Lead acid batteries do not like to deal with high discharge loads such as inverters. When you apply a load larger 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 rating load is determined by; Ah rating divided by 20.


100Ah battery / 20 = 5A

125Ah battery / 20 = 6.25

225Ah battery /2- = 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, then the Ah screen is simply not giving you an accurate representation of SOC..


All batteries have different Peukert constants. Some AGM's 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. Now we are going to look at just the Peukert effect on two different 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


So 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 load would never be 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, but they 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


100Ah Battery - 3A Load = 113.6 Ah Capacity


100Ah Battery With - 1A Load = 149.5 Ah Capacity



I highlighted the 5A load because that is exactly what the divide Ah capacity by 20 gets you to and where the battery is "rated".


As you can see any load above the rated capacity at the 20 hour Ah rating results in less Ah capacity. Any load below the 20 hour capacity rating and you have more available Ah 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...


Complicated? Sure is.... (wink)