article from Blue Sea Systems
Installing an Automatic Charge Relay in a System with a Dual- or Triple-Output Battery Charger
Most multi-output battery chargers have only one program running to control all outputs. Therefore, the outputs proceed through bulk, absorption, float, and equalizing charging stages together. The charging unit will shift from one stage to the next when the last battery has satisfied the conditions to move from the previous stage.
Even if the Start battery is much smaller than the House battery, connecting the batteries together with an ACR or with a manual switch while fed from a multi-output charger causes no ill effects. In fact, overall system efficiency may improve. When a charger such as a Xantrax TrueCharge 20 is feeding both batteries, the Start battery may readily come up to voltage. The excess capacity from that charging output can transfer through the ACR to the more needy House battery. Then, it is getting the benefit of both outputs until it catches up with the Start battery. I have a Blue Sea Systems' BatteryLink™ 7600 on my boat with two battery banks, and a WM version of the Xantrax Truecharge 20. It works fine. Xantrex concurs with this assessment.
In the Fall of 2005, Xantrax introduced a new multi-output charger series XC that has separate intelligence and control for each output. Having an ACR installed with this system will result in taking away some of the potential benefits of having the distinct charging profile control. Nothing will be damaged, but it may take longer to get a full charge on all batteries, or the batteries may stay in the absorption phase longer than they should. This increases the chances for over charging and may possibly lead to a shorter battery life.
If you have a multi-output multi-program charger, you may want to consult the manufacturer for proper connections.
When connected to shore power, you probably should disable the ACR. This can be done automatically by installing an AC driven relay to close a contact to ground when AC is available. If a manual switch is also used, the relay should be SPDT (single pole double throw) so that when AC is not present, the control line passes through the relay to the manual switch. When AC is present, the relay disconnects the ACR manual control line from the switch and connects it to system negative, shutting the ACR off. A good candidate for such a relay is the Omron L1YF-AC110/120 that costs less than $7 from Digi-Key.
Vice President R & D
Original article from Blue Sea Systems