The Balmar Smartgauge

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PREFACE:
This article is quite in-depth but much less so than it could have been. I have given a very brief overview of my actual testing procedures but enough to explain the methodology. Other than the EnerSys white paper this is the only other independent testing of this product that I know of.

Unlike EnerSys I tested new, used and even well-used marine batteries. If the product had failed my testing, this article would not be here. I don’t believe in writing articles to simply bash a product, unless of course it is a safety hazard.. The Smartgauge really surprised me and I am not easily satisfied. To be 100% honest I went into this testing with a slight bias that it would not or could not work. I was proven wrong.

Ah / Coulomb Counters:

For many years I have been a big proponent of Ah or Coulomb counting battery monitors and still am, for the right owner. These devices calculate and keep track of the current flowing into and out of a battery bank so you can attempt to keep track of SOC & Ah’s consumed, or at least get a rough approximation.

Coulomb counters give you lots of useful information; voltage, current, Ah’s consumed and SOC to name a few. Some can even give you historical data. These are nice features all packed into one compact unit. They are however not the easiest devices to program, not the easiest to wire and lose accuracy at an alarming rate, if not kept on top of.

Ah counters can be so problematic to use correctly that I had to write an entire article on installing and wiring them properly as well as a sister article to that one on proper programming. Despite both of these articles people still email me because they are confused. The Smartgauge is a simple two or three wire hook up!

Despite their complexity Ah counters have led almost every one of my customers to longer battery life, when properly used & wired. However, in some cases they have become so out of synch that they have led to erroneous readings that are simply meaningless.

Last summer I had a link 2000 reading -1100 Ah’s on a 65 Ah starting battery… (smirk) That can’t really happen now can it….? (wink)

Up until recently, this type of battery monitor was the best we had. The only other option was an ROCV reading (resting open circuit voltage) or SG measurement (specific gravity) neither of which lend themselves to prudent practical use or accuracy when actually using the vessel.

The problem with traditional Ah or Coulomb counters is keeping them accurate. As batteries age their capacity changes, the charge efficiency changes as does the Peukert’s constant. A battery is an ever moving target, so the 100Ah battery you bought three years ago may now only be a 75Ah battery.

If your battery monitor is still programmed for a 100 Ah capacity, and you are drawing 50% of the assumed Ah capacity out of it, based on this 100Ah’s, you are really drawing the bank to just 25% SOC, rather than the well accepted safe discharge level of 50%.

Follow me on this one.

• You had 100Ah programmed into the Ah counter for a 100Ah battery
• Your battery, due to age and use, is now 75Ah’s not 100Ah’s
• You now draw an assumed 50% of 100Ah’s out of the battery
• Instead of being at 50% SOC you wind up at 25% SOC

75Ah - 50Ah = 25Ah remaining or approx 25% SOC.

Now if you threw Peukert’s exponent into the mix you may actually be lower or higher depending upon the actual load at which it was drawn.

Holy cow Ah counters are confusing???? (head bonk)

This is but one example where an Ah or Coulomb counter easily and regularly become inaccurate.

A trick many of us in the industry might use is to start with a lower programmed Ah capacity than the bank is rated for. The best option is to physically test the batteries for 20 hour capacity, but this is expensive and time consuming.

For the 100Ah battery I might initially program it at 95Ah’s so the owner is never actually drawing to 50% SOC that first year (self protective feature). The next year I might remove another 3% – 5% off the capacity etc. etc.. These are just rough guesstimates, but they are never perfect. I then also count on the fact the battery bank is being drawn at average currents that are below the 20 hour rated load. This can lead to slightly more usable bank capacity, but again this gets CONFUSING for the average boater to understand any time we bring our good friend Mr. Peukert into the equation.

The only way to accurately know the actual battery capacity is to perform a physical 20 hour load test. This is complicated, time consuming, and very few boaters are willing to do this.

To be honest I don’t know of a single boater who actually has conducted an accurate 20 hour load test. Ah / Coulomb counters rely on the actual 20 hour capacity figures being accurate, to actually remain accurate, over time. No accurate 20 hour capacity figure, no reasonable accuracy in the Ah counter, only a “close enough” range. This may not be half-bad but is a a long way from accurate.

Unfortunately the scenario I laid out for a Coulomb/Ah counter is just one of the many ways these devices can become tripped up and lead to inaccurate readings. There are many more “gotcha” scenarios that can rear their ugly head, including shunt wiring mistakes, battery temperature, false re-synchs caused by solar or wind, incorrect programing etc. etc.. For the last 20+ years however, these are all we’ve had, and they are certainly better than nothing at all.

I suspect the big reason they are, and have been better, is because they make owners more aware of their bank. More awareness of your bank and charge source performance is important, and can play a larger role than we may otherwise assume.

For years I have been trouble shooting and helping owners try to use these devices in a smarter and more accurate manner. When owners understand it they can be very useful and as I said most all of my customers have had longer battery life as a result.

• FACT: Traditional Ah / Coulomb counters become LESS ACCURATE as time goes on when related to SOC.

• FACT: The Balmar Smartgauge gets MORE ACCURATE as time goes on for SOC

The Balmar Smartgauge is a major paradigm shift in battery SOC monitoring! Read on to find out why..

Peukert Effect & Ah Counting

Deep cycle lead acid batteries generally don’t like to deal with high discharge loads such as inverters, windlass motors water makers or electric winches. When you apply a discharge load above than the 20 hour Ah rating the actual usable capacity of the bank shrinks. Conversely if you consistently draw the capacity from the bank at below the 20 hour rating you will get slightly more capacity from the bank. Click the image to make it larger and see what I mean.

Deep cycle lead acid batteries, in the US, are rated at a 20 hour rating. This means a 100Ah battery can supply a 5A load for 20 hours, at 77-80F, before hitting a terminal voltage of 10.5V.

A 400Ah bank can supply a 20A load for 20 hours before hitting 10.5V. Any loads applied that are above the 20 hour rating, diminish the capacity of the bank and loads below the rated load result in slightly more usable capacity.

The 20 hour discharge rate is determined by; Ah rating ÷ by 20.

100Ah battery ÷ 20 = 5A
125Ah battery ÷ 20 = 6.25
225Ah battery÷ 20 = 11.25A

From this it is easy to see why simply looking at the Ah consumed screen of a Coulomb counter can be misleading at best. This is why an Ah counter that can correct for temp, Peukert, charge efficiency etc. will be the most accurate, when properly programmed, and the SOC screen is used.

Unless you have a consistent load that precisely matches the 20 hour rating, of your bank, and the battery is at 77F, then the Ah screen is simply not giving you an accurate representation of SOC.

Peukert:

All lead acid batteries have different Peukert constants. Some AGM batteries are as low as 1.11 and some flooded deep cycles as high as 1.50+.

Lets assume you have an Ah counter and a 100Ah bank and all you look at is the -Ah consumed screen.

The Peukert effect on two 100Ah banks at the same average 9A load:

* Bank 1 100Ah – Peukert 1.11, 9A load, 77F = Capacity at 9A Load = 94 Ah
* Bank 2 100Ah – Peukert 1.35, 9A load, 77F = Capacity at 9A Load = 81.5 Ah

If you only used the -Ah consumed screen on the 1.35 Peukert bank you would have:

* Non-Reality: -50Ah’s = “assumed” 50% SOC (100Ah – 50Ah = 50% SOC)

* Reality: -50Ah’s = 31.5% SOC not 50% SOC. (81.5Ah (9A load) – 50Ah = 31.5% SOC)

Of course your discharge load would never be a steady 9A continuously, and you would never use the entire capacity of the bank, so the numbers and examples are not precise, just as boat use related to Peukert, temp etc. is not precise. They do however give you a good idea of why proper programming, calibration and using the right screen can be the best way to use an Ah counter.

Here is another more simplistic look at it:

100 Ah Battery – Peukert 1.25:

100Ah Battery @ 80 Load = 50 Ah Capacity
100Ah Battery @ 50A Load = 56.23 Ah Capacity
100Ah Battery @ 40A Load =59.5 Ah Capacity
100Ah Battery @ 30A Load = 63.9 Ah Capacity
100Ah Battery @ 20A Load = 70.7 Ah Capacity
100Ah Battery @ 10A Load = 84 Ah Capacity
100Ah Battery @ 5A Load =100 Ah Capacity (20 Hour Discharge Rate)
100Ah Battery @ 3A Load = 113.6 Ah Capacity
100Ah Battery @ 1A Load = 149.5 Ah Capacity

I highlighted the 5A load because that is exactly what Ah capacity ÷ 20 gets you to, and where the battery is “rated”.

Peukert’s Effect =

* Discharge loads above the 20 hour rate result in less usable capacity.

* Discharge loads below the 20 hour rate result in slightly more usable capacity.

This is exactly why using the -Ah consumed screen is simply not an accurate representation of SOC.

Using the SOC screen, which has been properly programed, will result in the most accurate use of an Ah/Coulomb counter.

Are Ah counters complicated? You bet they are…. (wink)

The Smartgauge – Simple & Effective

There’s a new Sheriff in town and his aim, and ability to hit closer to the center of the target, is better than the old Sheriff..

The Smartgauge is actually a pretty amazing tool for SoC. I am a die hard skeptic, go figure, so when new toys like this come along I need to see, touch, poke, prod, test and put them through the paces. I need to see things for myself not what the marketing guy intended for me to see.

As a result of my skepticism I just finished multiple MONTHS of testing the Smartgauge. I tested it on AGM, FLA (flooded lead acid), GEL and LiFePO4 batteries.

The Smartgauge does exactly what it says it says it does with AGM, GEL and FLA batteries.

It falls flat on its face with LiFePO4 batteries, but this was to be expected because LiFePO4 has such a flat voltage curve as to be apparently unlearn-able. I suspect if an algorithm was created specifically for LFP batteries it may eventually learn the bank, but I doubt Smartgauge will do that for such a small niche market.

PHOTO: The Smartgauge is plain & simple, it displays battery SOC for the house bank and battery voltage for an AUX bank.

Here is is showing the bank at 92% SOC. In the mindset of keep it simple voltage is displayed in 0.05V increments and SOC, displayed as “C”, from 0-100%. That is all it does, see, simple.

The difference is the Smartgauge does this quite accurately and tracks SOC regardless of temperature, battery age etc.. The longer you use the Smartgauge, and leave it connected to the bank, the more accurate it becomes.

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The Testing Station

We don’t own a six figure Digatron like EnerSys does, so this test station had to suffice for the best accuracy we could do.

Test Bench:

• Charging – Mastech 3050EX & BK Precision 1900
• Charging – 80W Solar Panel – Rogue MPPT 3048
• Ah Counting – Victron BMV-602, Array DC Load Center, PentaMetric Data Logger
• Discharging – Array 3721A 40A DC Electronic Load & 400W Inverter
• Data Logging – PentaMetric Multi-Input Ah/Coulomb Counter & USB Interface
• Misc. – Fluke 179 DVM (NIST Calibrated)

TEST METHODOLOGY:

#1   Determine actual 20 hour capacity for each of the batteries tested through 20 hour discharge capacity testing. Both used and new batteries were tested. New batteries do not deliver full rated capacity, until broken in, so as-is condition for 20 hour Ah capacity had to be determined.

#2   Program Ah counters with the new 20 hour capacity and run load tests side by side with the Smartgauge.

#3   Test Smartgauge for accuracy during charging & discharging events, including solar, inverter loads and DC loads. Batteries were determined full when charge current fell to less than .5% of Ah capacity at target absorption voltage. (absorption charging voltages varied by battery type).

#4  Control room temperature to 75F to remove the temperature equations from the test calculations. (77F was just too damn hot). Part way through I added a temp controlled water bath to more accurately maintain battery temp, and keep my room a bit cooler.

#5   Re-test batteries for physical Ah capacity at the end of testing to note if changes were noted or the batteries declined in capacity during testing. Two batteries actually increased Ah capacity by approx 4% (new flooded batteries not yet “broken in”) and the rest were under 1% changes or well within my range of error resolution to even calculate.

#6   All used batteries were equalized and serviced before being put into testing.

NOTE: This testing took approximately four months to complete. This is not quick or easy work if you want to get accuracy as close as you can for the equipment you have on hand. It was a real eye opener as to how inaccurate traditional Ah counters can be, in regards to SOC.

Smartgauge Wiring

Remember when I said easy? This is it. You run a duplex 14GA wire to the house bank cross-connecting it as shown. Place the negative on one end of the bank and and the fused positive on the other end of the bank. For the start battery voltage a single 14GA wire goes to the B2 terminal.

IMPORTANT: Do not wire the Smartgauge to any place other than the actual physical battery terminals.  In order for the Smartgauge to work accurately the positive & negative leads from the Smartgauge need to attach directly to the physical battery terminals, not a positive or negative bus, or anywhere other than the actual battery terminals.

The Smartgauge’s negative and positive and battery bank system take off positive and negative points must be wired as shown in this diagram. The Smartgauge and system wiring must not simply pull from an end battery of a parallel bank. Negatives off one end and positives off the other end.

WARNING: The Balmar Manual is actually incorrect on how it shows the Smartgauge to be connected, this happens. Please use the connection method shown here if you want optimal accuracy.

Even if you have a traditional battery monitor you need to bypass the shunt and wire the neg to the neg battery terminal. The consumption draw/load of the Smartgauge is so small a traditional Ah counter shunt can’t even accurately see it or count it. The Smartgauge will however track its own miniscule consumption, over time, even when a traditional Ah counter can not.

The Smartgauge self consumption is less than 5mA (0.005A) when the display is asleep and just 15mA (0.015A)when the display is lit.

In 24 hours the Smartgauge consumes only 0.12Ah. In an entire week it consumes just .84Ah.

Bullet Point: A traditional battery monitor, using a 500A/50mV shunt, does not have the resolution to track the consumption of the Smartgauge.

Because the Smartgauge tracks voltage, not Ah’s, it can actually track its own self consumption. The self consumption of the Smartgauge can actually be less than a flooded batteries own self discharge in warm weather.

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Wire Gauge Confusion

There is a lot of confusion around which size wire to use when installing the Smartgauge. The actual manufacturer states that 18AWG is a bare minimum for wiring the Smartgauge. They also say that bigger is always better and originally indicated to Balmar that 14AWG should be used. As can be seen in the screen shot of the original Smartgauge manual.

Early Smartgauge manuals suggested 14AWG as seen in this image. 14AWG is what we at Compass Marine Inc. have used on every single Smartgauge installation. Typical of many manufacturers edits to manuals get made or they are not as clearly researched as they should have been by the individual making the edits.. On the hard drive here, we have no less than three Smartgauge manuals one suggesting 14AWG, one suggesting 16AWG and yet another suggesting 18AWG.

18AWG is the bare minimum. 16AWG is a happy medium and meets ABYC standards and 14AWG is even better and it fits near perfectly into the terminals.

The Results

In this image I am testing the Smartgauge with a 400Ah LiFeP04 battery bank. This is the only type of battery it failed to track accurately. Not a big deal as it was never designed for Li-Ion batteries and very few boat owners use LiFePo4 at this point in time.

I found it quite interesting that, while trying to find the actual capacity of some of the used lead acid batteries, the Smartgauge was already accurate by the second cycle and I was on my third complete discharge capacity test before finding an accurate new 20 hour rating for the battery to test it.

How accurate? It is tough to say precisely because I really don’t know how much I trust the Ah counters. Suffice it to say it was most likely as accurate as the Ah counters and probably below a 3% variation in SoC. I don’t have the test equipment resolution to make claims of 1% like EnerSys does, but even if under 5% this is simply outstanding.

The Smartgauge was lining up with the Ah counters, once the banks were well calibrated to the Ah counters (arghh what a process), to under a 2% – 3% variance. In many cases the Smartgauge beat me to the actual SOC. The Smartgauge found the SOC of the used batteries faster than I could by conducting actual physical 20 hour capacity tests. This is truly amazing.

The Smartgauge seems to work as advertised on GEL, AGM and FLA batteries in discharge mode.

What does that mean?

It means that I did see the Smartgauge get a bit confused when the bank was being charged. It can’t really track the capacity of a battery charger now can it…? However we are only talking about 10-12% variation from the Ah counters during charging, and not a huge deal when you consider how simple this battery monitoring unit is. Another issue with tracking SoC during charging is charge efficiency variations, so it was much easier to do this testing on the discharge side of the equation.

As soon as the charge source was discontinued, the Smartgauge fairly quickly identified the accurate SoC of the bank, and was back within approx 2% – 3% of the two painstakingly calibrated Ah counters.

WHAT THE SMARTGAUGE DOES:

1. It tracks the voltage of the battery bank up to 1500 times per second and over time learns bank behavior, with no human intervention & no complicated programming. As time goes on it gets more and more accurate. This is good!
2. It needs a good three to four deep-cycles for it to hone in on SoC. The longer it remains connected the more accurate it gets. I attempted 5-8 cycles on each bank beyond the capacity tests.
3.  It provides voltage of the HOUSE and START/AUX banks in 0.05V increments.
4.  It identifies SoC of the HOUSE bank irrespective of age or condition.
5.  It requires no programing beyond selecting the battery type and wiring it directly to the battery positive and negative terminals of the HOUSE bank.
6.  It removes the guess work and tediousness of programing a traditional Ah counter.
7.  It is the easiest to use battery monitoring unit I have ever used or installed.
8.  It works with either 12V or 24V banks and automatically detects this when connected.
9.  It offers low and high voltage alarm relay trigger ports. These ports can even be used to start a generator, if your vessel is so equipped.
10.  It requires no shunts or heavy gauge wiring and installs with simple 14GA wire. No battery lugs to crimp.
11.  It tells you all you really want or need to know in order to maximize your battery banks cycle life, the state of charge. It really does not matter what your capacity is just that what ever it is, you are not constantly pulling the bank below 50% SoC.

Program The Ah Capacity

This article features the older Victron BMV-602. The 602 has been replaced by the BMV-7oo series (700, 702 & 712). Everything in this article is still relevant to the current series of BMV-7xx battery monitors.

WARNING: This article is long and in-depth. Please do not misconstrue the points here and think we are trying to talk you out of an Ah counting battery monitor, nothing is further from the truth. Coulomb counting battery monitors give you great information but we also strongly believe owners should better understand;

#1 How they work

#2 How to make them work more accurately

First, let’s preface this article by saying that; I am personally big fan of Ah counting battery monitors. They can give an owner tremendous amounts of excellent and useful data at a glance or with a tap of a button. They can teach you about charging performance, on-board energy usage and even show you historical data. The newer models, such as the Victron BMV-712 (LINK) or the new Victron SmartShunt even have built in Bluetooth.

Where Ah counters frustrate myself, our employees and the average boat owner, is in the ability to accurately track SoC (State of Charge) for the way a cruiser typically uses their vessel.

This article is primarily written for Lead Acid Batteries

LiFePO4 is easier to track:

EVERYTHING BELOW THIS POINT DEALS WITH STATE OF CHARGE (SoC) TRACKING PERFORMANCE

When I first wrote the article on installing and wiring of a battery monitor, (See; Installing a Battery Monitor) I had originally intended to write this sister article to it, but never got to it. Unfortunately this is the article that should have been written first as it’s actually the far more important part of installing and using a battery monitor.

The SoC (state of charge) tracking problems with Ah counters are well known among us electrical geeks, throughout industry, military etc.. Unfortunately no one ever really discusses it frankly or in an in-depth manner. While Ah counters are extremely accurate at counting ampere hours (Ah) it is what the Ah counter is counting these ampere hours against, an ever moving target called a battery, that creates the problems. This article is going to show you why PROPER PROGRAMING PAYS.

To sum it up in simple terms we have yet to come more than a handful of properly installed and properly calibrated Ah counters. Most of the time the failure of the install is in the programming, typically a lack thereof. However,  the shunt wiring is confusing for some boat owners who conduct a DIY install as well as some professionals. I know this sounds shockingly surprising, but when you fully understand and comprehend how Coulomb counting works, and how a battery ages, it becomes a lot clearer as to why such craziness can be stated.

As a marine electrical business we are physically set up to test batteries for their true Ah capacity. Unfortunately in this industry I can count on one hand the number of marine electrical contractors or business that are set up to do this. To track this performance we simply note the SOC screen, and energy consumed screen on the battery monitor, when the batteries are removed from the vessel for testing.

This is simple; We have not yet seen an Ah counter’s SoC be within 10% of actual tested capacity and most are far worse than a 10% error. Is this the fault of the Ah counter? No, it’s not. It’s the fault of improper use, lack of understanding and poor programming.

FACT: If you do not keep up with programming your Ah counter, on a routine basis, Ah counters do not theoretically get out of sync with your battery, they physically get out of sync with your battery. This is a real problem that does happen, not a made up problem that might happen.

FACT: Ah or Coulomb Counters are very accurate at counting Ah’s but they do not track changes in your batteries health. They are simply calculators that rely on proper programming to yield a more accurate output.

This article is not going to explain the actual button pushes to adjust your particular monitor, there are far too many products out there to do this with, but we are going to discuss the importance of each piece of the programming puzzle, and why that part is critical to the performance of your Ah counter. Not all monitors offer all the programming features that may be discussed here. For example, the earlier Victron BMV-600 & BMV-602 offered no temperature adjustments or temp sensor options, newer models do. Some Ah counters, early Victron’s and Xantrex battery monitors, offered an automatic calculation for charge efficiency and for others you’ll  need to manually program the charge efficiency into the unit.

Bare Minimum Programming Steps:

-Program an Accurate Ah Capacity of Your Bank (best to obtain through testing)

-Program Your Banks Peukert’s Constant (obtain from battery manufacturer or calculate)

-Program Your Banks Charge Efficiency (obtain from battery manufacturer)

-Program Your Battery Temp (Should be done at least monthly if temps change, or use model with temp sensor)

The Battery, an Ever Moving Target

This section deals with the battery and how it changes with age, temp or other external factors as related to how an Ah or Coulomb counter attempts to track battery SoC.

#1 Battery Capacity:

The amp hour capacity of all deep cycle lead acid batteries is an ever moving target and the only way to track this moving target is for you to properly program it. The typical Coulomb counting battery monitor cannot, and does not, track changes in capacity, changes in charge efficiency (though some can) or changes to Peukert. It is the job of the owner to track these changes and update the programming of the monitor so that it has a more accurate data-set to calculate from.

Unfortunately deep cycle batteries never stay in one spot for very long and from day one they begin changing. How fast the capacity degrades is partially up to the banks owner and partially due to cycles, temperature and a host of other factors that all converge to eat away at your batteries actual Ah capacity compared to its rated Ah capacity. Different battery technologies will also yield differing slide-rates into the abyss. Certain battery technologies will also cycle-up to capacity faster than others? Cycle-up to capacity? Yes, cycle-up.

Incorrect assumption #1:

“My batteries are 100Ah, because the sticker says so.”

Rule #1 for Ah counter accuracy; never assume your capacity. Typical flooded lead acid batteries won’t deliver the full 20 hour capacity right out of the box, as many often assume they do. Batteries, especially deep cycle thick-plate flooded batteries, take as many as 50+ deep-cycles to attain their rated capacity. The problem is that many boat owners have already worn them out or ruined them before they’ve actually had a chance to cycle-up to full rated capacity. In other words many owners never even attain the full rating of the battery due to abuses.

Programming in the full rated sticker-capacity of the battery, without first knowing if that rating is accurate or true, may not be accurate. The sticker is good guidance, you as the owner would be best served to ascertain if that guidance is in fact correct, especially if you want any level of accuracy from your Ah counter.

AGM and GEL batteries cycle-up to rated capacity in considerably less cycles due to the way the battery plates are formed. Most commercially available AGM’s will cycle to full rated capacity in 3-7 deep cycles & GEL’s often within 30 cycles..

The problem is that not all batteries you purchase will always deliver the rated capacity. Often they are within 1-3% of the rating, some exceed it slightly, but we have tested many brand new VRLA batteries, or just broken in batteries, that are 4-15% off their claimed Ah capacity figures, right off the shelf. While 1-3% is acceptable 4-15% off rated capacity is not and can lead to Ah counting and SoC prediction problems.

Deep cycle flooded batteries are even more of a mystery, as related to Ah capacity. Identifying Ah capacity on flooded batteries is more difficult because of the way they slowly cycle-up, level out, and then begin trending down. What’s an owner to do, if you don’t physically capacity test your batteries? How will you know the actual capacity figure to put into the battery monitor? The simple answer is that, you don’t. Is this bad? Not necessarily with new batteries, but as they age it is.

The Typical Ah Counter is a Simple Calculator

The Ah counter is a factual item, like a calculator, and it is important to remember this. In fact, it is a calculator, and that is really all it is. The Ah counter relies on your input being accurate, so it can display a useful calculation. If you wanted to know 9X9=XX on a calculator you would type in 9X9 hit = and you’d get the answer of 81. A battery monitor is no different, it is nothing more than a calculator.

If you plug in 200Ah’s, for Ah capacity, it believes what you told it to be correct, even if your batteries are only 160Ah’s in their current state of health. Just like a calculator the Ah counter will give you accurate information based on what you have plugged into it. If you accidentally keyed in 6X9 and got 54 would that be the correct answer for 9X9?

Programming in erroneous data results in incorrect answers…

Programming in a correct Ah capacity is really just the tip of the ice-burg, but perhaps the most important part…

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Typical Programmed Capacity

Sadly this is typically how I see a good portion of battery monitors set up by using the stickered rating of the batteries. The typical battery monitor is initially programmed for say 400 Ah’s, as shown, and left there indefinitely.

Do you think that at year two, three, four, five, six etc. this bank still has 400Ah’s of capacity..?

They Even Explained How to Capacity Test Your Batteries

Again, this was in a nearly 18 year old Link 1000 manual. I don’t know of any current Ah counter manuals that include this information today.

What’s the big deal if my Ah counters capacity does not match my batteries?

Let’s look at the case of this pair of “100 Ah” batteries. These batteries were barely three years old when I tested them for Ah capacity. They had a tested capacity of just 69 & 70 Ah’s respectively, as tested at 77F, under a 5A constant load. The owner of this bank still had the battery monitor programmed for 200Ah’s, as many boaters often do at year three.

If we add 69 Ah’s + 70 Ah’s we can see that his actual capacity, of this bank, was now just 139 Ah’s not the 200 Ah’s the owner assumed it was. Herein lies problem, the owner was still discharging this bank as if it was still a 200 Ah battery bank. D’oh!

Coulombic Efficiency

This happens to be the set up screen for a Victron BMV-602 for the Charge Efficiency Factor or CEF. Here it is set for 90% which means it will calculate for 110% of the ampere hours that were removed, being returned.

Coulombic efficiency is the charge efficiency of your battery. In a perfect battery we would take 50 amp hours out and put 50 amp hours back in and this would be a Coulombic efficiency of 100%. With lead acid batteries it does not work like this and just like the Ah capacity changes with age, so does charge efficiency. Coulombic efficiency, like Ah capacity, is also a moving target.

Even the most charge efficient lead acid batteries often require 110% of removed capacity to be returned, and some batteries require 130% or more. As batteries age, and sulfate, charge efficiency can get worse. This means that if you use 100Ah’s you need to put back 110Ah’s to 130Ah’s plus in order to reach full charge.

Another issue with charge efficiency is that it’s different at different stages & different points in the SOC curve. Coulombic efficiency is also impacted by battery temperature as well. In bulk charging, when the battery is taking constant-current, the charge efficiency can be pretty close to 98-99% for lead acid batteries yet the last 5% of the SOC curve can see charge efficiencies at less than 50% efficient.

If you discharged to 50% SOC then replaced 20Ah’s, while never attaining a limiting voltage, you could take almost all of that 20Ah’s back out of the battery and wind up at the same exact SOC. BULK charging is a very efficient stage of charging, but absorption is not.

Sadly most Ah counters have no way to discern a difference between bulk and absorption or the gassing stages of charging where Coulombic efficiency is the worst.

Begin Quote:
“Charge Efficiency Factor: This is the traditional method used by the battery monitor, and the method used by some other similar monitors. This counts the amp-hours discharged exactly, at 100% rate, but when adding amp hours back (charging) they are counted at a lesser value, for example only 94% of actual charge. This requires that to make the “battery % full” display go back up to 100% charged, slightly more charge (6% more in this case) must be put back compared to what was discharged. This is the effect produced by the “efficiency factor” setting which is set at the factory to a recommended value of 94%, however you may program this to any value you wish from 60 to 100%.”
End Quote:

Please let that quote from a battery monitor manual sink in. If you understand anything about charge efficiency you’ll recognize that it’s not the same throughout the SOC curve. By applying a fixed percentage subtracted, as the Ah’s are counted back up results in counting errors if you don’t go all the way back to 100% SOC after each discharge. Simple stuff.

On boats we also use different charge sources, with different charge rates, all of which impact the overall charge efficiency.

• What if you stop charging in bulk, which is darn near 100% efficient, but the monitor has already counted your SOC back up with a 10% handicap?
• What if you only discharge to 94% SOC and the Ah counter subtracts 10% on the re-count but the charge efficiency was closer to 50% at high SOC?

Coulombic efficiency is sort of like Peukert, the rate of re-charge current can also cause changes in charge efficiency. A low charge rate of say 4% of battery capacity, 4A for a 100Ah flooded battery, the battery will have a better overall charge efficiency than a charge rate of 25% of Ah capacity or 25A for a 100Ah flooded battery. Charge efficiency also changes with battery chemistry such as GEL, AGM or flooded and also changes as the batteries age.

Unfortunately the typical Ah counter cannot track nor can it calculate for any of these variables in charge efficiencies, it is not designed to do so. To further compound the issue many boats have multiple forms of charging, some with low rates such as solar and some with very high rates such as alternators.

With a Coulomb counter we simply need to set the charge efficiency the best we can. Some Ah counters can set CEF automatically and I strongly recommend using this feature but it relies on you getting back to 100% SOC regularly and can quickly get out of sync during PSOC cycling.

A charge efficiency factor is often set as 75%, 87%, 90% etc., if you choose to do it manually. This means a battery with a factory return ampere hour number of 110% gets set at 90%. Is this perfect? No, but it is the best we can do. All lead acid batteries require more ampere hours to be returned than were taken out.

Unfortunately Ah counters are not programmed to know the difference between BULK/constant-current and ABSORPTION/constant-voltage charging as related to the charge efficiency settings. The higher you go in the SOC range the less efficient the conversion of input energy to stored energy is. Charge efficiency is not simply 110%, 115%, 120% returned, at all stages of charging. The Ah counter only has one number to apply and that is the charge efficiency you set it to apply or that it has chosen to apply. This charge efficiency factor is applied regardless of where you are in the charge state, BULK or ABSORPTION, what the battery temp was, and regardless of rate of re-charge in current. It also ignores where you stopped charging in the SOC curve so PSOC cycling can throw a monkey wrench into the applied calculation.

Unless you do a full 100% recharge cycle, every time you discharge, the Coulombic efficiency setting will lead to counting errors when calculated to reflect SOC. If you partial state of charge cycle (PSOC) in BULK, when off cruising, these errors can add up  quickly.

Testing for charge efficiency is considerably more complicated than Ah capacity so best to simply use the battery manufacturers stated charge efficiency factor or let the Ah counter calculate for you by discharging more than 10% of capacity and then doing a full 100% re-charge. The Xantrex Link monitors, the old Victron BMV-501 and a few others can “auto-calculate” the charge efficiency factor.

Many battery makers don’t publish a charge efficiency number so before buying batteries make sure you can get these numbers. Here’s a tip, don’t buy marine batteries from a company that can’t provide you a Peukert’s Constant, a general charge efficiency factor or the 20 hour Ah capacity figure.

Here’s an example of why a single number charge efficiency setting will lead to counting errors or drift in an Ah counter..

Begin Quote:
Lifeline Battery – “The amount of energy necessary for a complete recharge depends on the depth of discharge, rate of recharge, and temperature. Typically, between 102% and 110% of the discharged ampere-hours must be returned for full recharge.”
End Quote:

A single number for charge efficiency, such as 85% or 90%, is just not realistic in terms of overall accuracy on a vessel that PSOC cycles. However, that is what the battery monitor is set up for, and what we have to work with, so do your best to get it as close to accurate as you can. Finding this number may involve a call to your battery manufacturer or some deep internet searching. The Charge efficiency setting will cause out of sync events with an aging battery, & PSOC use, there’s just no way around this. Do the best you can to ascertain the best charge efficiency suggestion for your batteries. Again, if your Ah counter offers an automatic charge efficiency calculation, I suggest using it.

CAUTION: Not all battery monitors allow you to program for charge efficiency and they may try to internally apply a preset efficiency to all batteries. Be careful with monitors that leave this important programming option out or ones that do not offer an automatic CEF calculation.

I will leave charge efficiency with this quote from a US Government study of flooded lead acid batteries. In this test it was a Trojan G-31 flooded battery. Think about how an Ah counter would be calculating for CEF if you cycled your battery in the top 30% of the batteries capacity meaning 100% SOC to 70% SOC or say 60% to 90% SOC.

Begin Quote:
“Preliminary results agree well with established general understanding that the charge efficiency of flooded lead-antimony batteries declines with increasing state-of-charge, and that charge efficiency is a non-linear function of battery state-of-charge. These tests indicate that from zero SOC to 84% SOC the average overall battery charging efficiency is 91%, and that the incremental battery charging efficiency from 79% to 84% is only 55%.”
End Quote:

Think about that statement and how a single number charge efficiency could impact your bank if you shallow cycled it in the top 30% of the SOC range or routinely cycled in the most efficient range from 50% to 80%SOC. The charge efficiency algorithm in many Ah counters was designed with a 100% re-charge in mind not PSOC cycling as we do on boats.

Do the best you can with this one..

Peukert Effect on Capacity At Varying Loads

There is no better way to explain this than to show you an actual hourly capacity chart from a battery maker who publishes good reliable data. Don’t expect good data like this from Wal*Mart / Johnson Controls products or no-name batteries, it usually does not exist. This battery happens to be a very heavy duty L-16 deep cycle battery with a Peukert of 1.50.

So what can we glean from this chart?

100 Hour Discharge Rate – If we go all the way to the top we can see that if we apply a small load of 4.86A to this 375Ah battery it will run for 100 hours before hitting 10.5V and deliver 486 ampere hours of usable capacity! Yes 486Ah’s from a 375Ah battery when drawn at a low rate. This is the net positive side of the Peukert effect. By drawing this battery at 4.86A vs. 18.75A we’ve netted a usable capacity gain of almost 30%. If your battery monitor is not programmed to calculate for this, how accurate do you think your SOC will be….?

WARNING: Most Ah counters only compensate Peukert for HIGH DISCHARGE RATES. They completely leave out the low discharge side of the Peukert effect which can  increase the banks usable capacity at a LOW DISCHARGE RATE. Most all Ah counters have no way to compensate for a 30, 40, 50 or 100 hour discharge rate.

Now look at the rate of discharge compared to our typical house-bank discharge rates on cruising boats. A good many cruising boaters discharge their bank at about 25-30% of the 20 hour rate? Well 4.86A is 26% of the 20 hour rate for this particular battery.

20 Hour Discharge Rate – This is a 375Ah rated battery and at an 18.75A  load it will run for 20 hours and deliver its rated capacity. 375Ah ÷ 20 hours = 18.75A discharge rate. This is Peukert net neutral.

1 Hour Discharge Rate – Conversely if we apply a 135A load to this battery we can only get 1 hour of run time from it before it hits 10.5V. This is the net negative side of Peukert.

As can be seen from this battery manufacturers hourly discharge rate chart the Peukert effect is a setting that simply should not be ignored. Even if it only compensates one sided it still pays to program it as best you can. You really need to program your battery monitor for the correct Peukert in order to get the best information from it.

Let’s contrast that 1.50 Peukert L-16 flooded battery to a Lifeline AGM battery, which has a Peukert of 1.12. Using a 120 hour discharge rate, on the Lifeline AGM battery, will yield nearly 114% of its 20 hour rating (Lifeline does not publish a 100 hour rate). This is a large difference in the actual SOC of these two batteries especially if you can’t physically properly program the Peukert in your Ah counter for low discharge rates..

Peukert Effect Means:

-Loads above the 20 hour discharge rate = Less usable Ah capacity

-Loads at the 20 hour discharge rate = Rated battery Ah capacity

-*Loads below the 20 hour discharge rate = More usable Ah capacity

*Most Ah counting battery monitors do not account or correct for loads below the 20 hour discharge rate.

Smarter Use of Your Battery Monitor

“But RC I have heard you say many times battery monitors generally lead folks to longer battery life, how can that be?”

That is correct and I still actually stand by that statement despite a lack of accurate programming. Let me share some reasons why I can say this…

Reason #1

Prior to having battery monitors many of my customers had simple analog volt meters. Most of these analog volt meters were quite inaccurate and many owners often discharged to well below 12.0V before recharging. They were in essence taking their battery to 70-80% DOD with each cycle.. Battery life for many was the typical 2-3 years we see with abused batteries.

Along comes the Ah counting battery monitor and all of a sudden the owner has a screen to watch, buttons to press and has some insight into the battery that never before existed. A good portion of these customers now started to be really careful about battery depth of discharge (DOD). The 50% DOD they assumed previously, which was really closer to 70 or 80% DOD, now became “Wow I am at 65% SOC I think I will recharge.” Human attention was brought to batteries that never had such a fan base.

Reason #2

Previous to the Ah counting battery monitor most owners had not a clue as to what their alternator or charging systems were doing. They watched voltage and when it got to 14.4V or so they stopped charging. With the battery monitor they could at least hold off until a good portion of the -Ah’s had been returned. Is this accurate? No, not at all, but it resulted in batteries getting to a higher SOC than the owners previously & regularly attained thus leading to overall healthier batteries.

Reason #3

Peukert rears his head once again. Remember when we talked about drawing the batteries at a slower rate of discharge and getting slightly more usable capacity from the bank due to the Peukert Effect? Keep in mind that many boat owners are discharging their banks at approximately 20-30% of the 20 hour discharge rate. A bank of 450 Ah’s can support a 22.5A load for 20 hours at 77F. If we draw that bank at 6A at 82F it will run longer than 20 hours at that load. This can make a typical deep cycle battery bank, as installed on cruising boats, appear slightly larger based on the low rate discharge. Many owners look only at the –Ah’s screen and then unknowingly mentally assume a Peukert of 1.00, which does not exist, even in LiFePO4 batteries. With this type of low current discharge use, the actual DOD before recharge was slightly higher than was assumed by looking only at the -Ah’s screen & thus a shallower DOD before recharge and a higher SOC after recharge.

With those three points it is easy to see how we take an average battery life of 2-3 years and extend it to 4-5 or more, even with a poorly programmed monitor. There of course does eventually come a point where the battery monitor is actually harming the batteries when actual capacity fade has not been accounted for and these batteries tend to litreally fall off the proverbial cliff, when this occurs.

We can do better, if we program more accurately.

“ RC I want to  keep my Ah counter more accurate, keep it on its toes so to speak, but I am just not inclined to do an actual 20 hour capacity test? What else can I do?”

The *Old School SOC Test:

• Check the SOC screen and write it down
• Check the –Ah’s screen and write it down (Hint: -Ah’s and SOC should almost never agree)
• On a day when the batteries are between 75 & 80F disconnect them
• Allow them to sit for at least 24 hours (AGM & GEL may take longer)
• Test the specific gravity (SG) or the open circuit voltage (OCV) at the battery terminals
• Compare this voltage or SG reading to your manufacturers;  SG to SOC or OCV to SOC scales. (Avoid batteries that don’t have this data)
• How does this compare to your Ah counter?

*A marginal method for checking for the level of honesty in your battery monitor at tracking SOC

It’s important to understand that every 10% of battery capacity is represented by an approx 0.1V change in the rested open circuit voltage (OCV) reading. A short rest or an incorrect OCV reading is no better than an improperly calibrated battery monitor at finding your approximate SOC.

Different batteries from different manufacturers will have a slightly different rested OCV or SG for determining the approximate SOC. Generic scales for this, often found on the internet, may not apply well to your particular batteries. Always get your OCV to SOC or SG to SOC charts directly from the manufacturer of your batteries.

Please also understand also that the test above only tells you how the monitor is tracking against SOC, not what your current Ah capacity is. Specific gravity (SG) or OCV readings tell you nothing about the actual Ah capacity your battery can deliver. For determining Ah capacity the only thing you can do is run a physical 20 hour Ah capacity test.

The battery bank represented by the meter reading, shown at 12.76V, had rested for approx 3.5 hours completely disconnected, at 80F. After 24 hours it was giving an OCV reading of 12.54V, or approx 84% SOC for this battery bank. The Ah counter said the battery was 100% full, a 16% difference in the relative SOC readings. If we had accepted the 3.5 hour OCV reading we would have incorrectly assumed the Ah counter was accurate when it was really 16% off.

“But RC 16% is not that much is it?”

This was a 450Ah rated bank so a 16% counting error = a 72 Ah variance. A 72Ah mistake is an entire day’s energy use for this particular boat. To compound on that counting error, when these batteries were tested they were really only 383Ah’s…. This is how the snowball rolls…

How Did I Arrive At All This?

I know it is a lot to absorb and to comprehend. Much of it I know will go directly against what you thought you knew about Ah counters, and I’ apologize for that.

How did I arrive at all this? It’s what I do and part of my job to understand this stuff. I actually test an awful lot of batteries for 20 hour capacity. All my tested batteries are run through the paces in a controlled temp “water bath” set for 77F (unless testing certain GEL batteries) and tested with laboratory grade calibrated test equipment. The battery being tested in this image is at the very tail end of a 20 hour capacity test. It is a Lifeline GPL-31T rated at 105Ah’s. As can easily be seen it delivered slightly over 105Ah’s and the 5.25A load is just about to cut off at 10.499V..

With this information an Ah counter can be more accurately programmed. Without accurate programming you can use your Ah counter but just don’t expect more than a general ball-park accuracy. Is this bad? Heck no, but I think you’ll agree we can always do better.

Don’t fret it though, there are many ways to look at this:

#1 You now have a better understanding of Ah counters and will be more careful trusting yours empirically, as you may have in the past.

#2 You are perfectly happy with your banks performance already so if you can tweak it just a bit, and get better life, you’ll be in 7th heaven.

#3 You’re a a geek at heart and you want this damn thing to be as accurate as you paid for…. Good luck.. (wink)

#4 There must be an easier way?

There is now an easier product for monitoring SOC, it is called the Smartgauge.

The Smart Gauge gives you less information but a far more accurate representation of SOC. In the whole realm of battery health, and your system running efficiently, SOC is the only parameter that really matters. You can add a Smartgauge and use it side by side with any Ah counter but if you do that I would suggest that you ignore the Ah counter for anything more than amps, -Ah’s and volts, unless you want to go all tech geek and actually program it accurately….

This article does not change anything about Ah counters, they have been like this since they were invented. I am hopeful it did give you a better understanding of how they actually work and how and why they drift or develop counting errors over time. It is my hope that you can use this information to make your Ah counter more accurate for your battery bank.

Good luck!