Quote: "100 hours out of a 220 AH battery is 55% SOC, out of a 180 AH battery is 45%. Nevertheless it is still 100 AH that has to be replaced by some means, for a sailboat cruiser this is what is important."

It is not, unless you drew it out at 75-80F, at the 20 hour rate and had that capacity to begin with, which most batteries don't, once used.. This is pretty simple stuff... Loads are very, very rarely discharged on house banks at anywhere near the 20 hour rate thus we have to rely on Peukert corrections and temp corrections to get a more accurate idea of SOC/DOD... The upside is that we draw the banks down less than we "assume" we did so our SOC is actually usually higher than we assume it is, if the bank has the capacity it is programmed for.

220 Ah Bank Peukert 1.25 Assumes 75-80F battery temp, load constant

100Ah's at 100A = Peukert Corrected Load 173A - Bank Capacity at 100A load = 126Ah

100Ah's at 50A = Peukert Corrected Load 73A - Bank Capacity at 50A load = 151Ah

100Ah's at 30A = Peukert Corrected Load 39A - Bank Capacity at 30 load = 171Ah

100Ah's at 11A = Peukert Corrected Load 11A - Bank Capacity at 11A load = 220Ah

100Ah's at 5A = Peukert Corrected Load 4.1A - Bank Capacity at 5A load = 268Ah

If one were to use just the Ah screen with a 100A load you would be way off..

If one were to use the Ah screen at 5A you would also be quite a off....

If not doing manual resets when the bank is known full these counting errors only tend to get worse...

Of course loads vary widely on boats so the Ah screen alone is really only a hip shoot....

If someone is competent enough to use an Ah counter, as you are, then they are terrific tools and wonderfully accurate for a period of time until capacity, temp etc. changes again. The vast majority of boat owners, in my experience, are not competent enough or willing enough to get good accuracy out of an Ah counter but they are still great tools..

The point is that 100Ah out of 220Ah battery is only 55% SOC if the temperature, load and capacity all line up with what you assume to be correct.... Considering you are an EE I find your statements quite interesting in that you can't understand how counting Ah's, without considering Peukert, temp, actual capacity etc. is anything even close to accurate... The only way to use an Ah counter, and get good accuracy, is to use the SOC screen, or equivalent, which accounts for temp & Peukert. The Ah is just a guess, and may even be close, but won't be accurate unless proper programming has taken place and temp and Peukert are accounted for.

Some elcheapo Ah counters predetermine the Peukert constant for you. This really sucks when you get into a bank of Rolls batteries, or some Odyssey TPPL's, and there is no way it is going to be accurate. This does not even begin to touch on "gotcha" issues like solar or wind replicating the "charged parameters" and resetting the Ah counter to 100% well before the bank is even close to full. I see this one all the time, especially with solar.. It also does not even touch on charge efficiency, which also changes with age, or self discharge which a Coulomb counter can accurately predict. All of these lead back to manual control of "known full resets" as a primary way to keep these devices close to accurate.

Even for the Link Pro used on my LiFePO4 bank I had to calculate the Peukert by conducting physical load tests. Because the Peukert of this bank is so low I ideally could use the Ah screen but on a bank with a 1.25+ you really get lost... The Link Pro is now quite accurate in relation to my Li bank but it is still always manually re-set and the bank undergoes yearly capacity tests to ensure the Link Pro is giving as accurate data as possible (actually it has had 8 capacity tests in 24 months due to accelerated cycle testing). If they could develop a Smart Gauge type device that worked for Li I would use it in a heart beat.

The best counting error I saw was on a Sabre. The Ah counter was reading -1100Ah's on a 65Ah starting battery and the owner had long ago given up on the Ah or % charged screen and simply reverted to voltage.....

You are absolutely right that 50% is not a magic number. The shallower the discharge the more cycles you get and Ah counters on large house banks tend to trend in favor of shallower discharges, for a while, until it flip flops as battery capacity diminishes, the capacity has not been reprogrammed, and then it really begins to snow ball....... I had one customer with a 690Ah bank with a 1.27 factory stated Peukert. His average load was about 8.5A. He wanted to cycle to 60% SOC and used the Ah screen to do so not the SOC screen. Problem is that at his average load his bank was closer to 1000Ah's not the 690 he was doing the math on by looking at the Ah consumed screen. He was discharging far less than the 60% SOC he wanted to be at before starting the motor as a result he ran the motor more than he wanted to and was in the very inefficient absorption charging range rather than bulk like he wanted/assumed..

The reality for me is that I see so many Ah counters wired wrong, programmed wrong, wrong assumed capacity etc. etc. that the Smart Gauge is going to be a great tool for many boaters who don't have the ability to use an Ah counter as accurately as they would wish..

It could also be argued that an accurate SOC is really all one needs to maximize cycle life of the bank and even at that it really only needs to be known once per day to adjust usage & charging...