Battery Rating vs Capacity Monitoring

[BFL13]

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 Wh

I 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 levels

Battery type, !00%, and 0%

LFP- 13.6, 10
SiO2- 13, 11.1
AGM -13, 11.5
FLA- 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.

[time2roll]

When it gets low, charge. When it gets enough charge relax.
Trust the monitor and have less worries. I don't need to know how every component functions... just so it does.

Like fuel on the road, YMMV.

[FWC]

Because the voltages are different - the LiFePO4 provides more energy (Wh) than the AGM which provides more energy than the FLA.

If you pull 90Ah out, your are at 10% SOC in terms of Ah, but you are actually at 8-9% SOC in terms of Wh.

A battery monitor keeps track of Wh the same way it does with Ah. You start at zero when fully charged, and then you subtract what you have used. If you use 120W for 1 hour you have are at -120Wh. The SOC is arbitrary number based on what you entered into the monitor. Say you told it you had a 1200Wh battery - your SOC would be (1200 - 120)/1200 = 0.9 = 90%. Just the same as Ah.

[BFL13]

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?

[FWC]

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.

[BFL13]

I am just using the Wh to AH conversions where AH x V = Wh

Seems 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 BB
scroll down to the table--it says 13.6 resting is 100%

https://1t1pye1e13di20waq11old70-wpengine.netdna-ssl.com/wp-content/uploads/2019/01/BB10012-2021-Spe...

for 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.

[FWC]

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.

[BFL13]

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 90AH
SiO2- 11.46v x 10 = 114.6 from 1300 = 1185.4 Wh is 90AH
AGM-11.75v x 10 = 117.5 from 1300.0 = 1182.5 Wh is 90AH
FLA- 11.51v x10 = 115.1 from 1273.0 = 1157.9 Wh is 90AH