Ultra (green stripe models) comes out of the box ready to be used in automatic voltage activation mode. By not using the ignition terminal the unit will only turn on once voltage has attained the turn-on voltage of 13.2V, and a short delay timer clock has been run out. The timer delay is to prevent on/off cycling as the bank approaches the voltage turn-on level. The on voltage and cut off voltage can be adjusted up or down but there will always be a 0.2V spread between the ON & OFF points. An adjustable turn-on voltage can be handy when dealing with a LiFePo4 house battery, that may be the “source bank”, and will have a significantly higher resting voltage than lead acid batteries do. In this case increasing the automatic voltage activation point to 13.5V – 13.6V will mean the B2B only boots when the LiFePo4 battery is actually being charged, but don’t forget that the OFF voltage is always 0.2V lower than the ON voltage.

For example; ON = 13.2V and OFF = 13.0V   or   ON = 13.6V and OFF = 13.4V

What about quiescent current draw?

The nice thing about the Pro Batt Ultra is that it automatically puts itself to sleep when input voltage is below the TURN OFF voltage set point (13.0V as it ships). If you fit a remote display the remote also goes to sleep. The quiescent draw or parasitic load that’s placed on the source battery, when the unit is sleeping, is just 1mA! 1mA = 0.001A. In an entire week asleep on standby the unit uses just 0.168 Ah’s. Pretty amazing really.

Alternative Uses for the ProBatt Ultra – LiFePO4

With LiFePO4 drop-in batteries now being heavily marketed, and prices falling to an acceptable level for many boaters, there are issues that can arise that need to be addressed before you can simply “drop them in“..  These issues involve “drop-in” type LiFePO4 batteries that feature a 100% sealed non-communicable internal  BMS (battery management system). The problem is not how the BMS manages the battery, it is in its ability to disconnect the battery from the vessel & charge sources. With most drop-in LFP batteries this can happen without any advanced warning.

A LiFePO4 drop-in batteries internal BMS can disconnect for the following reasons:

• Cell Over Voltage
• Cell Under Voltage
• Cell Temperature
• BMS Temperature
• BMS Current Limits Exceeded

If there is a bad cell, temperature too high, too much charge current, a glitch in the charging voltage settings or a cell imbalance issue creating an over-voltage condition, the battery will physically disconnect itself from the vessel. Most drop-in LiFePO4 batteries can disconnect themselves with no advanced warning to the vessel occupants. This is called a load disconnect or load dump.

A load disconnect or load dump is something a lead acid battery can’t physically do on its own, so this, by definition makes “drop-in” LFP batteries not so “drop-in” because we now need a ways to ensure our alternator or inverter/charger is not suffering load dumps. Of course you don’t need to take out word for it, so how about Balmar, the worlds largest specialty marine performance alternator and regulator manufacturer.

Sure, many an owner has moved a battery switch with the alternator charging and had the destroyed alternator to show for it but the battery did not do this without warning, and the owner made a simple, and often fatal to the alternator, mistake. If a BMS disconnect / load-dump occurs, when charging with an alternator, or even a large transformer based inverter/charger, the resulting *voltage transient,  can damage the charge source and also what ever is connected to the DC bus/system such as sensitive marine electronics.

*Voltage Transient – What occurs when a charge source such as an alternator is suddenly disconnected from the load (battery). The current now has nowhere to go sending the voltage through the roof. When the load (battery) is suddenly disconnected the voltage skyrockets to damaging levels in milliseconds.

During normal operation the alternator operates normally: (most drop-in batteries have the BMS disconnect on the negative side of the battery)

In a fault condition this is what can happen to the alternator:

What a load dump can look like:

How does the Sterling ProBatt Ultra play into this?

The unique aspects of the ProBatt Ultra are in its ability to:

1.  Have a charge profile that is suitable for LiFePO4 (many lead acid charge sources are not ideally suited for LFP)
2. Can withstand a load dump

This image below is why, when discussing the ProBatt Ultra, I prefer to use the terms SOURCE BANK and TARGET BANK. As can be seen we have reversed the way we would typically use the ProBatt Ultra in a lead acid installation and now the start battery is our  source bank and the ProBatt Ultra feeds the LiFePo4 bank or target bank.

“But RC a BB1260 is not enough charge current for my LiFePO4 system?”

Not a problem, simply parallel two or more Pro Batt Ultras together, providing your input charge sources are 30% larger or more, and you can now charge at significantly higher amperage.

While the ProBatt Ultra has been designed to withstand a load dump, other items on-board your vessel, connected to the loads bus, may not be. This is why we recommend a Sterling Alternator Protection Device for every vessel or RV etc. that has drop-in LFP batteries. The Alternator Protection Device clamps the transient to a safe level. We have tested these in our shop, on our alternator test bench, to 130A and not been able to kill one. Installation is very simple & straightforward two wire connection done close to the alternator B+ & B- terminals as shown below:
If you’re installing LFP drop-in batteries a Sterling Power Alternator Protection device is a must-have item: Purchase an Alternator Protection Device

Remote Display

Like any charge management device they are often installed where you can’t see them, and don’t know what is really going on. The optional BBURC is the remote display for the Pro Batt Ultra. It displays charge stage, input voltage, output voltage, unit temperature and battery temperature. It can also be set to alarm a user of a fault condition.

The words remote displayare meaningful because the BBURC is, unfortunately, not for programming the unit. Sterling Power keeps programming at the B2B charger so folks can’t fiddle with the remote and change settings. Little kids love to push buttons. Programming via the remote would be nice but I do fully understand the hesitation. Perhaps in the future there will be a lock-out on the remote to stop roaming fingers from changing settings?

Pro Batt Ultra Likes & Dislikes

No product is 100% perfect and we won’t pretend the Pro Batt Ultra is, but it is quite good and certainly a “best in class” product. Here at Compass Marine Inc. we are big fans of the Pro Batt Ultra because it can do things no other charge management device can.

Likes:

• Dedicated Voltage Sensing
• Adjustable Absorption Duration
• Forced Float Option
• Optional Remote Display
• 1 mA Parasitic Draw When in Sleep Mode
• Can Equalize
• True Fully Independent Multi-Stage Charging Output
• Buck or Boost
• 12V & 24V Mixed Voltage Models
• Compact Size for the Amperage
• Temp Sensor Included
• Can Withstand a Load Dump
• Ignition Excite Option
• Fully Custom Programmable
• Easy to Install
• Best in Class Product
• Pricing is Very Competitive
• Good for Drop-In LiFePo4 Charging
• 2 Year Warranty (many other DC to DC chargers are 90 Days)

Dislikes:

• Owners Manual
• Programming is a Bit Kludgy
• Terminal Strip Orientation for Temp/V-Sense/BMS is Awkwardly Located
• Fan Noise (only when working hard)
• Programming Buttons Vary in Location By Model

Overall the Pro Batt Ultra is a very unique product that no other company even comes close to. The Pro Batt Ultra is a product we are proud to offer to our readers in the MarineHowTo.com Web Store. Please remember the web store at MHT supports this site and keeps it FREE!

BB1230 – 12V 30A Input to 12V Output

BB1260 – 12V 60A Input 12V Output

BB122470 – 12V 70A Input to 24V 35A Output

BB242435 – 24V 35A Input to 24V Output

BB241235 – 24V 35A Input to 12V 70A Output

BB123670 – 12V 70A Input to 36V 23A Output

BBURC – Remote Display for Pro Batt Ultra

Happy boating!

Making Sense of the ACR

WARNING: The ACR’s in this article are not for use with LiFePO4 Batteries!

What is an ACR?

An ACR is nothing more than a fully automatic, voltage triggered, BOTH/PARALLEL switch that closes when charging voltage is present and opens when charge voltage is no longer present.

You read that correctly, in its simplest form, all an ACR really does is parallel batteries when charging is present and un-parallel batteries when there is no charging present. It does this automatically with no human forgetfulness.

In days of old a boat owner had to use the battery switch to route/direct charging to the bank or banks they desired to charge. The most ubiquitous of these methods was simply switching to the BOTH/ALL or 1+2 setting on a 1/BOTH/2 battery switch. This was all well and good, charging in parallel, so long as the motor was running. However, when the owner stopped the boat the switch was often forgotten about and left in the PARALLEL position thus draining both batteries while on the hook.

Many a boater has succumbed to two dead banks due to what we refer to as HEF (Human Error Factor). Back in the early 90’s the first of the voltage sensing relays were hitting the market, thus no longer requiring the owner to do anything to the battery switch in order to charge both battery banks. Unlike a diode type isolator, which causes an approximate 0.6V volt drop to the batteries being charged, the combiner/VSR’s were simple voltage triggered paralleling switches and both batteries could be charged without human intervention or the voltage drop associated with diode type isolators.

The Blue Sea Systems ACR’s (automatic charging relays) are one of the most common charge management devices in existence today. In a conversation with Wayne K. of Blue Sea Systems, a number of years ago, he suggested that over 500,000 ACR’s had been sold world wide. Wayne has been retired now, for at least a few years, and that number is now likely much larger. Blue Sea Systems is not the only manufacturer of “Combiner/VSR’s” and today the competition is actually quite wide spread including; Yandina, Sterling Power, Victron, BEP and many, more. Even Smartgauge makes a VSR and the Balmar Duo Charge can be wired to work as a simple VSR. This article deals specifically with the Blue Sea Systems ACR, because they are easily the number one seller in this class of CMD’s. Most VSR’s operate similarly but with varying voltage triggers or delays.

Definitions used in this article:

Charge Management Devices (CMD’s): Devices used to route or direct charging sources to a targeted battery or battery bank. They include ACR/VSR/Combiners, DC to DC buck or boots chargers, diode type battery isolators, DC to DC buck type chargers & DC to DC current limited voltage following devices.

Automatic Charging Relay (ACR): A Blue Sea Systems trade name for an electronic voltage triggered paralleling relay

Voltage Sensing Relay (VSR): A generic term for an electronic voltage triggered paralleling relay

Combiner: Another generic term for an electronic voltage triggered paralleling relay

The term ACR is a trademarked term by Blue Sea Systems for their version of a VSR or voltage sensing relay. The class as a whole is know by many names such as VSR’s, ACR’s, Combiners, Parallel Combiners etc. and they all do just about the same thing.

Despite the gross simplicity of the Blue Sea Systems ACR these little units are fraught with myth and lore. Let’s take a look at some of the myth & lore that are often incorrectly assumed;

ACR Myth & Lore:

1- “An ACR charges the start battery first then isolates it and charges the house” = FALSE

2- “An ACR gives priority charging to the start battery” = FALSE

3- “An ACR will over charge a start battery” = FALSE

4- “An ACR can’t be used with mixed chemistries” = PARTIALLY FALSE

5- “You can’t charge the house bank first or the start battery will never get charged” = FALSE

6- “Blue Sea says to wire charging to the start battery only.” = FALSE

7- “With a smart battery charger you must wire an ACR disable switch into the negative lead of the ACR” = MOSTLY FALSE

8- “When the ACR combines batteries the massive in-rush current can blow up a battery.“ = FALSE

9- “An ACR will allow the start battery to drain into the house battery and leave it depleted.” = FALSE

There are many, many more but, you get the point. Hopefully this article can show you why the above myth & lore are just that.

What is a Relay?

A relay is the control device used inside a combiner, VSR or ACR. It is nothing more than an electronic switch that is closed or opened using a relay coil. By energizing or de-energizing the coil the relay can change positions from OPEN to CLOSED or CLOSED to OPEN.

In this image we are looking at the guts of a combiner/VSR that was 7 years old. It had somewhere around 12,000 hours of parallel combined use on a world cruising boat with solar, wind, alternator and genset charging. As can be seen the contacts are still in perfect condition even after 7 straight years of 24/7/365 world cruiser live-aboard life. Despite these units not being sealed to anywhere near the level of the Blue Sea Systems ACR, the Blue Sea Systems ACR’s are fully epoxy potted, this relay is in superb condition.

What about VSR/ACR/Combiner reliability?

As a class these devices are one of the most reliable devices we’ve seen in the marine market. In fact I can’t recall a single Blue Sea Systems ACR, that we’ve seen, actually fail. This particular VSR, a Yandina, carries an unconditional lifetime guarantee. You can’t guarantee a product like this for life if they are not reliable. We actually see more manually operated battery switches go bad than we do Combiner/VSR/ACR’s. Some ACR’s, such as the Blue Sea Systems 7622 ML-ACR  can be used as a manually operated parallel battery switch and a fully automatic ACR.

Inside a VSR/Combiner:

Contacts: We can see the natural state of the VSR and that is with the contacts normally open or what is referred to as “NO”. A relay with a natural resting state of closed would be called an “NC” relay. If a NO relay loses power it isolates or un-parallels the batteries. When the contacts are closed the batteries are in parallel.

Batt 1 & Batt 2: These heavy duty plated copper buses are directly connected to the Battery 1 and Battery 2 Terminals outside the unit.

Voltage Control Logic Board: This is the smarts of the combiner/VSR. This logic board simply measures the voltage at both Batt 1 and Batt 2 terminals and then tells the coil when to energize or de-energize to close or open the relay. Good quality combiners/VSR/ACR’s also have combine/un-combine delay logic and over or under voltage lockouts built in.

Relay Coil: The relay coil is what causes the contacts to move from open to closed or from closed to open. It is controlled by the logic board. Energizing this coil closed the relay and places the batteries in parallel. De-energizing the coil allows the relay to open and un-parallel the batteries.

ACR Parameters for Combine/Closed/Parallel

First we need to understand where to place/install an ACR. Placement matters. This image is intentionally over-simplified to show relay closed parameters and relay installation wiring and location. If you notice there are no battery switches, chargers, alternators etc. shown in this drawing. This is done purposely. Despite Blue Sea Systems heavily marketing their Add A Battery Kit (7650) an ACR or other combiner/VSR is completely independent of any battery switches. You do not need to purchase an additional battery switch to make an ACR work!

Where You Should Not Install an ACR:
An ACR does not get wired between battery chargers, solar, alternators, wind  etc.. Don’t laugh, we have seen this done. As an example we had a customer wire the alternator output to one side of the ACR and the other side to the start battery and house battery with both house and start bank positive lugs stacked onto the “A” terminal. By placing the start and house battery positive wires on terminal “A” it meant the start and house banks were now hard wired in permanent parallel. This was oops #1.

For oops #2 the owner wound up blowing the diodes in his alternator twice before calling us. With the relay open the alternator output had nowhere to go and the voltage, almost instantaneously, surpassed the 16V over-voltage threshold and the ACR entered “over-voltage lock out“. With the ACR locked open alternator voltage kept climbing until the diodes in the alternator were blown. Bottom line? A Blue Sea Systems ACR is never installed directly into a charge devices positive output path.

Where You Should install an ACR:
As can be seen in the image above an ACR is wired between the positive terminals of each battery bank with the only thing in its path being the fuses located within 7″ of each positive battery terminal. These fuses are there to protect the ACR positive wiring from the battery bank should they short to ground.

TECH TIP: If you make the “A” & “B” terminal wires for the ACR the same gauge as the house and start bank wiring eg: 2/0 and 2/0 the ACR can share the house and start bank fuses, if so equipped.  Start banks are not required to have over-current protection but it never hurts.

The above image is how an ACR parallels:

13.0V for 90 Seconds: If either the B or A terminals of the ACR sense 13.0V for more than 90 seconds the ACR will close and parallel the batteries. The green arrow is pointing to the relay being closed and the banks are in parallel.

13.6V for 30 Seconds: If either the B or A terminals of the ACR sense 13.6V for more than 30 seconds the ACR will also close and parallel the batteries. The green arrow is pointing to the relay being closed and the banks are in parallel.

Question: “I thought the ACR only sensed the start battery?”

Answer: The Blue Sea Systems ACR is a bi-sensing relay meaning it can sense/monitor charging or non-charging voltages at both the “A” & “B” terminals in order to parallel the banks or to un-parallel the banks.

ACR Parameters for Un-Combine/Open/Un-Parallel

Just like the logic used for closing the relay Blue Sea Systems also has logic to control when the relay opens.

12.75V for 30 Seconds – If either the “A” or “B” terminal sense a voltage below 12.75V for more than 30 seconds the relay will open/un-parallel the batteries.

12.35V for 10 Seconds – If either the “A” or “B” terminal sense a voltage below 12.35V for more than 10 seconds the relay will open/un-parallel the batteries. IF voltage is trending upwards and attains 12.75V before 10 seconds has elapsed the relay will remain closed. This logic is here to enure a large load will not cause the relay to open when it creates a short duration voltage sag. It is also there to help minimize “relay cycling” which we will discuss later.

Start Isolation – The SI or “Start Isolation” feature is a unique to the Blue Sea Systems line of ACR’s. The start isolation feature momentarily opens or un-parallels both banks when the starter motor is engaged. The SI terminal of the ACR is wired to the momentary “start position” of the engine switch (see above image) or to the starter button. It is never wired to the “run” position. Doing this will keep the relay open indefinitely. Again, we’ve seen this done. When the starter motor is engaged the ACR’s relay opens so any voltage sag is not transferred to the house bank, where low voltage may cause electronics to shut down. For the SI feature to work as intended you need a dedicated starting battery and a dedicated house bank. The SI feature does not work with a 1/BOTH/2 switch where starting and house loads are shared by the same bank.

16.00V Over-Voltage Lockout – Over-voltage lockout is just what it implies. If the sensed voltage at either the “A” or “B” terminal is 16.00V or higher the ACR will lock out and open itself.

9.50V Under-Voltage Lockout – Under-voltage lockout is just what it implies. If the sensed voltage at either the “A” or “B” terminal is 9.50VV or lower the ACR will lock out and open itself. If you’ve drawn one battery too low don’t expect the ACR to combine until the battery gets back above 9.50V. In a case like this simply use your manual battery switch set to Both or your manual emergency parallel switch.

The above has covered myth & lore 1 & 2

#1 “An ACR charges the start battery first then isolates it and charges the house”.

As can be seen above the ACR does not in any way do this it is either in parallel or it is not in parallel. Very simple..

#2 “An ACR gives priority charging to the start battery”

Please understand that even if you feed charging to the start battery first, which is not advised on a cruising boat with disparately sized banks, 30 seconds of charging is not charging, even for a minimally depleted start battery. A battery at 99% SOC is in the worst range of charge efficiency. Despite being minimally depleted it still takes a good bit of time to reach an actual 100% SOC again. Each Ah we deliver to the battery, at a high SOC, is not being stored at 100% or even 50% due to the horrible Coulombic efficiencies at high SOC. The logic delays in the ACR are not there to create “priority charging” for a start battery or house battery they are there to eliminate relay-chatter and to help minimize relay-cycling or on/off/on/off/on/off behavior.

Q: “Why are there two different parallel voltages and delays?”

It is all about depth of discharge and when it is prudent to parallel the batteries. The lower combine voltage is there for a very good reason. It is there so that a deeply discharged bank does not take very long to attain the combine point. The higher combine voltage is there for a bank that’s not been deeply discharged and rises in voltage near instantly.

This all goes back to myth & lore #5 “You can’t charge the house bank first or the start battery will never get combined & charged” 

Let’s put this myth & lore to bed…..

To address the question of the house bank taking a long time to combine with the start battery, we first need to consider a start batteries actual energy usage.

Start battery Energy Use?
A battery used for starting an engine is using very little stored energy to do this job. It is a very short duration but also high amperage. Most engines will use considerably less than 0.5Ah to start. This is due to the cranking duration, loaded starter to unloaded starter, averaging 0.75 seconds to about 1.5 seconds (averages measured over 70+ marine diesel engines using the Midtronics EXP-1000HD). This means your previously full start battery will still be at about 99%+ SOC after you’ve started the engine. A 99% SOC battery does not really require immediate charging or priority charging and has many, many, many more starts left in it before any charging would even become necessary.

In the image above we have a 44HP diesels cranking diagnostics:

Averaged cranking voltage = 12.04V

Averaged cranking Amps = 286A

Loaded to Unloaded Cranking Duration = 0.765 Seconds

Even if we round up the cranking duration to 2 full seconds we are using just 0.17Ah. If we correct for Peukert, due to the high load on the battery, we are looking at a max fudge factor number of about 0.29Ah’s to start this engine.

Experimentation: For the sake of experimentation we cut power to an external voltage regulator then proceeded to start a Yanmar 4Jh forty-six times before finally getting bored.  The battery used was a single Trojan SCS-200 Group 27 “Deep-Cycle” 12V battery. Not once did this group 27 “deep cycle” battery even so much as wince at starting this motor forty-six times, in less than one hour, without any charging what so ever.

The experiment above was only done to illustrate to an employee why we charge house bank first, not the other way around, on cruising boats. When you run the actual numbers, and see how little energy is used to actually start a motor, it becomes much clearer.

Q: “But how long does it take to attain a combine/parallel voltage?”

From 50% DOD/SOC, the max depth of discharge recommended by most lead acid battery manufacturers, it takes a bit less than 2 minutes at a .2C charge rate to attain 13.0V and this experiment was done on a high acceptance AGM battery.

Q: What is .2C?

The term .2C simply means 20% of the battery banks Ah capacity in charge current. Blue Sea Systems knows how simple it is to attain 13.0V, even for a deeply discharged bank, and this is why their ACR’s feature two differing combine/parallel voltage points, one at 13.0V & 90 seconds and one at 13.6V & 30 seconds.

This battery began charging at 50% DOD/SOC when the clock read 12:00. The charge rate was .2C or the bare minimum recommended charge current for this Lifeline AGM battery. As can be seen, after just 2 minutes of charging at 21A, it is already at 13.1V and the ACR can now combine. If your start battery is going to suffer not being charged for two +/- minutes, you have other issues.

Rumor/myth & lore #5 goes something like this: By using a battery combiner, on “high acceptance” AGM batteries, and feeding the alternator or battery chargers charging current directly to the house battery bank first, “it will leave your start battery under charged“ because “it will never get to the combine voltage or will take too long to get there”.

If you are practicing good battery management, and have even the minimum suggested charge current for an AGM or flooded battery, this is really a non-issue. In 2 minutes of charging, at .2C or 20% of Ah capacity from 50% SOC, the AGM battery voltage is already at the parallel/combine level for the Blue Sea Systems ACR. Even at .1C or 10% of Ah capacity the time to attain 13.0V is not very long, just a few minutes more. To get from 13.0V to 14.4V+ does take quite a bit more time but the relay has already combined at 13.0V and both banks are now being charged.

Battery voltage will rise pretty slowly from the low 13’s on but, to get to an ACR’s combine level, is relatively quick and easy, especially if you have your system set up properly. When hearing this rumor we need to also consider that Echo Chargers, Duo Chargers and a number of other DC to DC chargers also turn on at similar voltages and those devices require all charge sources to be fed directly to the house bank. On cruising boats with disparate sized banks Blue Sea Systems recommends feeding charge current to house first, not start, but you have the option to choose start first if you really feel the desire and you don’t with other products such as the Echo Charger, Digital Duo Charger etc….

Correctly Wiring an ACR on a Cruising Boat

While a three wire device, four if you use the SI feature or five if you use the remote LED indicator, seems simple to install, there are some areas that can trip you up. One of the most common blunders we see on cruising boats is leaving the alternator wired to charge the starting battery first.  This is most often the result of the Blue Sea Systems instructions not being very clear. The majority of these are sold for use on boats where the battery banks are nearly identical in size. They are very, very popular on center-console and walk-around fishing boats where the start battery and house batter are nearly identical in size and the motor is started and stopped multiple times per day while fishing and owners don’t want the sounders and plotters to drop out during starting (SI). With both banks nearly the same size feeding the start battery first works pretty well. Because most builders sell boats wired this way, alternator feeding start battery, this is how they are typically wired. In an ideal world the charging would be fed directly to the bank that gets most depleted.

On a cruising boat, with a large house bank and small start or start/reserve bank, the best way to wire an ACR is to have the alternator charge the house bank directly.

“Why is it best to charge the house first?”

There are a number of reasons to do this but the best reason is to ensure the bank that needs the most charging is actually getting it and getting it as efficiently as possible.

#1 With large house banks wiring charging sources to the HOUSE bank means more efficient charging and more optimal voltage sensing for the alternator.

#2 With large house banks, wiring charging sources to the HOUSE bank means less chance of what is called relay-cycling. Please take the time to read the link below. Blue Sea Systems covers relay cycling very well so there is no sense in us repeating it.

Preventing Cycling in Battery Combiners, Voltage Sensitive Relays, and Automatic Charging Relays

#3 By wiring charge sources to the larger HOUSE bank the relay contacts need to pass just a few amps at best in order to charge the start battery. By feeding all charging to START means the relay must be able to handle the full rated current of the alternator and we are utilizing it at max duty cycle. We are also passing the charging current through multiple more terminations and fuses and there will be additional voltage drop.

One part of the instructions that most installers miss is this:

What About Fusing / Over Current Protection?

One topic that comes up rather routinely is ACR fusing/over-current protection. Because the ACR or VSR is connected directly to the battery banks + terminals, or their respective *close-by charge / always on distribution bus, the wires themselves need over current protection. There is some confusion regarding ACR fusing, even among some professionals, that the fuse is intended to protect the ACR, and it is not. The fuse is there to protect the wire as Blue Sea Systems clearly illustrates below. If we are following ABYC standards these fuses should be within 7″ of the banks positive terminals or their bus.

(*Within 7" of the banks + terminal)

Fuse Sizing

One mistake we see all too often is a 120A 7610SI ACR installed with a 120A alternator feeding directly to a start battery and the relay is protected by a 120A fuse. If the house bank is heavily depleted the relay can conceivably pass close to 120A across it for a short period of time or until the alternator heats up and can no longer produce its cold rating. Also keep in mind that many alternators can exceed the cold rating for short periods by as much as 15%. I think you can see why a 120A alternator with a 120A fuse would spell disaster for the fuse especially when fed to the start bank first.

The fusing is there to protect the wire not the ACR, so if you have a 120A alternator the minimum fuse & wire size should be 175A & 1 AWG minimum. Fuses should not be run at 100% of their rating or they will eventually nuisance trip. This is why Blue Sea Systems calls for a 175A fuse for a 120SI ACR when being used with a 120A rated alternator. Of course if you wire it correctly, for a cruising boat, and the alternator feeds the house bank first, the relay will never see 120A across it except during very brief inrush duration’s that are not long enough in duration to trip the fuse.

TECH TIP:

If your house and start banks already have over current protection you can simply use the same size wire for ACR “A” and “B” terminals as the bank is wired with. In other-words you can share those fuses for protecting the A & B ACR wires, provided you use the same size wire. If both banks are already fuse protected this can mean no additional fusing costs for the ACR installation. If you make use of an already fused charge/always on bus, as shown below,  you can just connect the ACR to that bus with the same size wire the banks are already using. In the example below the banks are wired with 2/0 wire and fused at 300A.

The use of a charge / always on bus is certainly a best practice and one more professionals and DIY’s should do more often. A charge/always on bus prevents messy bank wiring & incorrect lug stacking and makes for a neat and tidy installation anyone can easily troubleshoot.

Connecting Other Charging Sources

One of the major benefits of an ACR is that it works with any and ALL CHARGE SOURCES. Because an ACR is triggered by voltage changes it means that its an extremely valuable tool for charge management. Unlike a diode type isolator, that can really *only work with an alternator, the ACR can work with alternators, wind, solar, hydro, fuel cells, and AC chargers.

*Diode Isolators – Diode type isolators do not have a voltage reference on the input stud. By voltage reference I mean if you place your DVM on the input stud of a diode type isolator you will read 0V. This is one of the number one trouble shoot calls we get from folks trying to integrate solar or wind to multiple battery banks using a diode isolator. A diode isolator can’t be used with most charge sources that need to see a DC voltage before booting up. Today most voltage regulation charge sources have a feature that does not allow them to boot into no voltage where a typical “dumb alternator regulator” will. This is a safety feature so you’re not charging into a failed battery. Today there are very few good uses on a boat for a diode type isolator.

The question of other charge sources ,and an ACR, comes up a lot. Due to marketing it can be a bit murky wading through it all. The bottom line, for simplicity & operational sake on a cruising boat, is that you want to wire all your charging sources to the largest bank eg: the house bank. This would include, alternator, AC powered battery chargers, inverter/chargers, solar, wind, hydro or fuel cells. It is critically important to wire low-current charge sources such as solar, wind, hydro, fuel cells or small battery chargers directly to the house bank to prevent relay cycling.

In the image below we can see a cruising boats foundation wiring with a 500Ah AGM HOUSE bank and a 125Ah AGM START/RESERVE bank. As can be seen all charge sources feed the house bank and the ACR parallels in the start bank when 13.0V or 13.6V is attained.

What about twin engine boats?

On twin engine boats one alternator, usually the largest and most capable, can feed the house bank directly and one can directly feed the start bank directly. The addition of an ACR means that both alternators will contribute to the house bank charging during bulk. Without the ACR the start bank alternators full capability is just being wasted by feeding a few amps at best to the start battery. By adding an ACR we can make much more effective use of both alternators and charge the house bank faster.

Myth & Lore #10- “With a smart battery charger you must wire an ACR disable switch into the negative lead of the ACR”

This one can be a bit confusing but all boils down to what is actually inside a “smart-charger“. If your smart charger actually has multiple voltage regulators and multiple power supplies inside it, then a switch in the negative lead can allow the charger to charge each bank with its own fully independent charge profile. The catch here, and why this is MOSTLY FALSE, is because finding a smart charger with two or three fully independent chargers inside one box is about as likely as Hillary Clinton switching parties and becoming a Republican. Follow me for a moment..

What you think you’ve paid for:

What you actually have:

Another way to view most multi-output chargers would be like this:

With this image it becomes more clear how the single voltage regulation and single power supply can be connected to multiple batteries through “isolated outputs”. For this example I drew simple diodes, an electrical one-way check valve, but most chargers these days are using FET’s on the outputs to achieve the same effect. The only purpose of the FET or diodes on each output leg of the charger is to prevent the batteries from back-draining (in parallel with each other) into each other when the charger is turned off. You guessed it all batteries get the exact same charge profile just as they would if you fed charger output #1 to HOUSE and then used an ACR to charge the START battery.

Let’s discuss myth & lore #3: “An ACR will over-charge a start battery”

Please examine the above images and let them hit home. Now ask your self a simple question; How is a “smart charger”, a model that uses one voltage regulator and one power supply and two or three diode or Mosfet (FET) isolated outputs, any different than the BOTH setting on your battery switch or the combined mode of an ACR? If you landed on “its not any different” reach over your shoulder and pat yourself on the back. The diodes or FET’s on the single circuit of a multi-output charger are only there to prevent parallel back-drain when the charger is turned off. An ACR achieves the same exact outcome, preventing back-drain, by opening the relay when no charging is present.

The same guys who walk the docks and profess that an ACR will over-charge a start battery are quite often the same guys professing why you need a smart charger to charge your multiple on-board battery banks. I know this because one of these guys attempted to re–edumacate me on a dock one day, & he used this very argument. The charger on his own boats was a muti-output single power supply single voltage regulation unit. The funny part about this re-edumacation was the start battery on the boat I was working on was 8 years old and had been charged via a Blue Sea Systems ACR for the entire 8 years. It had been charged via multiple charge sources, including a shore charger, solar & alternator. According to the “dockspert” that start battery had been murdered 7 years ago by the ACR yet in the real world it was still going strong at year 8.

It was not worth trying to explain the concept to him, in a short period of time, and besides he’d already made up his mind on the subject. Little do folks realize there is usually no difference between using the multiple-outputs of a battery charger vs. using just one output of the charger and an ACR.. The ACR simply prevents back-drain by opening the relay when there is no charging & the smart charger uses diodes or FET’s to prevent back-drain. Whether you use the isolated outputs of the charger or one leg of it, and an ACR, there is really no difference.

The vast majority of multi-output smart chargers are one charger hiding behind two or three back-feed prevented (diodes or FET) outputs. If you want to charge multiple banks, and you already have an ACR, use the ACR, as it will work with all charge sources. This will save you charger to bank wiring and an extra fuse/s. To get around the multi-output charger and differing bank voltage profile conundrum, a situation where neither the multi-output charger nor the ACR would be a good choice, Sterling Power products offers their Battery Chemistry Module.

Ok back to our dockspert for a moment.

If your single power supply, single voltage regulation smart-charger is not over-charging your start battery, how is it that an ACR would?

Think about it…… Even Blue Sea Systems own “P-Series” chargers are one single voltage regulator and one single power supply. They market the product describing how it can float one bank while charging the other at absorption. While this is certainly a nice selling feature we still have millions of single VR/single power supply multi-output “smart chargers” out there that don’t do this, and yet we don’t have start batteries being routinely over-charged & murdered.

What Blue Sea Systems is actually doing in the P-Series is switching in an additional diode to the start battery output leg. Switching in an extra diode causes a 0.6V drop on the start battery output. It is not a truly independent smart charge profile but rather a 0.6V drop from the absorption voltage & a nice selling feature for sure. To do truly smart-charging, the type most boat owners assume they have, the charger would need multiple voltage regulators and multiple power supplies something very, very few chargers actually have.

Still, if you desire to allow your “smart-charger” to do it’s thing, or you use a Sterling Power Battery Chemistry Module or Blue Sea “P-Series” and feel it does a better job than the ACR, by all means insert a simple ON/OFF switch into the negative lead of the ACR, to disable it, or just flip the switch of the ML-ACR to OFF..

When to Use a VSR/ACR/Combiner

To keep this simple, when charging lead acid batteries, is that it’s all about the appropriate charging voltages. Also we can’t forget that GEL, AGM, TPPL AGM and Flooded Deep Cycle batteries are all lead acid chemistry.

With that in mind;

If both banks can be charged within 0.1V to 0.2V of each other, an ACR is a fine choice

Same Chemistry & Same Charging Voltages = √

Same Chemistry & Very Similar Charge Voltages =  √

*Mixed Chemistry & Same Charging Voltages = √

*Mixed Chemistry & Very Similar Charge Voltages =  √

*Excludes mixing lead acid and Li-Ion batteries

Most lead acid batteries will have a safe voltage range for absorption & float. If we compare a bank of Trojan golf car batteries and a Trojan Group 31 12V battery it’s clear to see they share the same charging voltage range and thus a VSR/ACR/Combiner is a good chocie for this application.

When Not to Use a VSR/ACR/Combiner

If we go back to Myth & Lore #4; “An ACR can’t be used with mixed chemistries”

We describe this as “partially false” and here’s why…

Let’s assume you have a GEL house bank, an excellent deep cycling battery, and a TPPL AGM windlass bank and excellent high current capable windlass or thruster bank. The GEL battery should not be charged at over 14.1V to 14.2V so the primary charging sources, solar, wind, alternator, chargers etc., would all be set up for a maximum of 14.1V to 14.2V. The problem here is that the ideal charging voltage for a TPPL AGM bank is closer to 14.7V. In this case, if charging is set up for 14.1V to 14.2V, we will wind up chronically undercharging the TPPL AGM bank via the ACR.

If we reverse this scenario, and the charging is tailored to the TPPL AGM bank, we will quickly destroy the gel battery by over-charging it.

Same Chemistry & *Differing Charging Voltages = X

Mixed Chemistry & *Differing Charging Voltages = X

*Differing by more than 0.3V

If specified charging voltages are the same or similar then an ACR/VSR/Combiner is a worthy choice. Once we get beyond about a 0.3V difference, it starts to make more sense to move to a DC to DC charger such as a Sterling Power Battery to Battery Charger where we can get a true fully independent smart charge profile.

“But an ACR will Discharge my Start Bank.”

This statement takes us directly to Myth & Lore #9 “An ACR will allow the start battery to drain into the house battery and leave it depleted.”

The fully charged resting voltage of a typical lead acid battery is about 12.72V. At any voltage above this point there is really no usable energy stored when discharging (see image below).

By now I know you are understanding it, but if not, this one is really quite simple. The ACR normally opens/un-parallels at just above or just at the 100% SOC point of a lead acid battery. If either battery bank dips below 12.8V the relay opens within 30 seconds. If it dips to 12.35V, the relay opens in just 10 seconds. The answer to this myth is that it is indeed false that an ACR will allow the start battery to discharge into the house bank. Your start battery cannot discharge into your house bank in 10 or 30 seconds.

The discharge graph below (voltage is the red line) was a typical marine battery undergoing a 20 hour capacity test. The battery was fully charged, equalized and had an open circuit voltage before the discharge test of 12.95V or what we refer to as a “surface charge“, despite having rested for a full 24 hours prior to the test. Because this was a 130Ah rated battery the discharge rate was 6.50A (130Ah / 20 hours = 6.5A discharge rate). At data point #1 the battery was at 12.95V and by data point 2 the battery was already below the open/isolated/un-parallel voltage of the ACR at 12.76V.

You are seeing that correctly, by the time this battery hit 12.76V a paltry 0.002Ah worth of energy had been removed. If the ACR opens at 12.8V how much can we discharge either bank by? This answer is nothing worth even really discussing…

If the relay opens at 12.8V it can’t remove any Ah quantifiable capacity from either bank before the banks are isolated.

This does bring us to another myth we have heard and that is when the banks are combined, in parallel, they can transfer energy between each other. This one is also a pretty simple explanation.

The ACR combines at well above the natural resting voltage of a lead acid battery. Due to this, charge current can only flow in one direction and that is into the battery. At 13.0V or higher current flows is into the battery from the charge source and the charge source would take up both the DC loads and the charging.

The combine point of 13.0V is a charging voltage and when a battery is charging it can not also be discharging. It can only be discharging when voltage is below charging level. Very, very simple.

The final point we should discuss is myth & lore #8;

“When the ACR combines batteries the massive in-rush current can blow up a battery.”

The easiest way to answer this question was to create a scenario on the test bench that could show these “massive” currents, currents so huge they can blow up a battery. (sigh)

The math could easily be calculated to show why this is not a concern but, it is often easiest to physically see it in action. In the video below we created and tested for the scenario that created the highest in-rush current we could create. The term in-rush, as related to this example, just means the absolute peak current measured over a very *brief time period,  between banks, at the widest voltage spread.

*The Fluke 376 captures current transients at 0.1 second or one tenth of one second.

The point where the banks are combined, and voltage spread is widest, is the point where the most current transfer is created. This in-rush lasts less than .2 seconds and current transfer, between banks, literally nose dives very rapidly as the bank voltages close in on each other and attain parity.

We created this scenario based on a very popular 315Ah AGM house bank (3 G-31’s) with an AGM starting battery. In order to try and capture the most in-rush we could, an Odyssey Thin Plate Pure Lead AGM, or TPPL AGM, was chosen as the start battery. The Odyssey PC2150M is a battery that can deliver over 5000A of current into a dead short, 2150A of cranking current at 77F and 1150A of cranking current at 0F. The 172A peak current it delivered to the house bank, for about .2 seconds, is literal child’s play. The test delivered the maximum recorded in-rush current at about 30% state of charge or a 70% depth of discharge. This is a depth of discharge you should not be routinely seeing with typical AGM batteries. Interestingly enough when we discharged the large house bank to 80% or 90% DOD the maximum in-rush was actually lower than it was at about 70% DOD. We chose to show the maximum in-rush we could create.

If we have 315Ah’s of AGM batteries, and we play pretend fairy-tale stories and assume the 172A in-rush could last for even 30 seconds (it can’t physically do this) this equals a charge rate of about 0.54C or just 54% of Ah capacity. In the Practical Sailor PSOC testing all the AGM batteries were actually charged, not an in-rush, at .46C (46% of Ah capacity) and the batteries barely even got warm to the touch. Of course this extremely short in-rush is not a “charging current” it’s a fraction of a second peak-transient current and is in no way dangerous to your battery bank. Yes, myth & lore #8 is indeed false and there are no “dangerous” in-rush currents created when the banks parallel with each other.

Another example of why this is not dangerous, Lifeline battery states their 100A AGM battery can handle 500A of in-rush charge current with ease. This equates to a charge rate of 5C or 5 times Ah capacity. As you’ll see below the ACR switching closed could only transfer 54% of Ah capacity in this test for a very brief 0.2 second transient.

In this video you will also see the effects of relay cycling and why hooking up a cruising bank incorrectly can create this phenomenon..

Choosing an ACR

Blue Sea Systems offers two distinct types of ACR models, the basic fully automated SI-ACR’s and the larger, manual or fully automated ML-ACR’s. Both charging relays feature fully potted electronics, heavy duty 3/8″ studs and the ability to charge two banks.

ML-ACR – The ML-ACR is a step up from the basic SI-ACR, and also costs a bit more. For a bit more money you get a lot more in features and current carrying capability and it can handle as much as 500A continuously. The ML-ACR also allows for manual paralleling of banks via a dash mounted toggle switch or the yellow knob on top of the ML-ACR. This means the ML-ACR could take the place of your emergency paralleling battery switch and do double duty as both an ACR and an emergency paralleling switch. The 500A continuous rating makes it the ideal product for boosting the available current to a bow bank used for a windlass or a bow thruster. With the flip of the toggle switch it can go from automated ACR charging, which would open on voltage sag, to manually locked in parallel. This means your bow bank, house bank & alternator can now all contribute to your windlass or bow thruster performance and you can rather drastically improve bow thruster or windlass performance. The ML-ACR also features SI or start isolation. Standby draw on the ML-ACR is a scant 13mA or just 0.013A.

SI-ACR – This is the basic fully automated model and is a relatively inexpensive upgrade. The 7610 SI-ACR can handle 120A continuously, has SI (start isolation), fully potted electronics and heavy duty 3/8″ studs. If you don’t need the high amperage or manual paralleling feature of the ML-ACR the SI-ACR is a great value. Standby draw on the SI-ACR is just 15mA or 0.015A.

Today there are lots of options for charging multiple banks, without suffering from voltage drop, and the ACR/VSR/Combiner is just one of them. We stock quite a few charge management devices in the MarineHowTo.com webs store.

MarineHowTo.com – Charge Management Devices

Good luck with your project & happy boating!