Battery switches

[Stuart Thomson <stuart.thomson@domain.elided>]

Hi All,

Whilst installing a battery isolator switch in the racecar I was 
considering which line to cut, positive or negative.

After pondering for a while I chopped the positive line, as this will 
cut the motor as well (if you cut the coil power supply as well).  
Pretty smart I thought, just saved myself about $85.  "Proper" battery 
isolation switches from our local racecar parts supplier are about 
$100, they have two large terminals for cutting the battery, and four 
others for ancillary systems.  The one I bought from a marine supply 
shop only had the two large terminals and cost about $15.

I thought about the safety of the design later...

I now have a large wire going to an exposed terminal running up inside 
the dashboard which is always hot.  Also in the near vicinity I have 
several exposed metal surfaces, which are always grounded.  In the 
event of a crash (it won't be an accident, I'll just have pushed too 
far) I don't think I want that hot wire in the dash.

Switched the battery leads over, so the switch now cuts the negative 
lead.

I now have a large wire going to an exposed terminal, always earthed, 
same as the rest of the body metal.   I think much safer.  But ...

Now whilst the engine is running there is no way to cut the power to 
the coil (as the alternator is earthed through the body), so my battery 
isolator switch does just that, nothing more, it won't stop the engine 
running.  I figure that if I've stacked the car seriously enough for 
someone to pull the isolator switch there's a fair chance the motor 
isn't going to be running anyway.

Does anyone know how I can make it work for both though??

BTW those green screw knob battery switches are great, whenever I work 
on the car I just unscrew the switch, should put one in the 75 as well. 
 Always put those on the negative terminal as well, for safety reasons.

Cheers
Stuart
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