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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Reducing contact resistance for low volt use?
Date: Thu, 12 Dec 2002 20:16:02 GMT
Organization: MindSpring Enterprises
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X-Server-Date: 12 Dec 2002 20:15:34 GMT
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>>>>>>Putting batteries in series is completely useless and would give very
>>>>>>misleading results as far as I can understand.
>>>>>>The individual discharge curves are different in Amps, Voltage, and
>>>>>>Time. The series method assumes incorrectly that only the voltage
>>>>>>factor would be different between the two discharge curves.
>>>>>>But all three factors will be different for each individual battery.
>>>Darn good point. Unless the cells were matched very closely, this
>>>wouldn't work. Most of the cells track very closely but I couldn't
>>>tell you if it was +/-1% or +/-10% without doing some serious
>>>testing....something the new discharger would be great for. :-)
I gave this some thought and I think I've changed my mind. It doesn't
seem that differing discharge characteristics/curves for each of the
2-3 series cells would be a problem.
If one cell drops its voltage faster than the others, the MOSFETs
would automatically drop their Rds-on (via monitoring of the shunt
voltage and an op-amp comparing that voltage to a reference) to
compensate. As long as the three cells maintained a voltage high
enough to compensate for the drop across the internal resistances,
100A would flow...I think.
The only problem I can see is that if one cell dropped its voltage far
enough (before the others) to cause it to reverse polarity and react
very exothermically. But, holding the final discharging voltage to
above 0.9V/cell (2.7V for a 3-cell string) is a standard recommended
by the battery manufacturers to prevent this from happening with
series strings of cells...at least at lower discharge current levels
(I've done it up to 40A without problems so far).
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