Measuring a battery's capacity

[Pengwuino]

I need a bit of help here guys :D

I have a something here that's effectively a battery that can supply around 1.5V I believe at a few micro-amps to a few milli-amps for an unknown amount of time. The problem is... they are all infact unknown quantities and I want to figure out what this batteries capacity is. I hear that the way battery companies measure their batteries capacities are to drain a battery at a fixed current until the voltage reaches "terminal voltage" and they use that time in hours to determine the capacity by multiplying the current draw and the time it took to drop down to terminal voltage.

Now the problem is there's no information given on how they actually do that and I was wondering if anyone know how to create the circuit necessary to draw a constant current so I can determine the capacity. If anyone knows what this "terminal voltage" is as well, that would be helpful to know :) Thanks!

 

[berkeman]

The terminal voltage you are referring to is really the "final cutoff voltage", where the capacity of the battery is pretty well spent, and the output voltage starts to droop faster. Look at the discharge curves at the battery websites (like Eveready) to see how they mark the cutoff voltage.

As for a constant current drain circuit, there are lots of ways to make one. The simplest is to use a resistor, if the output voltage is fairly constant over the draining characterisic of the battery. That's probably the easiest first cut at doing the measurement for you. For a more accurate number, just make a constant current sink circuit with an opamp or something (look up constant current source/sink in an opamp textbook if you're not familiar with them yet).

 

[Pengwuino]

Well unfortunately, I have no idea how to do any of this, it's uncharted territory for me. I have a circuit that will allow me to figure out the final cutoff voltage however thanks to one of my professors. I guess I can just just occasionally adjust the resistance with a rheostat since I am probably looking at trials that will take 10-30 hours each.