# Calculating total energy of battery (J)

Hi. How would you calculate the total electric potential energy of a battery containing two cells (just an example), with for instance 20 V (cumulative, if cells are in series) and runs on a 60 A circuit.

Is it required to have the charge of the battery to calculate total energy in joules?

The whole point of this is to calculate how long a 100 W motor would run on a certain combination of batteries.

(By the way, these are all hypothetical situations)

Thanks

One measure is the watt hour rating of the battery. This is voltage times current times the number of hours it can provide that.

So an approximate answer is provided by the product of the rated voltage of the battery times the amp hour rating....

http://en.wikipedia.org/wiki/Automotive_battery#Terms_and_ratings

If you want to understand more, also check Peukert's law.

In practice A battery actually disppates energy (during charge and discharge) via its own internal resistance...hence a wet cell lead acid battery with a higher internal resistance is not actually as efficient as a low resistance AGM battery

Oh okay thanks. But wouldn't a batter last longer running a 50 W bulb compared to a 75 W bulb?

I assumed that if you knew the total number of joules of energy that a battery could produce, then you could calculate the total time it would take to discharge.

Hm I guess it's a little more complicated then that, I'll have to do some research