>     Are you trying to build a NiMH charger ?

Yes, eventually.  As I said, the test circuit is a long way from the final
charge circuit.  Right now I'm trying to get a better handle on how these
batteries behave under various charging conditions.

>     If so, you should'nt be charging the NiMH battery
>     this way. You should use constant current, typically
>     at 1C, i.e. if you battery is 300mAH, you should
>     charge at a constant current of 300mA. Therefore
>     if from fully discharge, it should take around 1 hour
>     to charge your battery.

1C is the maximum rapid charge current.  There are a bunch of restrictions
on that.  You can't just charge at 1C without careful monitoring and knowing
where you are on the charge curve.  The trickle charge current is apparently
.033C to .05C.  I was starting with a simple circuit at the lower end of the
allowable charge current range just to see how the battery voltage behaved.
I intend to crank up the current on successive tests.  I'm running another
test with higher charging current right now.

Eventually I want to build a charger for these battery packs.  I don't care
much how fast the charger is.  I'm more interested in doing the best quality
job.  I think that means getting the batteries as fully charged as possible
without damaging them, and being able to start with packs of unknown charge.
If necessary, I could have the charger discharge a pack to a measurable
state if that makes the charging easier or is better for the battery.

I'd like to avoid measuring the battery temperature, so I hope there is a
reliable algorithm to determine how much current to use when and when to
stop by looking just at the voltage.  This is the ultimate goal, but I'm
also curious about these things and want to see how they behave under
different conditions.  I've done a bunch of research on the web.  I found a
lot of information (the Panasonic site was particularly helpful), but there
are still unanswered questions.  For example, it's not always clear when
data sheets talk about the battery voltage at various stages whether thaYQgm
open circuit or while charging.  Does it matter?  If so, what are the
differences?  So far I haven't seen much difference, but that may change as
I get to higher current.

>     Your ON/OFF current circuit is not suitable
>     for this, you probably need a PWM-controlled
>     DC-DC switcher.

I think you missed the point.  I'm not trying to control charging current by
switching.  I am only switching the current on and off so that I can measure
the battery voltage under both conditions.  One thing I don't know yet is
whether the battery needs to "stabalize" somehow after the charging current
is shut off.  For now I am waiting 100mS after shutting of the current
before starting to measure the voltage, but I have no evidence that this is
too short or completely unnecessary.

Another thing I'd like to know is can PWM be used to charge these batteries.
At what PWM frequency, if any, will the battery "see" the average current
instead of individual bursts of high current with dead times of no current
in between?  I haven't seen any discussion of this at all so far.  PWM would
be nice, and it would also be nice not to need large inductors - but will
that work?  I'll post more information as I find it.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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