Excerpts from audi: 3-Apr-95 Re: Big Battery = Destroy S.. by David
Schwarze@superc.no 
> > 
> > This raises the question of how to wire our auxiliary batteries. If
you wire
>  
> > two 12 volt batteries together, or allow them to connect together during
> > starting only as I have done, then it would seem you risk frying
your starte
> r. 
> > Hummm...
Hmm...  I have been told that on some cars, that because the voltage
drops to 6 or 8 volts when the starter is cranking, the
computer/ignition/whatever has to be able to operate at that level, so
they put a voltage regulator in front of the F.I. computers so that the
computers is always running of 8 volts. By designing the computer to run
at the lowest possbile voltage, they make it easier to design the
starting circuitry. This could explain why many cars will run even with
completely dead battery, even without a working alternator for a
surprisingly long time once they are started(push started/jumped
whatever) as long as you don't turn on the lights.(how much juice can
the fuel pump and ingnition use).  

On another note, I rented a large International Diesel U-Haul Truck once
and it was plastered with giant stickers which never ever jump start
anything with this truck.  This seems to corroborate with what Derek got
out of the Bosch manual.  Assuming the resistance of a starter is
constant, the power used by the starter is proportional to the square of
the voltage across it.  If the resistance of the starter is .1 ohms,
voltage is 8 volts(current is 80 amps), the power is 640 watts.  If the
starter was hitched to a giant battery(or two batteries) which could
maintain ~14(current is now 140 amps) volts regardless of the load, the
power would be 1960 watts... about three times the juice.

James

'81 Audi 5000 Diesel
'69 Squareback
Wannabus