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Re: MMC resister problem



Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Gary.Lau-at-hp-dot-com>
> 
> I fully agree that battery packs can achieve reverse-charges on cells, but
> this is very different than a string of caps.  A NiCd cell is not a
> capacitor and is not at all a linear device.  A "bad" cell had radically
> different V-I properties than the remaining good cells.  Even good cells
> have different V-I characteristics depending upon their state of charge.
> But in MMC strings, we don't have a "bad" cap that is responsible for the
> imbalance.  As far as we understand it, all caps are electrically the same.
>  The observed results of course suggest otherwise, but I've not yet seen a
> model suggested that can explain the effect.
> 
> While iron-core inductors exhibit hysteresis, I wasn't aware that caps had
> this property?
> 
> Gary Lau
> MA, USA

	If the capacitances aren't identical, when the series is shorted (or
discharged through a resistor), the smaller capacitors will have
reversed polarity if the voltages across each capacitor have been
equalized by a resistor shunted across each one.  Just ohm's law. 
Consider a large capacitor and a small capacitor in series with the
series string charged to some voltage.  If the voltages on each are the
same (again by means of a resistor divider) then the larger one will
have more charge than the small one.  On discharge the current will be
the same for each capacitor, and the smaller capacitor will end up with
reverse voltage.

	In general, capacitors also have some hysteresis due to a process
called dielectric absorption.  This manifests itself as a rise in
terminal voltage after the capacitor is shorted and then open
circuited.  For good dielectrics (quartz, polystyrene, teflon) the
effect is very small.  For ordinary oil-filled paper capacitors the
recovery can be several percent of the original voltage.  It is fairly
common practice to wrap a shorting wire around the terminals of HV
filter capacitors, to prevent serious shock.  The effect is often
modeled by considering the capacitor to be composed of one perfect
capacitor in parallel with a number of different series RC circuits of
smaller capacitance and different time constants.  I have measured very
good capacitors intended for use in precision analog differentiators,
and even for them current flow (very small at the end) can be measured
for hours.

Ed