Here's a handful of  ideas:

1. Check out Raychems polyswitches - although the SRP series is rated
too low for you application, you may
    be able to use the RUE series (assuming you can put this outside of
the battery pack).

2. Put a mosfet in series with the battery voltage, sense the battery
voltage with a micro/low power voltage
    comparator, and then turn the mosfet off when the voltage begins to
collapse - or sense the current w/ a
    low power shunt and do the same.  You would probably us a p-channel
fet and pull the gate low during normal
    operation, this way you won't need a charge pump to drive the
n-channel above the battery voltage.

3. Unitrode/Linear tech have some ic's that do something similar to 2. I
use the LTC1153 in a similar application.

There aren't too many more solutions out there - I think 2 above is the
slickest way to do it.

Daniel Najemy - Data General Corporation,  Numaliine Power Systems

> -----Original Message-----
> From: David Lions [SMTP:bjlions@MAGNA.COM.AU]
> Sent: Wednesday, January 21, 1998 5:53 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Battery short circuit protection [OT]
>
> Hi,
>
> May I draw on the eternal wisdom of the list?  I have a problem with
> battery protection.
>
> Our product uses a 12V 3AHr NiMh battery pack (and 3 PIC's).  The
> battery pack is designed for use in rough environments, and for this
> and
> other reasons, the terminals have been left relatively well exposed.
> This is a problem if the battery is short circuited.  The batteries
> are
> a small but expensive part of the unit.
>
> Can anyone suggest a way of short circuit proofing the batteries?  The
> method has to :
>
> 1. Be user-resettable after a short circuit, or self-resetting.
>
> 2. Respond very quickly after the current limit is exceeded (batteries
> are quickly damaged).  Ideally the current limit will hardly be
> exceeded
> at all, even for an instant.
>
> 3. Draw very little or preferably no quiescent current (NiMH
> self-discharge automatically :-).
>
> Some "resettable" fuses that we've tried are way too slow.  The
> current
> limit by the way is 9A.
>
> Any ideas?
>
>
> Thanks.
> David Lions
> bjlions@magna.com.au.NOSPAM