Re: charging 10 cell nimh pack 6 cells at a time?

acannell@xxxxxxx wrote:

<snip>


- Show quoted text -

Okay then two seperate LM317's charging the banks seperately, one
charge controller that uses dt/dt (BQ2002T, ~$1 @ 1k qty), a relay for
connecting it all up, and there you have a way to charge 10 nimh cells
with only 12V and no switching power supply, and you do it by charging
5 cell banks simulatenously. Should be cheap.

Comments?

It seems if you are not careful, you can get the two different groups
of batteries at different potentials. As you discharge the batteries,
you only have one pack. I think this can lead to a scenario where you
get cell reversal. Take a look at this:http://www.batteryuniversity.com/print-partone-16.htm

The nice thing about charging the pack as one group is if the
batteries are well matched, they see the same charge and discharge
current, so there is less of a likelihood of cell reversal.- Hide quoted text -

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Remember though, the charge termination comes from one thermistor,
which is in thermal contact with both "banks", so the charge
termination will occur at the same point as if you were charging all
the cells as one pack. I'm not sure there is any way, from the cells
perspective, that a difference between charging all the cells at once
and charging two banks at once could be perceived.

When one of the banks reaches the DT/dt set point, it shuts
down charging. The other bank does not get a full charge,
except by chance. You need individual control and sensing
of each bank.

If you have the capability to separate the thing into
2 banks as you mentioned in your earlier post:
"Okay then two seperate LM317's charging the banks seperately,"
then there is no issue. Run one supply into two separate
controllers, with equal termination set points. The
controllers can share some circuitry, but each bank needs
to be terminated depending on the signal from its own
sensor.

There's drawbacks/compromises/etc to whichever way you do
it. I like your idea separate LM317's, provided you can
separate the pack into two individual banks of 5 cells each,
and you terminate the charge individually.

Actually, since anything you do is a compromise when you
split the pack, I'd set each 317 for constant current with
a taper charge when terminal voltage is reached. The easy
way is to set a TL431 to the terminal voltage, and have it
shunt current from the source when that voltage is reached.
That way, you can continue to pump constant current from
the 317, but divert some portion of it through the 431, and
provide the NiMh with whatever C/X you decide.

A nice side benefit is that you can light a red led and
extinguish a green led with that approach, so that you see
that the pack is fully charged, and use the current through
the led as part of what you divert. You can also put a
resistor in parallel with the led to divert more current.

The TL431 has a max of 100 mA AIRC - if you need to shunt
more than say 75 mA, you can control a transistor with the
431.

Is that the best possible approach for charging an NiMh.
No. But it is simple, it works, and you are forced to
compromise in your situation, so it seems a reasonable
way.

Ed
.

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