Calculating wattage output of battery bank

[parthian_shot]

Hello, I'm a complete newb, please forgive my ignorance.

I'm trying to figure out how to calculate the wattage a battery bank can output. The Tesla and Enphase batteries all tell you what their output is so it's easy to understand how a solar installation matches up to the batteries you have. But I'm looking at stringing together 40 nickel iron cells to make a 48-volt battery.

If the batteries are hooked up to a 220 amp circuit at 48 volts then that's 10.56kW of power. If I have 40 of the 1200ah cells, does that mean I have 48,000ah total? And then 48,000ah/220a = ~218 hours of 10.56kW? That sounds like way too much!

So the discharge rate here has to do with the amps, but some batteries can't discharge as fast as others, so maybe you can't just hook up the battery to whatever amp circuit you want.

And there's probably something about AC versus DC I'm completely missing.

Here's a spec sheet on these kinds of batteries: https://ironedison.com/wp-content/uploads/2020/12/Iron_Edison-Nickel_Iron_TN_SPEC_Sheet.pdf

Any help is appreciated! Thanks.

 

[sunshine_eggo]

If the batteries are hooked up to a 220 amp circuit at 48 volts then that's 10.56kW of power. If I have 40 of the 1200ah cells, does that mean I have 48,000ah total? And then 48,000ah/220a = ~218 hours of 10.56kW? That sounds like way too much!

Capacity only adds if cells/batteries are in parallel. If they are in series, voltage adds.

Wh is easier to keep things consistent.

40 1200ah 1.2V cells would be 40 * 1200Ah * 1.2V = 57,600kWh, so it could sustain a 10,560kW load for about 5.5 hours.

You would have a big 48V 1200Ah battery

A 100A load would run for 1200Ah/100A = 12 hours.

A 220A load would run for 1200Ah/220 = 5.5 hours (seen that number before).

The datasheet breaks down a lot of that for you.

So the discharge rate here has to do with the amps, but some batteries can't discharge as fast as others, so maybe you can't just hook up the battery to whatever amp circuit you want.

Not really sure what you're asking.

And there's probably something about AC versus DC I'm completely missing.

hard to comment without specifics.

 

[parthian_shot]

Thanks for the reply, that's very helpful.

I guess I'm confused because the powerwall and enphase batteries give you their output and surge output in kilowatts. So you can add up the wattage of all your loads and figure out how many batteries you would need to run everything at the same time. Then you can add up the kilowatt hours of each battery and figure out the length of time you can run everything simultaneously. With the nickel iron batteries it seems like I can calculate that based on the amps of the load, but there must be a limit to how fast you can discharge. Like if hook them up to power a 1200-amp load they could run for an hour. If I hook them up to a 72,000-amp load they can run for a minute. But I probably can't do that! Is there a way to know what would be too much of a load for the batteries to handle?

 

[sunshine_eggo]

Page 2 lists specs including max discharge. Listed as C/2. That would be 600A for a 1200Ah battery. To get power, multiply amps *48V

 

[parthian_shot]

Awesome, thanks for the help!