What about connecting a negative lead direct from the battery to the engine.
Also disconnect the alternator, it may have fault.
Did you previously mention the voltage regulater was not reading properly.
We may have a few faults, I would go for battery power first.
The battery should be so full of life, ( something must be wrong with it, ) is it guaranteed or even the correct type.This could even be a starter motor problem.
I would say that the battery should turn the engine over really fast for quite a long time. If not sort that problem out first.
I also believe that the prevous owner knows about this problem, any chance of contacting them.
Keep up the good work.
Roy
