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Compass Marine How To | all galleries >> Welcome To >> Keeping Your Battery Monitor More Accurate > Program The Ah Capacity
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Program The Ah Capacity

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

#1 How they work

#2 How to make them work more accurately

First let me say that I really, really like Ah counters for the following:

Voltage Information
Consumed Ampere Hours Information
Amperage In/Out Information

What I dislike like about Ah counters is:

Tracking State of Charge

The first three items alone make an Ah counter worth having on board. This article however deals almost entirely with how to make them track your batteries State of Charge (SOC) more accurately.

When I first wrote the article on installing and wiring of a battery monitor, I had originally intended to write this sister article to it, but I never got to it. Time flew and I am finally getting around to it now. Unfortunately this is the article that should have been written first as it is actually the far more important part of installing and using a battery monitor.

The problems with Ah counters are well known among us electrical geeks, though no one ever really discusses it frankly or in an in-depth manner. While Ah counters are extremely accurate at counting ampere hours (Ah’s) it is what the Ah counter is counting these Ah’s against, an ever moving target called a battery, that creates the problems. This article is going to show you why PROPER PPROGRAMING PAYS.

To sum it up in simple terms I have yet to come across a properly installed and properly calibrated Ah counter. I know this sounds shockingly surprising but when you fully understand and comprehend how these devices work, and how a battery ages, it becomes a lot clearer as to why I can suggest such craziness.

As a marine electrician who is physically set up to test batteries for their true Ah capacity, I note the screen on the battery monitor when I remove batteries and bring them to my shop for testing. This is simple; I have not yet seen an Ah counter programmed to within 10% of actual tested capacity and most are far worse than a 10% error. Is this the fault of the Ah counter? No, not entirely, it is the fault of improper use, poor programming and the inherent limitations of the typical Ah counter.

FACT: If you do not keep up with programming your Ah counter, on a continual 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 (they actually count kWh but display Ah) but they can not track changes or declines in your batteries actual energy capacity. They are simply calculators that rely on proper programming to yield a more accurate output.

In this article I am not going to explain the actual button pushes to adjust your actual monitor, there are far too many products out there to do this with, but I am 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 I will discuss. For example the earlier Victron BMV-600 & BMV-602 offered no temperature adjustments or temp sensor options.

Bare Minimum Programming Steps:

-Program Accurate Ah Capacity Of Your Bank (obtain through testing)

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

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

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

I first want to discuss 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, the battery owner, to properly program it. The battery monitor cannot, and does not, track changes in capacity or changes to the Peukert's exponent. Ah / Coulomb counters do not track changes in your battery capacity as it ages.

As a programmable tool 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.

Batteries never stay in one spot for very long and from day one they begin changing. How fast the capacity slides or 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 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.

Poor assumption #1: “My batteries are 100Ah’s because the sticker says so.”

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 50-150+ cycles to attain their rated capacity. The problem is that many boat owners, especially those who are in an abusive PSOC (partial state of charge) use cycle, have already used them up before they’ve actually had a chance to cycle-up to full capacity.

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 SOC prediction 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 I have tested many brand new VRLA batteries or just barely broken in 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 errors..

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 can be.

In regards to flooded Group sizes 24, 27, 31 and 4D & 8D there are only three brands I have tested that actually met or came close to their advertised Ah rating and those are; Trojan, Crown & US Battery.

I have yet to test any other Group 24, 27, 31, 4D or 8D that met or even came within 5% of it's rated Ah capacity and this testing is new, broken in & well cared for batteries. If you own flooded batteries such as group 24, 27, 31, 4D or 8D you MUST test them for capacity before plugging that capacity into the Ah counter.

The Ah counter is a factual device, 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=?, 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 type 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 typed 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 the correct Ah capacity is the tip of the ice burg but perhaps the most important part...

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