http://bluesea.com/viewresource/92
Battery Isolators are a common method of distributing charging current to multiple battery banks while assuring that the batteries remain electrically isolated during discharge. These devices are electrical "one-way check valves" that allow current flow to, but not from, the battery. Their disadvantage is that the diodes used in these devices cause a voltage drop that consumes charging energy, creates heat, and causes batteries to be undercharged. Alternators with external voltage sensing can correct for the undercharging problem, but voltage drop and the heat generated remain a problem. Battery isolators generally cannot be used with outboard motors that share a common wire for charging and starting.
Automatic Charge Relays (ACR's) are becoming a popular method for achieving the same goal as isolators, but they work on a different principle. Instead of using diodes to block current from flowing in both directions, ACR's use mechanical relays controlled by a circuit that senses when a charging source is being applied to either battery. When a charge is being applied, the ACR closes after a short time delay and connects the two batteries. When the circuit senses that the charge is no longer present, the ACR opens after a short time delay. This delay assures that the ACR does not open during temporary voltage sags due to load start-ups. ACR***8217;s can be used with outboard motors and inboard/outboard (I/O) installations.