watt hours probably dumb question

[oldmaninsaline]

Dumb question but maybe other newbies are wondering as well - I've never seen this addressed. And yes, I know, cell voltage, etc..

I'm wondering something about lifepo4 batteries regarding watt hours.

(Ah)*(V) = (Wh)

I've seen "12 volt" lifepo4 batteries, 100 amp hours @ 1200 watt hours which makes no sense to me.

They spend most of the discharge cycle at 13.1 volts - once mine hits 12.8, I recharge so a 12 volt calculation doesn't reflect the voltage range it operates at.

Given that most of the time they're 13.6 to 13.0 with the super flat discharge at 13.1, wouldn't the amp hour to watt hour be calculated at 13.1 volts because once it hits 12, it's pretty much depleted?

100ah*13.1v=1310wh

 

[12VoltInstalls]

“12 Volt” batteries are named as 12V nominal volts.
It probably is “practically” accurate to calculate at nominal voltage, although - depending on manufacturer- you may be technically correct, the watthours at 12V nominal are the number you should go by

 

[Mattb4]

Much about batteries are approximations. Being chemical devices they react based on factors that change both what they can give and for how long they give it. Do not think of them like a volume measurement.

 

[wiseacre]

I like the nominal volts comment, it gives me some positive fudge factor when calculating my Wh

I have (4) 24v, 200 ah batteries
at 24v I should have 19,200wh
but 24v is only - 3v per cell
nominal full voltage would be 3.2/3.4 volts (25.6/27.2v) - (20,480/ 21,760 Wh)

That bit of ignored extra gives me comfort ?

 

[oldmaninsaline]

I like the nominal volts comment, it gives me some positive fudge factor when calculating my Wh

I have (4) 24v, 200 ah batteries
at 24v I should have 19,200wh
but 24v is only - 3v per cell
nominal full voltage would be 3.2/3.4 volts (25.6/27.2v) - (20,480/ 21,760 Wh)

That bit of ignored extra gives me comfort ?

That's the way I'm looking at it.

 

[pekka8]

I was just thinking about this too. So 200 Ah 12V lifepo4 is probably closer to 2600 Wh.