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

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.)

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

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.

• I think the OP is asking how he might determine the characteristics of the battery experimentally without a datasheet... Commented Mar 22, 2013 at 21:39

On the face of it, it sounds like your USB port is supplementing the battery energy wise but i suspect it might be something else that is causing your problem. The key to this is the random restart of the processor - this doesn't sound like a battery problem but more like interference coming thru the power rails from the servos.

Maybe connecting the USB port helps avoid this by providing more power supply capacitance or by some other means. It's hard to tell based on the info you have provided. I'll leave this one open and maybe you can think about the possibilities i'm talking about.

• I meant that it works fine when it's hooked up only to a USB port. It's not that the USB port supplements the battery, but that the USB power works fine by itself. Commented Mar 22, 2013 at 22:05
• @DevinR - then maybe the USB port is providing some extra protection when it is connected like decoupling caps across the micro's supply OR maybe the lower power level is preventing the servos from really generating some EM noise when they are going full tilt. I'm fairly sure it's not a battery power issue dude Commented Mar 22, 2013 at 22:11
• Interesting thought, with the lower noise when not going full tilt. I'll mess around with that idea. Commented Mar 22, 2013 at 22:15