article from Blue Sea Systems
In multiple battery bank systems, Charge Management Devices (CMD's) automate charging. These devices provide a means for combining two battery banks when a charge source is present, while keeping the battery banks isolated from each other when no charge source is present. This assures that even if one battery bank is depleted, there will always be a charged battery bank available for engine starting. Some devices also provide a manual means of connecting both battery banks together for additional power while starting engines. There are many types of CDM's that fulfill this role. The two main categories are battery isolators, and Automatic Charge Relays (ACR's).
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
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's can be used with outboard motors and inboard/outboard (I/O) installations.
Original article from Blue Sea Systems