On 10/05/2012 21:03, Tim Lamb wrote:
Newish car battery employed to power pony field electric fence. Likely
to have been left connected well beyond the fully discharged state and
possibly exposed to overnight frost.

Won't take a charge! Yes, I know, throw it away. However, horse person
has gone off with my sound spare. Is *over* discharging an issue? or...?

regards

Car batteries are designed to deliver lots of cranking amps. They are
generally destroyed by a deep discharge, or the very least have their
capacity severely reduced.

Use a leisure battery which is more rugged and can cope with deep
discharge, or should I say are less damaged by a deep discharge. I'm
pretty certain you can get devices which switches off a load once
battery volts get below 11V.

There is a thought that leisure batteries can be part recovered by
charging at 15V or so to reverse sulphation, but I don't have any
experience of this or if applies to car batteries. My experience of car
batteries is that unless they stay fully charged, they are relatively
fickle beasts.