Full capacity in watt hours from 100Ah AGM and LFP

[krby]

I haven't been able to find this info anywhere so I thought I would ask here.
I'm trying to spreadsheet out various battery costs, comparing different sizes, LFP vs AGM vs SLA as well as comparing different discharge depth. What I'd like to be able to do is know something like:
a LiFePO4 4 cell (the common config for 12V systems) 100Ah battery has X watt-hours total. So if I set my discharge limit at 80% discharged then I get roughly X * 0.8 watt-hours total in DC (assume I account for various losses somewhere else in my spreadsheet). This should be easy, right? But what voltage do I use for Ah * V = Wh? 3.3V per LFP cell? Something else? Why?

I want to answer the same question for SLA batteries (specifically AGM or Gel). It seems even harder to guess a V for SLA chemistry as the voltage range is larger from 100%-0% than it is for the very flat LFP discharge curve.


I understand the trade-offs between depth and number of usable cycles, I'd like to be able to compare various discharge depths and then cost out what I would need for a particular application. It's going to be a fairly low load, 60W AC with maybe spikes of 110W AC. So the current from "12V-ish" batteries will be in the range of 6-10A


I'm leaning towards SLA because I'm building a DIY UPS that will be plugged into mains AC nearly all of the time, with the batteries being left topped off for months at a time. I won't need to cycle them often. Also, wait isn't a concern. If I get 10 cycles out of them over the next 5 years, that would be awesome. But, I'm curious about the overall costs if I assume various depth of discharge .

 

[pollenface]

Generally

LFP
100*12.8*0.8 = 1024wh

AGM (a type of SLA)
100*12*0.5 = 600wh

For standby systems (UPS) lead acid is ideal because it likes to be kept at 100% charge

 

[krby]

Thanks for the quick reply, where did you get those numbers? Why 3.2V per cell for LFP and 2.0V for SLA? Is that where the cells spend most of their time when delivering power or is it just a handy average?

Also, the 0.8 and 0.5 discharge depths are recommendations, right? I can go lower on an LFP and AGM, I just shorten their useable lifetimes. For my application (very little cycling, nearly all the time spent at 100% charge) I was thinking about 75% discharge for the AGM and then just planning on having to replace them after 10 cycles or so.

 

[pollenface]

3.2v/cell is nominal for LFP

2.0v/cell is nominal for lead acid

 

[Statistical]

And to clarify that is exactly what the nominal voltage means. The voltage of the cell will decrease over the discharge cycle from 100% to 0%. So you can't just use the 100% SoC voltage or 0% SoC Voltage. If the cell discharges 100Ah it will be 100Ah over a falling voltage. Initially it would be something like 3.4V and then 3.38V etc etc. With a coulomb count the energy delivered over the discharge works out to 3200Wh and at 100Ah the weighted average voltage is 3.2V during the discharge cycle from 100% to 0% so that is all nominal voltage means. It is a placeholder to get from Ah to Wh. It is exactly what you are looking for. This is why you will see a LFP 4 cell "12V" 100Ah battery advertised as 1,280Wh (12.8V nominal voltage * 100Ah nameplate capacity).

One minor thing is that the nominal voltage is for 100% to 0% discharge at 100% to 20% the average voltage will be a bit higher but 3.2V (12.8V for 4 cell "12V" battery) is close enough.

 

[pollenface]

All systems nominal