# How to calculate battery bank Watts and amps



## gemmuj (Feb 22, 2008)

Hiya guys, 

as the title suggests, I am just trying to work out how to calculate the watts and amps of a 144V battery bank.

I know that 

Watts = Amps*Volts

and 

Amps = Watts/Volts

but how do i know the Watts of this pack. i'm using trojan Lead acid batteries.

thank you


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## rillip3 (Jun 19, 2009)

gemmuj said:


> Hiya guys,
> 
> as the title suggests, I am just trying to work out how to calculate the watts and amps of a 144V battery bank.
> 
> ...


Amps is a function of the Amphour of the batter. For example, take a 100 Amphour flooded lead-acid battery. You could do a rating like this

100A x 144V = 14.4kW.

However, due to the Peukert's law, you can't get that 100 Ahs, which was measured over 20 hours, in the 1 hour you're using. The wiki indicates this limits you to 55% of that 100 Ah, meaning 55x144. 

But then you have to consider thatyou don't want to drain them down to 100% all the time. The recommended max DoD is 80%, leaving you with 

55A x 144V x .80 = 6.336kW.

But keep in mind that you can also pull up to 3C for limited periods of time on lead-acid batteries, meaning that you are not limited to 100 amps all the time. You also have to keep in mind that 144v is the nominal voltage, it iwll actually be more like 156 when fully charged.

So... in all... I don't think I helped!

Maybe you should read this, might be more cohesive:

http://www.diyelectriccar.com/forums/showthread.php?t=11709


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## major (Apr 4, 2008)

gemmuj said:


> I am just trying to work out how to calculate the watts and amps of a 144V battery bank.
> 
> but how do i know the Watts of this pack. i'm using trojan Lead acid batteries.


Hi gem,

It depends on the load. Lead-acid batteries will put out very high current. Take your car's cranking battery. It probably can put out 1000 amps. It may only be a 50 or 60 Ah (ampere hour) battery. The current the battery delivers is load dependent. And so is the power (watts). Because, like you said, Watts = Volts times Amps.

But when figuring the power, you must consider the fact that the battery voltage decreases with increasing current. This is due to the internal resistance of the battery. Take the car cranking battery for example. It may have an internal resistance of 0.008 ohms. And one normally uses an open circuit voltage of 12.8 volts at the battery terminals.

So the current from the battery will be 12.8 volts divided by (the internal battery resistance plus the load resistance). The maximum current the battery is capable of is called short circuit current. This would occur if you connected the battery positive terminal to the negative terminal with a zero resistance wire (like a solid copper bar or large cable). In this case, short circuit current is 12.8V/0.008ohms = 1600A. That is the maximum current the battery can produce.

Obviously, a short circuit can do no work for you, so this is a useless situation, and not too good for the battery either. You can go thru some math and determine that maximum power from the battery will occur at one half of the short circuit current and that will occur at one half of the open circuit voltage. So, maximum power from the example battery = 6.4V times 800A = 5120W = 5.12kW.

The power versus current curve is parabolic. As such, the maximum power occurs on a declining slope of watts per amp. This means you seldom want to go over 35 percent of short circuit current to calculate usable maximum power from a battery. 

So you have a 144V set. Maybe you can find the actual resistance, but let's use 0.08 ohms. Say the open circuit voltage is 150V. The short circuit current is 1875A. 35 percent of that is 656A. Battery voltage at 656A = 97.52V. Power at that point is 63.97kW.

I've used some assumed figures, like for internal resistance. But that is how I'd figure maximum current and power from a battery set. Of course, your actual power at any given moment will depend on your load.

Regards,

major


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