Durite do a nice heavy duty voltage sensing relay which isn't a fortune, ( I think its about 100 Amp ) Or if you just need something light duty, you could use one of the nasty types used by towbar fitting places. ( about 10 - 20 Amps )

I would use the heavy duty version and heavy cables ( 65's at least ) as the current flowing from a Charged / Charging battery into a flat battery can be surprisingly high and is only really restricted by the cable size and length.

H.T.H Tony.