How to Charge an Auxiliary Battery in a Tow Vehicle


Charging the auxiliary battery in your tow vehicle or RV automatically will eliminate the risk of a dead battery after running accessories with the engine off. Wiring your vehicle to charge this battery can be done inexpensively in only a short period of time by the average do-it-yourself car owner.

Things You'll Need

  • Continuous duty solenoid 16-gauge primary wire Battery cable Starter cable
  • Disconnect the negative cable from the vehicle battery.

  • Mount the continuous duty solenoid in a convenient location in the engine compartment using self tapping screws.

  • Connect a battery cable from the positive post of the auxiliary battery to the input stud of the continuous duty solenoid. The nut to attach the cable to the stud is included with the solenoid. Tighten until snug, being careful not to over tighten because the post is soft and the threads are easily stripped..

  • Connect a starter cable from the out terminal of the continuous duty solenoid to the positive cable of the main battery. This can be connected at the battery or at the starter solenoid in the same location as the positive cable from the main battery. To do this carefully remove the nut which attaches the battery cable to the starter solenoid, install both cables and replace the nut being careful to avoid over tightening.

  • Use a test light and locate a fused circuit which is powered only when the ignition key is on. The best circuits for this would be the heater fan or radio power.

  • Using a Scotchlok connector or fuse tap connect this switched power source to the small terminal on the continuous duty solenoid. Use a solderless ring terminal to attach the wire to the solenoid terminal.

  • Connect accessories to the auxiliary battery. This will allow operation of those accessories with the ignition key off with no fear of draining the main battery, while at the same time, allowing the battery to be charged when the engine is running.

  • Reconnect the negative cable on the vehicle battery.

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