Active8 said:
I said "I'd think...".
I plan to whenever I get around to that deep cycle charger I'll
need. No need to worry about NiCds now.
I'm going to put each separate thought in it's own paragraph so you
don't misunderstand me. Read them as if the others weren't there.
Thanks, I need all the help I can get.
Nothing I said was intended to suggest that it would. I opined that
a constant current all the way up to the end might heat it up.
I said that IIRC the internal resistance of a cell decreases as it
reached full charge.
I said I think that a current through a small valued resistor causes
more heat (heat = work = energy = P*T) than the same current through
a larger value resistor.
Mr. Ohm and those starting from his equations are all turning over in
their graves about now.
I don't have any charge curves handy AFAIK but I'm guessing (hell, I
can almost picture curves I've seen in the past) that when it's
almost up to full charge, dV/dt is less than when it started
charging.
I'm not guessing, my battery charger plots the curves every time it charges.
For NiCds dV/dt peaks before it levels off then turns negative.
I take it you mean it's always under constant current charge for the
dV/dt in question.
That's a reasonable assumption.
My cordless drill batts have a thermistor inside. I thought that was
the *only* thing that stops the charge.
It is. There are many cheap chargers that do just that. They charge
the thing until it gets hot enough to turn off. Great way to cook
batteries to death. I've rebuilt about a dozen drill packs. Usually,
all the cells have vented.
FWIW, this technique works reasonably if you
fully discharge the pack...but not too fully to reverse a cell...before
charging. During the full recharge time, there's enough heat built up
to bring the thermal mass up to cutoff without serious damage. Problem
is when you put a 70% charged pack on charge. The internal temperature
comes up too fast for the external sensor to trip. Cell vents.
A key phrase seems to have been snipped from my earlier post"
"In my experience, a properly charged cell..."
Temperature only cutoff is not a properly designed charger.
Today, you'd use that only if you cared about the lowest possible
product sales price, and you made a bundle off replacement batteries.
Now I have a minus sign to ponder.
Google 0-delta-v or -delta-v or some permutation of spaces and dashes.
Is that the -dV/dt above? The cell or string starts uncharging?
Nope, still charging. There's lots of stuff on the web on proper
charging of NiCd cells.
Google 0-delta-v or -delta-v or some permutation of spaces and dashes.
Or start at the cadex website.
I'd like to know how to detect and/or deal with that situation. I'd
guess that I'd have to determine that the string never reached it's
expected voltage at the expected time.
No problem. Make the charge current independent of pack voltage.
Charge at the proper current relative to cell capacity. (there's
argument over exactly what this is. I usually use C"
Terminate on voltage slope rather than voltage.
Have three safety cutoffs
1)over temperature
2)over voltage
3)over charge I x T > 1.2 x pack capacity
And apply a trickle charge if the pack voltage is below some minimum
voltage. Only start fast charge if the pack voltage is within some
min/max range and the temperature is within some min/max range.
The only reason my charger needs to know the voltage is so it can
properly normalize the graph so every pack looks the same on screen
and I can immediately recognize a problem.
mike
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