NiMH Battery Testing Question

Zigzago

Enlightened
Joined
Aug 29, 2003
Messages
439
Location
Wisconsin, USA
I am new to the wonderful world of rechargable NiMN batteries. After watching the "Charging" indicator light go off on my charger (Rayovac 15 minute) I had the nagging question "How do I know it's really charged?"

What do you measure to verify that it's fully charged? Voltage? Current? Or is it more complicated? I read the thread here about "flash current" but it seemed to refer to lithium batteries.

Is this kind of cheap battery tester of any use in this case? Radio Shack Or do I need to get a multimeter?

I guess I don't need precise data, I just want to know if I have a bad cell or if the charger isn't doing its job.

Any tips are appreciated.
 

jayflash

Flashlight Enthusiast
Joined
Oct 4, 2003
Messages
3,909
Location
Two Rivers, Wisconsin
A digital multi-meter is inexpensive and worth the time and $$ to use. With a DMM you can measure the amount of current going into the cells and multiply that times .25 (hour) for 15 minutes of charge time. Deduct aprox 30% from that and you will have a rough estimate of what went into the cells but it won't tell you how much charge the cells accepted.

Better still, get a smart charger or test each charged cell (or pair) by timing its discharge into a known load. For example, a fully charged 2000mAh cell should light a 500 mA lamp for about four hours if it's accepting a full charge.
 

MrAl

Flashlight Enthusiast
Joined
Sep 9, 2001
Messages
3,144
Location
New Jersey
Hello there,

There are at least three ways to tell when the battery is
"fully charged".

One way is to drain it down to 1.00 volts and watch
the current. When it reaches 1.00 volts you can
calculate the amount of stored capacity. This
method is good for testing a cell for the first time
since you bought it to make sure it lives up to it's
rated capacity. It has to be charged up before you
do this test of course.

Obviously that method completely drains the cell so you
only want to do it when your wondering if the cell can
hold a charge.

The second method is to watch the voltage with a digital
meter on each cell when being charged with a fast charger.
The fast charger will apply a constant current to the
cell and when the voltage gets up to a certain point
the voltage will then start to decline. At the moment
of the decline, the cell is considered charged fully.
(If you see momentary drops occurring every so often,
ignore them -- it's the charger interrupting the cycle).

A third method watches the change in temperature of
the cell. When the cell is charging, the temperature
might increase a little at a time as the cell takes
more and more charge. At one point all of a sudden the
temperature of the case starts to rise sharply. This is
the point where the cell starts to convert most of the
applied energy into heat and tells you the cell is
fully charged.


Take care,
Al
 
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