Donovan Parks wrote:
>
> Hello,
>
> We are planning to drive the motors at 12 or 24V from some NiCd batteries.
> The maximum continuous current draw is around 12A for the motors we are
> using (peak current around 100A).  Three questions:
>
>     1) Can NiCd accept "reverse charging" (they are obviously rechargable,
> but I'm guessing this isn't exactly the same thing)?
>     2) Can NiCd accept this sort of heavy current draw?
>     3) How can I find the answers to this questions for myself? (I've read
> several websites on batteries, but they did not address these issues)


My suggestion is to NOT use the NiCds as part of the
recirculation current path. They are a chemical device
and do wear out, besides having lousy charging efficiency.

You should decouple the bridge and recirculating current
with LARGE filter caps. The pulses of current are supported
by the caps, NOT the batteries. These caps should be close
to the bridge which in turn should be close to the motor.
You need low ESR electros, one cheap way to do this is to
parallel a lot of smaller electros.

The batteries can be separated from the caps by a low-ohms
resistor which spares the batteries from much of the
pulse current and also doubles as a current measurement
resistor, ie will show average current from/to the batteries.

You ratio of peak motor current to average curent is 100:12
which seems high and somewhat unnecessary? If these motors
are for submarine propulsion can't you soft-ramp them
under PIC control? This will keep your h-bridge cost
lower and reliability higher, it also reduces drain from
the batteries upon starting and current forced back into
the batteries upon deceleration.

There are two main methods of handling recirculating
current, one involves just switching the FETs off and
letting the body diodes pass the current back to the
filter caps. This is better when you need high rates of
change of the current, like fast accelerations and
decelerations. In your case you are probably better off
with the other method (which you suggested) of turning
on the bottom pair (or top pair) of FETs and keeping
the current as slow-decay (ie in the motor).

Have you tried a google search for "high power h-bridge
design" ?? :o)
-Roman

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