Battery Testing

Hey all, I started using rechargable, mostly Eneloop AA and AAA about a year ago and like it. I'm a lil techie and have a volt meter and was wondering how to test my batts? I noticed that primaries are rated at 1.5 volts and the NIMH's are rated at 1.2 volts. Is testing the voltage to see if its close to the rated amount the way to test how much juice is left in them or do you need to do other tests ie amps etc? Thanks for any help or helpful links.

Unfortunately for you there is no correspondence between voltage and amount of charger in NiMH cells.

Some brands/types have a higher maximun voltage (for example eneloops) and for most of their capacity the cell is almost static in a certain voltage until the end. For that reason some digital cameras that try to determine the remaining capacity of the batteries show "almost empty" with some NiMH (the camera expects alkalines) but it keep going with "almost empty" cells.

Another thing to take in account is that the voltage in a "idle" cell is different (higher) than the voltage under discharge (when you use it). AFAIK the only way to know the actual remaining capacity in a NiMH cell is discharging it and measuring how much energy it had.

There is one thing you can know using a voltimeter: if the voltage drops below 0.9 V the cell can be damaged.

Measuring the voltage of NimH AA or aaa cells when not connected
to anything (ie Unloaded) can give you the wrong idea of
its state of charge.

First connect a 10 ohm resistor across the cell so its discharging through
it. This loads the cell. (ie Loaded)
About a 120mA drain.
Now measure the voltage of the Loaded cell.

1.35volts means its full because its just been recharged
1.00volts means almost empty and time to recharge.

-approximately-

There really isn't a way to test NiMH cells with a simple DIMM, except as hopkins suggested.

A lot of folks use a ZTS tester, or the equivalent thereof (I use one of these), which does apply a load under test, but really, these only give you a rough idea of the cells state of charge. The accuracy can be as bad as +/- 50%. As has been pointed out, a voltage reading alone, is pretty much meaningless.

Dave

sweetlight said:

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Click to expand...

Have a look at the "battery" section of this guide to using your meter. Here he loads a 1 ohm battery in parallel with meter.

To interpret these measurements using a figure of merit called the "on resistance". Take a look here for theory an d 2ohm results .

PeAK