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After answering numerous questions about different battery isolator schemes, I decided it would be easier to just build a web page... Below you will find the basic design of 3 types of battery isolators with the pro's and con's of each.
I got an email from someone scolding me that I was not telling the whole story and claiming diagrams were simplistic (overly simple). That is all true. The diagram below are intended as an overview and some details are missing from the diagrams.
The Diode Battery Isolator
The diode type battery isolator uses semiconductor diodes to split the current from the alternator or generator and charge 2 or more batteries at the same time. One battery is used to start the engine the other is used to run the accessories. The load on the accessory battery does not drain the starting battery so it remains charged even when the accessory battery is run down.
Pros: no user action needed to operate.
Cons: May requires some wiring changes -- you need to separate starter/ignition from accessory wiring. This could be complex in a modern vehicle. Some vehicles will need the VSense line connected to alternator.
The Battery Switch
The battery switch allows you to switch between 1 or more batteries and sometimes between a combination of batteries. A common battery switch lets you choose battery 1, battery 2 or both. Which ever battery you choose is connected directly to the engine, alternator and accessories. The switch lets you drain one battery, then switch to another to start the engine. You must then switch to "both" in order to charge both batteries or have a separate diode-type battery isolator.
Pros: No need to separate starter/ignition wiring from accessory wiring.
Cons: Requires user to manually switch between batteries.
Heavy battery cables must be run to the switch
Auxiliary battery solenoid The solenoid isolator uses a continuous duty solenoid to connect the auxiliary battery during certain times (like starting and charging) then disconnects it when not in use. The solenoid can be switched to be on all the time, off, or on with ignition depending on how the switch is wired.