Using a Continuous Duty Solenoid to Isolate a Second Battery


Heres a schematic of how to use a continuous duty solenoid as an isolator for a second battery. In this configuration, a single SPDT switch is used to choose between one of 3 isolation modes and 3 SPST switches are used to alter the operation of the radio, off-road lights and A/C inverter.




To connect the battery to the solenoid and the solenoid to the stock electrical system, a minimum of 2AWG cable should be used. The schematic doesnt show it, but there should definitely be a fuse close to the battery.


The 3 modes of isolation are normal, isolated, and connected. In order to switch between them, the center contact of a SPDT switch is connected to one of the coil terminals on the solenoid. For the other two contacts, one is wired to the ignition circuit and the other to the second battery. In this configuration, when in the normal position, the second battery is connected only when the key is in the START or RUN position. The middle position turns everything off thus disconnecting the second battery and the third position uses power from the second battery to force the second battery to be connected. Power from the second battery is used here because should the second battery deep cycle, it will not have the current to switch the solenoid and the only way to connect the two is to use the normal position when the engine is running. This prevents the second battery from draining the main battery when deep cycled.


In order to run the radio and CB with the engine off, a SPST switch was used. The center terminal is wired to the switched power to the radio, one side is wired to the ACCESSORY circuit (on with key in RUN and ACC positions) and the other is wired to the second battery. With the switch in the normal position, the radio turns off when the key is removed from the ignition but in the other position it allows the radio to run from the second battery without the key in the ignition so the main battery is not drained. The constant power for the radio is used for memory only and does not need to be switched to the second battery. The CB is connected to the output from this switch so it behaves like the radio.


For the off-road lights, its a similar situation except one side of the switch is connected to the parking light circuit and the other to the second battery with the center terminal going to another switch on the dash. This switch then connects to a relay that controls the high power supply to the lights. A schematic of the off-road light wiring can be found at Its a lot of switches, but it works and is relatively easy to use.


The best place to mount the battery in a 2000 Ranger was behind the passenger headlight. This requires removal of the air box, replacing it with a 70mm MAF adapter and a K&N RU-3130 cone filter. This leaves enough room for an Optima yellow top D51 either standing up or on its side. I chose on its side so it would be more stable for off-roading.



1 85amp Continuous duty solenoid

4 Red 2AWG marine battery cable

1.5 Black 2AWG marine battery cable

6 2AWG welding terminals

6 - 12AWG hookup wire

2 Battery terminals

1 SPDT toggle switch

3 SPST toggle switches

10 10AWG cable

1 6 position fuse panel



Content: Tom Barcia at

MAF adapter: Robert Emery at

K&N Air Filters:

Optima Batteries: