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).

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