Measure Battery Remaining Capacity

I have a multimeter and would like some advice on using it to measure the remaining life in a 40D or a XTi Li-ON battery. What should I measure? I presume I need a voltage reading.

I can fully charge a battery to get a baseline so to speak, but does anyone have advice on what I should look for in a measurement to determine the amount of charge remaining after a period use or storage?

Thanks!

Maxdave

fully charge them, measure voltage

wait til its run out, then measure voltage again?

so you know voltage at full and voltage at empty, will be able to hazard a guess at whats left in them no?

Yeah, there will be a voltage drop as the battery depletes. I doubt it's linear, but it's probably enough to get a rough idea of remaining capacity.

Don't forget that you need to take that measurement under some kind of load.

With Li-ion batteries it doesn't work well. Li ion batteries hold their voltage up very close to the top until they are almost depleted, then the voltage crashes. You need to basically watch the current usage and sum it up against the known battery capacity to be accurate. There are fuel gauge chips that do that for you but you will not be very happy with the results from a volt meter. It is the same problem with the gauge on the camera - shows full until it a few frames later it starts to drop rapidly.

Li Ion batteries do not self dishcharge fast so you can charge them up as soon as you are done using them and they will be ready to go even if they sit for a long time.

J.