## 2011-03-28

### EDUCATION: Calculating battery life

Just a quick note on battery life.

When putting together a circuit where you need to know how long it will run on a battery, you need to know how many amps the circuit draws. Once you have that, you need to know how many amp-hours a battery is rated for.

For example, if your circuit draws 1 amp of power and you have a battery that is rated for 1 amp-hour, that battery will run the circuit for about one hour.

Wikipedia has a list of batteries and their typical amp-hour ratings at http://en.wikipedia.org/wiki/List_of_battery_sizes

What I found surprising was that a typical 9-volt battery is rated for 565 mA/hr (milliamps, not amps), while a typical alkaline AA battery is rated for 2.7 A/hr.

However, a word of caution: circuits also require VOLTS. Therefore, if your circuit pulls 270 mA/hr but requires 3 volts, you will still need two AA batteries, as each battery is roughly 1.5 volts.

At this point, I am assuming batteries in series have the same A/hr rating, but that's a wild guess at this point