Dave Liquorice wrote:

Any sort of reasonable connection between the two batteries will do to
charge.

You can't really charge a battery from another battery. It works for a
while but the recipient battery ends up still pretty well flat. The
voltage of the good battery will not overcome the surface charge that
builds up on the recipient battery and the current flow ceases. Also,
when the voltage difference between a battery being charged and the
charging source is almost zero, as in this case, the resistance of the
connecting leads becomes very significant. You need around 15V to charge
a car battery at a reasonable rate. 11 or 12V will do nothing.

Bill