[olePigeon]

Can I just use a multimeter to test the charge of a battery? I have some 1/2 AA 3.6v batteries, and I tried 3 different electronics stores (including Fry's), and none of them had a battery tester that would work with the batteries.

I'm trying out a digital multimeter, and nearly all the 3.6v batteries measure 3.7v on the dot. Two or three measure 3.67v or 3.68v.

The reason I'm testing them is because they have a shelf life of 10 years, and they're all 10 years old. Wanted to make sure they're still good. However, I wasn't sure if just connecting a multimeter is sufficient enough to test if a battery is good or not.

[reader50]

To test a battery, you need to place a load on it. A 30-50 ohm power resistor would work, creating ~100 mA load. Then measure the voltage. If it holds with the load, the battery is still good.

[olePigeon]

I'll go buy a resistor. Thanks.

[olePigeon]

OK, hooked up a 47-Ohm resistor and tested the batteries. All but 6 came up at exactly 3.7v. So it looks like they're fully charged. 6 of them came up between 3.67v and 3.69v consistently, so I set them aside. They're 10-years-old, so maybe those 6 are on their way out. The others look like they're in perfect condition still.

[reader50]

Under a load of 78 mA, anything above 3.6 volts is probably good. Differences of a couple percent of a volt I would not consider significant.

It sounds like your batteries were stored well.

[residentEvil]

i concur. good response reader50.

i would add though; what do you expect from the batteries when it is time to use them? i tend not to keep stored batteries longer then 2 years of it is something i need/want that is important out of them.

[olePigeon]

They're used in older Macs as PRAM batteries. I got them surplus from a parent of one of the students at my school who works at Apple. I just wanted to make sure they work. I was going to sell them cheap if anyone wanted some. They're $6 new at Fry's, but I was going to sell them for a couple bucks.

[ghporter]

A quick check of a battery's charge can be done with just a multimeter. You look for something markedly above the battery's rated capacity (1.7+ for straight AA, C, or D batteries, 9.2+ for 9v, etc.). If you have something below the rated voltage, you don't need to load test it-it's not going to work.

For a load, you need to do the math to find the right size resistor that will provide the appropriate load for the application. reader50 did that for olePigeon's specific use, though you can be pretty loose with how many amps a battery should be expected to supply. After that, you need, as reader50 says, a LOAD resistor. For a AA battery, a 1/4W resistor could be just fine. But what about a larger battery? Make sure you use that same math to determine how much power you'll need to handle and get a resistor rated WELL above that. This avoids the risk of burns (resistors get HOT!!!) and of burning up that resistor. You will never forget the smell of a composition resistor when it burns up.

[macaddict0001]

Open circuit voltage is a perfectly acceptable method of measuring state of charge. But it cannot measure capacity.