Battery Cut Off

[FJB203 at aol.com]

In a message dated 97-03-24 18:36:03 EST, you write:

> to the relay switch, but of course it will also do this when ths switch
>  is open, the other way round its blocks the current all the time so the
>  alternator never charges. Any suggestions

Another solution is to create a high-current regulated power supply using an
op-amp, power MOSFET or equivilant bi-polar transister, and a zener diode
that limits the input voltage threshold to 12 or 13volts.  This ensures the
electronics receive only what they are supposed to receive, and your battery
disconnection won't harm anything.

The side advantage (but also very small advantage) is the engine sensors and
electronics always will receive the same voltage, so at high RPMs when the
alternator is cranking out 14+ volts (hopefully not), all internal voltage
references in the computer stuff will be constant.

Just a suggestion if you are just trying to protect electronics stuff.

For battery/engine cutoff, I'd have a large relay with one SPST side between
the alternator and the vehicle, and one SPST side of the same relay between
the alternator and the battery.  When the cutoff is triggered, there is no
juice whatsoever anywhere.  I suggested seperate SPST relay contacts only to
ensure the battery doesn't feed into the ignition and keep the vehicle
running.  Guess that makes the relay requirement DPST. :)

Fred