Re: NiMH battery paralleling

On Wed, 18 Jul 2007 16:06:07 +0100, John Devereux
<jdREMOVE@xxxxxxxxxxxxxxxxxx> wrote:

Spehro Pefhany <speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx> writes:

Hi, all:-

I want to combine some (6 or 8) series strings of a bunch of NiMH
batteries in parallel to get more W-h. The voltage is high enough that
a diode drop won't affect the efficiency much.

Do you think it is better to just parallel them or to use a Schottky
diode to "or" each string? There will be a fuse or polyfuse on each
string, and a single string can supply the entire current if
necessary. Charging will be done independently, it's only the
discharge path I'm concerned with here.

I don't see any specific guidance on this matter from the battery
manufacturers.

Thanks for any suggestions.

I would be worried about a cell failing short-circuit. Isn't that the
normal failure mode? Then the other strings will discharge themselves
through the string with the faulty cell.


Hi, John:-

With NiCd cells shorting is the usual failure mode, I am not sure
about NiMH. The end-of-life is defined as a decrease in storage
capacity under specified conditions. I don't think I've seen one that
has failed completely (unlike NiCds), in my own use of them for
digicams etc.

That's my idea behind putting "or" diodes in there, so if the voltage
drops on one string due to a dead cell (open or short) then the
survivors should share the load (more or less).

But even without diodes, in case of a catastrophic (maybe a chain
reaction failure due to overcharging from other cells) the internal
fuse & vent in the cells chould go (but before that, the external fuse
should isolate the problem string and break the current). I suppose in
the case of a polyfuse it would cut in and out with some rep rate,
which isn't good.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.

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