On Thu, 10 May 2018 08:53:16 +0100, Harry Bloomfield
wrote:

on 10/05/2018, Bill Wright supposed :
I helped a friend (a friend in need...) who had a similar problem in a
motorhome. For some bizarre reason the starter battery was in a locker
instead of in the engine compartment. It wasn't all that far but during a
start the voltage at the starter motor was about 6V. In the end we put the
battery back where it should have been and that fixed it.

Many people do not understand volts drop and just how serious a problem
it can be at such low voltages. They base their calculations simply on
the current rating of the cable and even the pros ignore volts drop..

The good RC car racers do as they often go as far as soldering
directly the things like the speed controller to the motor and even to
the batteries using the shortest lengths of the largest CSA and best
quality copper wire that fits. As you say, if you only have 7.2V in
the first place ...

When I made up some extension leads for my electric outboard motor
(because for good boat trim I needed the batteries in the bow) I used
quality, copper, stranded starter motor / welding cable that was
probably 6 x the CSA of the original outboard cables.

When I built my electric endurance racing 'motorbike', even though the
main battery cable was pretty heavy, I was able to use the voltage
drop over it to drive a mini DVM calibrated in Amps.

Cheers, T i m