> RussellMc wrote:
>> If doing capacity testing then say 4C at 25% duty cycle may produce a
>> different result to 1c at 100%.

> This brings up a interesting point I have so far been unable to find
> adquately addressed in battery datasheets. =A0At what frequency does the =
4C at
> 25% duty cycle effectively look like steady 1C?
>
> This is not just a academic question. =A0If you are building a battery ch=
arger
> or maintainer and want to use effecient electronics, they are going to be
> pulse switched. =A0You could put enough inductance in series to smooth ou=
t the
> average current, but these currents can be substantial and the inductors
> therefore expensive. =A0Therefore knowing what frequencies the battery ca=
n
> "see" would be useful. =A0If you pulse it at a frequency above what it ca=
n
> see, then no series inductor should be needed.

That's a good and hard question.
You could guess that say 1 kHz would be reasonably safe, but, maybe not.

There are various effects.
Charging efficiency depends in part in internal IR drop, and loss
while charging is 4 times as high at 4C than at 1C (assuming VI loss
overall) . If efficiency is the only consideration it's not a major
one in almost all cases for battery charging. But heat generation can
be substantial within the cell at say 1C so at 4C it's notionally 16 x
as high but for 1/4 the time. So losses increase linearly but there
are bursts of heat production at 16x 1C rate. This may be enough to
drive local areas of water into vaporisation almost instantaneously -
a good knowledge of the thermomechanical build of the cell would be
needed to know what a "safe" level was.

Also, as cells reach end point gas generation occurs through
electrolysis and many modern cells(but less so in higher capacity
units) recombine the gases catalytically. Gas generation and the
ability to recover may be substantially different at 1C and 4C.

Modern high capacity AA allow minimal or zero trickle charge and offer
minimal gas reconversion when gassing at full charge, so 4C pulses
when fully charged will be extremely unwelcome.

It seems that there are  enough potentially bad second order effects
that judicious testing using parameters of relevance to your
application may be the only good choice - and the results may vary
between brand and model of battery.

One approach to smoothing charging pulses is to use RC smoothing
rather than inductive. RC is inherently lossy but this can be allowed
for in measurements and charging/discharging control by measurement at
the cell and by allowing for the known losses.

For AA NimH  cells maximum charge rates are about 3A (1C) although
some specialist cells allow 2C.
At 500 kHz+ inductors to handle 3A are not especially large of
expensive, so inductive filtering looks viable for a suitably high
frequency converter

           Russell

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