Some things:Shipping batteries in the US requires the Watt-Hour rating to be marked on the battery, and the rule is to use the "nominal" voltage. So LFP uses their 12.8v figure for that 100AH x 12.8 is 1280 WhI think ? the "nominal" voltage for other batteries is 12v and 6v, eg so a 100AH 12v would be 1200 Wh for shipping it.This has nothing to do with their capacities you want to monitor in an RV, where you start with the capacity as Full (100%) and you have voltage tables vs SOC as percentages of capacity. Using AH for capacity:The AH rating is at the 20 hour rate, which has the batt run down to 10.5 volts. A battery is not out of all capacity below the 10.5v, but the SOC tables use "residual capacity" at 0% at different voltage levelsBattery type, !00%, and 0%LFP- 13.6, 10SiO2- 13, 11.1AGM -13, 11.5FLA- 12.73, 10.5 (Trojan table only goes down to 10% at 11.51)Not clear how the 20 hr rate down to 10.5v works for an LFP that has zero at 10v or an AGM that has zero at 11.5v.You do have to enter something in your SOC monitor as what 100% is in AH and have an idea what your battery's voltage/SOC numbers are, as they are all over the map by battery type.LFP also have a BMS to make it interesting.

Anybody understand this? Converting the v/SOC tables to Wh from 100% and 10% to get the Wh used for 90% SOC = 90AH (all 100AH batts)\10% SOC is:LFP -12.1v so 10AH x 12.1v = 121 Wh from 1360 = 1239 Wh is 90AHSiO2- 11.46v x 10 = 114.6 from 1300 = 1185.4 Wh is 90AHAGM-11.75v x 10 = 117.5 from 1300.0 = 1182.5 Wh is 90AHFLA- 11.51v x10 = 115.1 from 1273.0 = 1157.9 Wh is 90AH

I think you have a misunderstanding of how a battery is rated in Watt hours, to provide an accurate ratings you need to measure how much energy is provided by the battery, just as you would for Amp hours. Now for regulatory reasons, often the 'nominal' voltage is just multiplied by the Amp hour rating, but this is an approximation at best. To determine the Ah capacity you discharge the battery (at a specified rate and specified temperature) until it reaches a specified end point while at the same time measuring the current and integrating over time. That is how a manufacturer knows their battery is a 100Ah battery. Same deal with Wh, you measure the power the battery provides during discharge (Watts) and integrate over time. PS the fully charged resting voltage of LiFePO4 is 3.2V/cell ~ 13.2V not 13.6V.

I am just using the Wh to AH conversions where AH x V = WhSeems to me if 90AH is used up that should come out the same amount in Wh for a percentage of SOC, but I am asking if I am doing that wrong or what is going on.I am told all LFPs use this same v/SOC table such as the one here for BBscroll down to the table--it says 13.6 resting is 100%https://1t1pye1e13di20waq11old70-wpengine.netdna-ssl.com/wp-content/uploads/2019/01/BB10012-2021-Spec-Sheet.pdffor a 100AH batt, you run it down at 5 amps and in 20 hrs it is at zero capacity and 10.5v. Mystery how they can measure that way with a batt that is zero at 11.5v.

As you point out, V is not constant, so you can't us Wh = Ah x V. You need to use Wh = (A x V) x t where t is time. With a lead acid battery you get noticeably more power from the top 50% of the battery than the bottom 50% as the voltage in the top 50% is about 0.6V higher.

They are all 100AH batts at the 20 hr rate of 5 amps. So when you use up 90AH and are now at 10% SOC in each case , why didn't you use up the same Wh in each case?it means that different types of battery have different capacities in Wh even though they have the same capacities in AH?So how can battery monitor keep track of capacity being used up in Wh like it can with AH?

Battery Rating vs Capacity Monitoring | Good Sam Community

BFL13
Explorer II
Mar 27, 2021
I tried out the Trimetric but my load was via inverter, so that kind of confuses things. I would rather just have big enough DC load like the furnace, but that was not convenient today. Another time.

anyway, the Trimetic swap to watts from amps was easy, and it did not change the AH reading, so it does not do Wh, only AH.

I want to see if the watts came to a different amount with the same amps when the battery was lower in SOC, but the inverter got in the way.

Starting with full 200AH pair of 100AH G27 SiO2 (much like AGMs) and using the "700w" MW as a load on the 2000w MSW inverter and taking into account whatever wiring zoo I have in the set-up, I got:

-12.1 loaded voltage and 700 watts at first, and 725w after two minutes.
-12.1 loaded voltage and 62.8A at first, and 63.3A after two minutes.

63 x 12.1 = 762.3

Ran the batts down more to get a lower voltage to go with the amps. Down 28AH - about 86% SOC (the Trimetric's % SOC not being used for this)

-11.8 loaded voltage and 913 watts swapped to
-11.8 loaded voltage and 77.0A allowing for time to swap the display.

77 x 11.8 = 908.6

I don't have the desired same amps to go with different watts I would like to test for, but

12.1/63 = 0.153, and 11.8/77 = 0.192 so the proportion of amps per watts is different, but I don't know how much is due to the non-resistive MW load with the inverter action mixed in.

So I think I need to use the furnace to see what I want next time camping when the batts will also get down to a much lower SOC. so the two sets of numbers will be outside any testing margins of error too.

If anybody can make sense of the numbers I did get, please share! Thanks.

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