Electronic – How to determine the maximum amp output of a battery pack

batteriesdischargepower supply

How can I determine the maximum amp output of a battery pack?

Some background: I've got a 5-AA battery pack hooked up to a microcontroller and some continuous rotation servos, but there's current weirdness when everything is drawing their maximum amounts. The microcontroller randomly restarts itself, and the servos stutter and jerk around instead of rotating smoothly. Suspiciously, everything runs fine, just a little more slowly, when hooked up to my laptop's high-speed USB port, despite that being lower voltage. This makes me suspect that I'm exceeding the current output of my battery pack at 7.2V.

How do I determine how much current my AAs are capable of pumping out? Do different battery materials have different output ratings?

(I know that LiPo batteries of the sort used in RC cars have a "C" rating that represents the discharge rate, but I've been unable to find a similar rating for household batteries.)

Best Answer

Batteries, like any source, can be modeled as a Thevenin-equivalent circuit. They have some equivalent impedance.

enter image description here

As you pull more current out of the battery, it's terminal voltage will drop. The datasheet for a particularly nice Energizer AA battery lists an internal resistance of 90-160 mOhm. You'll get some voltage drop just from the instantaneous current draw, which will go away once the load is gone. The sheet also gives other useful data, like discharge curves for different loads. I got that particular datasheet from Digikey, where they list several other similar batteries. I'd look for the manufacturer of your battery (or alternately, get a different battery for which you can find a datasheet) and see if they provide similar information.