Not per se, but it needs to operate on 12V or whatever that post on the alternator gives, and be able to handle lots of current.

Exactly HOW much it needs to be able to handle depends on what you hookup and how.
The minimum would be the charging current of the alternator.but even higher current can flow if the charge difference between the batteries is large (ie full vs empty) so I'd say 200A is a good target. Nprmal relay (Bosch relays. like used if you have lots of items to turn on with the radios remote output) typically handles 20-30A


"Battery isolators are relays made with this task in mind, so they're high current and have sturdy connecting posts for the heavy wires used.