This post is to document how to properly wire a three battery system in conjunction with a Blue Sea Add A Battery combo ( and a ProSport20 Dual Bank onboard battery charger ( Hopefully, this will give someone the confidence to do the install themselves. I thought it would be helpful to show the wiring diagram in three stages as it can get pretty confusing. This is the set up I have which I ran the past two summers without a hitch. I wil post some pics of my install later this week when I finish my current rewiring project.

So below is step 1, which is wiring the switch and the batteries. Note that stereo batteries 1 and 2 are wired together (in parallel) so they act like one, big 12 volt battery. Also note that the 4 pole Dual Circuit Plus Switch is very different that the three pole Perko switch that is commonly installed on our boats. The Dual Circuit Plus is like two separate switches in one housing and does not have a common post like the Perko does.

The idea is to wire all the stereo related stuff to the stereo battery bank via one side of the switch, and the starter and the helm buss to the starting battery via the other side of the switch. When the switch is in the ON position, both sides of the switch are closed, and are separate. In the OFF position, both sides of the switch are open so everything is dead. In the COMBINE position, both sides of the switch are closed and connected together so both batteries are feeding all the electrical loads.

Note that I show suggested wire sizes but I do not show any fuses but they must be installed as specified by the equipment manufacturers.

Step 2 is shown below, in which the Blue Sea ACR is added. Note that a jumper is added between the neg post of stereo battery 1 and the neg post of the starting battery.

The ACR automates the switching process so that you just set it to ON and forget it. When the ACR senses that either battery is receiving a charge from the alternator (in this case, the starting battery), the relay closes so that the charging current is also sent to the stereo battery bank. It does this once it senses 13.0 Volts for more than 2 minutes. So essentially, the relay closes two minutes after the engine is started. Once the engine is off and you are cranking your stereo, once the ACR senses that the batteries combined voltage has fallen below 12.75 Volts for 30 seconds, the relay opens, isolating the batteries. So now your stereo is only draining the stereo bank and you won't accidentally drain your starting battery.

Note that the ACR will not close if either battery bank is less than 10.8 volts. Doing so would cause the batteries to equalize, which means you now have two discharged batteries instead of one.

Step 3 is shown below, which shows the addition of the onboard dual bank smart charger. The ProSport20 is a 20A dual bank smart charger and can send up to 20A to the bank that needs it most. However, since the ACR will detect the charge and it's relay will close, both banks will receive the charging voltage equally and in essence, defeats the purpose of the smart charger. So it's important that a 100A resettable circuit breaker is installed between the ACR and each battery so that the breaker can be opened during onboard charging operation to disable the ACR.

Sorry if this is too much information. I think it will help once you see the pics of my actual install.