Re: NimH batteries

On Fri, 9 Sep 2005 11:09:46 -0400, Dave C. wrote:

> With the diode forward biased across the cell, discharge current is 1.19
> amps.
>
> Drop across the diode is 0.838 volts.
>
> Battery voltage with the diode conducting is 0.820.
>
> After one hour, battery is hardly warm and diode is very warm, and hot
> enough to hold. I figure about 100 degrees F or so.
>
> So, I would expect that the battery will discharge down until about 0.838 or
> so volts and then stop discharging expect for diode leakage.
>
> Does all this sound like a workable situation? If so, I would do this with
> spent batteries from my camera before recharging.

After one hour the cell (assuming that it's high capacity, over
2000mah) should be only about 1/2 depleted. Were the given voltages
taken at the beginning or after 1 hour? Also, the 0.838 voltage
drop across the diode depends on the current. As the battery nears
depletion, the voltage should drop well below 0.8 volts. Try adding
a 2.2k ohm resistor (or greater) to the circuit and see what kind of
voltage drop you'll see across the diode. Using a diode rated at
only 3 amps would have it operate dangerously close to its limits,
especially if it's not solidly mounted to something that can act as
an effective heat sink. If you use the diode to drain a cell that
has a much lower internal resistance, such as an NiMH C or D cell or
a good NiCad AA, you may notice the odor of burning silicon.
Lastly, draining batteries as quickly as this may shorten their
lives by a good deal more than an single charge/discharge cycle. If
it's not done frequently though, it probably wouldn't be worth
worrying about.


.

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