I used an ACR. I didn't buy the add-a-battery kit, as I found someone here that sold me a used ACR on the cheap, so I just bought an on/off switch that is inline between house battery and house breaker panel.

The ACR is in-between port start battery and house battery, but this is not a parallel circuit with starting battery. It is isolated from starting battery (by the ACR).

Several have chimed in above, but wanted to clarify my original post.

If you see my diagram above, the two ACR's are independent of one another when the battery switch is set to OFF. The ACR's work purely on the detection of voltage levels, ie: they close the relay when voltage above 12.75 is detected and open it when it drops below so in theory with one engine running and the battery switch to all, it might be possible for one bank to detect the voltage going up on the other. not sure if thats a problem or not.

For example, using my diagram.. say the port engine is running. The port ACR will detect voltage going up on the start battery from the alternator, close the relay and start charging House battery #1, if the switch is set to ALL, that current could travel across the switch to House battery #2 and the starboard ACR detecting voltage going up on House 2, will close the relay to combine the starboard bank as well. That scenario might be too much for a single alternator to handle. This is not an issue with the battery switch set to 1, 2 or off.. only when on ALL..

Good diagram HyFive, it actually makes sense. I can see the usefulness of the ACR's in that scenario. To your point, I want to accomplish this in a way where I can't make a mistake by switching something that creates a nuclear back feed! :)
Thanks again for the clarification!

I don't think there is any foolproof option here. If you want to add a house battery, you need a way to charge it. if you hook it up in parallel with another battery, that creates the possibility that one battery can drain the other. So you have two choices, use the engine to charge or a battery charger.

If you want your engine(s) to be the charging source, then you really should use an ACR to safely parallel the batteries when theyre charging and isolate them when discharging or you run the risk of one battery draining the other whether you're using the stereo or not. I would highly advise against configuring something that runs the risk of depleting the starting battery(s) because you are guaranteed for that to happen at the worst possible time (like a sudden raging thunderstorm)...If you don't want to go the ACR route, install two house batteries and use a charger but you'll have to remember to always charge them and deep cycle batteries can take a long time to recharge when run all the way down. In either case, you will have to remember to do something.

I think my diagram gives you a really good solution. You would have two house batteries available for your stereo that will be charged by your engines. You would have automatic isolation when they are discharging and not risk your starting batteries. And I'm not certain that having the switch in the "ALL" position is really a problem, thats the one unknown for me, but you could easily make shutting that switch part of your startup ritual.

Agreed! This also has some installation advantages with my current setup, and eliminates the awkward need to have passengers move off the bench so I can constantly switch switches. Memory is NOT my strong suit anymore, so having the ACR do the combining/isolating automatically is a big plus!

Going off the above posted diagram, put a battery solenoid between the 2 house battery's of the NC state and use your ignition signal as the power lead to switch the state to disconnect condition. This way when you are sitting with the motors off, the house batteries are paralleled. As soon as you start your motors, the relay will open and the house batteries will unparallel and be charged thru the ACRs. All automatic, no owner/operator input required. no switches need to be remembered to be switched.


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