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 The 602 has been replaced by the BMV-7oo series (700, 702 & 712).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.
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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