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What the World
Needs Now is -- NiMH
Battery Tester |
Rechargeable NiMH
batteries are
used in countless electronic products. These batteries make economic sense because one set
of rechargeable batteries can be charged and discharged 100s of times instead of throwing
away a depleted set of non-rechargeable types each time. There are lots of AA and AAA NiMH
battery chargers available. Some are designed to quickly charge a battery in as
little as 15 minutes. Although there are lots of ways to charge these batteries
there is no easy way to test them. How do you know if the 3-year-old batteries
you have been using are ready for the recycle bin? You discover that the 4 AA
cells which you use in a digital camera runs out of juice after only a few shots. You
know that there is one or more of the 4 cells which is bad, but which ones?
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| What I would like to see is a
battery tester unit. A really bad battery is easy to spot and to test but a battery
whose Amp-hour capacity has gradually reduced is harder to test. The only true way
to weed out the bad cells from the good ones is by loading them down with some current and
time how long they can support that current before reaching a critical cutoff voltage.
The tester I have in mind might be set up to test one to 4 cells at a time. A rough
sketch of such a device is shown below. A four stall AA or AAA battery holder is
plugged into the unit. The “end of test” cutoff voltage and the test current levels
would be adjustable, using two multi-turn potentiometers. A digital panel meter on
the tester would allow these settings to be measured. A “start” pushbutton switch
would begin a test. Once started, a fixed current would be drawn from the individual
cells in the holder. Four elapsed timers would begin measuring the test time for
each of the four batteries being tested to reach the cutoff voltage. A selector
switch could display the battery voltage and the test time for each cell being tested.
Indicator lights would display the status of each cell under test. When the recommended
battery cutoff voltage is reached, the load current for that battery would be
disconnected. The elapsed time for the test would then be stored and displayed on
the meter. Those that have low Amp-our capacities are dropped in the battery recycle
bin. |
| To accelerate the test, a heavy
current would be drawn from the batteries. A 500ma test might allow the batteries to
be tested in just a few hours. A more sophisticated tester, than my analog design,
might use a microprocessor to control and monitor the battery testing. It might use
some algorithms to accurately measure the Amp-hour capacity of each cell being tested,
based on known discharge curves. The user could load the battery holder with
batteries, start a test and come back a few hours later for the results. |
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