Ken,
Without getting into the chemistry of ni-cads, keep in mind these facts:
(New) ni-cads are typically good for about 1000 charge / discharge cycles.
Shelf life depletes the charge about 1% a day.
Occasional deep discharge does help you get your 1000 cycles.
Batteries may be kept on the charger continuously in some cases. Let me
explain.
Assume your "AA" battery has a listed current rating of 1100 milliamp hours
(Mah). This means it should deliver 1100 milliamps for 1 hour or 110
milliamps for 10 hours or 11 milliamps for 100 hours. You get the idea. The
1 hour rate (1100) is known as it's "1R" parameter.
Now, regarding chargers, you can charge a totally dead cell at the 1R rate
(1100 milliamps) and expect a complete charge in just over 1 hour. You can
also charge at the 1/10R rate for about 14 hours and get the same results.
The important factor to remember is that once a cell is completely charged,
a high charge rate will cause it's temperature to rise and that's a "bad
thing". The trick is to know when a cell is "full" so the charger current
can be reduced or stopped. Two ways exist to determine a full charge.
Carefully monitor the cell voltage or carefully monitor the internal
temperature (rectal and oral have no meaning here).
In specialized cells (think Motorola commercial walkie-talkies) where quick
charging is desirable, custom batteries are built with internal temperature
sensors. That's why these batteries have more than two contacts.
Customized (smart) chargers use these sensors to know when to reduce the
high charge rate (1R) to the 1/10R rate.
Finally, a ni-cad cell can be left connected to a 1/10R charger indefinitely
with no ill effects. The off-gassing that occurs is offset by the
re-combining chemistry within the cell so overheating does not occur at the
1/10R rate. How do you know if your wall wart is designed to deliver at the
1/10R rate? Easy. If the owners manual indicates a complete charge may
take 14 - 16 hours, you have one. If it says it can charge a dead battery
in substantially less time, you don't.
Nickel metal hydride (Nimh) and lithium ion cells have different rules.
Another time, perhaps.
"Ken" wrote in message
oups.com...
I have a beard trimmer that runs on a Ni-cad battery that I recharge
about once a month when the battery runs low. Recently, the battery
stopped being able to hold a charge. I charged it for ~12 hours,
tested the voltage, and got zero. OK, fine, so I soldered in a new
ni-cad, and everything works fine now. The trimmer is only a few years
old, so I thought the battery died earlier than I would have expected.
Then I got to thinking about the charger, which is just a simple wall
wart. The battery is one AA ni-cad, 1.2 V 600 mAh. The wall wart
charger is labeled 2.3 V, and I measured 7.5 V DC actual output. The
charger is what came with the trimmer, and molded into the trimmer is
something about "use only charger # such-and-such", which is also the
number on the charger. So I'm sure that the charger is the one that
the manufacturer intented to be used with the trimmer.
Did the higher than expected voltage on the charger lead to the early
demise of the ni-cad battery? Should I find a new wall wart that has a
voltage closer to 1.2 V? If so, what current output should it have?
(I save each and every wall wart from every dead appliance that I have
ever owned, so I have a wide selection to choose from a box in the
attic, although I think most of them are 5V and up.)
Ken
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