Re: Just an idea

Nothing wrong with gel cells. I'm only focusing on Nicads (or NiMH if
you will, same animal) only because of much lower source impedence,
twice the energy density, quick-charge capability, small form factor
(AA size) and 1/3 the weight of lead-acid batteries. The AH rating I
feel is very secondary; a 25 Amp pulse for 100 ms discharges only
0.3% of the charge stored in a .5 AH battery. This charge can be
fully restored during the quiescient phase of the duty cycle.

Regarding exploding NiCads; they will if shorted; counterintuitively
that is their advantage. Simply square the cell voltage, then divide
by the source impedance (1.25 * 1.25 / .02). The result is 80W
dissipation in a AA size package. It will blow. It is their ability
to provide high pulse currents, when abused, will make them explode.

Having overly powerfull motors was an objection to this approach. I
must respectfully disagree with this. Motors are expensive and should
be sized to the task. If a 1HP peak motor is used, one should expect
to get 1 HP at peak power. It isn't a case of overdriving the motor,
rather it is one of getting by with the smallest and least expensive
power supply.

To be practical, I would design safety features up the kazoo; MOSFET
switch based current limiting at 25 Amps backed up with a 30A fuse,
undervoltage and overtemp detect and use the latest NiMH charge
controller IC that is used to manage laptop computer batteries. The
MOSFET would also serve as a switch-over device; much more efficient
than even a shottky rectifier. I think it bears further work.

Mariss

> What about lead-acid gell cells? These run about $18.00 with 4AH

rating,

> have substantially longer life (no memory problem) than nicads and

are less

> prone to exploding under heavy loads. Ten security alarm back-up

batteries

> would yield 132V for $180. The gell cells don't seem to gas much

either.

>
> Doug