How to Charge an Extra Battery While Driving a Car

by Cassandra Tribe
Jim Bounds

Charging an extra battery while driving a car is easier than you might think. For years, RVs have done this--charging a "house" battery for use when the engine was off by using a battery isolator that allows the charge, but also protects your main battery. The main thing to consider is where to place your extra battery; once you have decided, you can install a system for charging an extra battery in about 20 minutes.

Step 1

Disconnect the cable from the negative post of your main battery.

Step 2

Place your extra battery in your vehicle and attach battery tie downs to hold it in place. Use existing bolts for an anchor for the tie downs or drill and attach the tie downs with sheet metal screws.

Step 3

Attach the battery isolator to the firewall of your engine compartment. If you cannot find an appropriate size bolt/screw to "piggy-back" off (removing the nut on an existing bolt and attaching the isolator to it), drive a sheet metal screw with a drill into the car frame to attach the isolator.

Step 4

Trace the wire from the battery post on the alternator (marked 'B' or 'B+') to your car's fuse box. On the opposite end of the fuse the alternator wires to, attach a wire and run this to the post marked "IN" on the battery isolator. Strip and crimp a ring connector on the end of your wire to attach it to the post, rather than coiling bare wire around the post; your connection will be more reliable this way.

Step 5

Attach a wire from the "OUT" post on the battery isolator and run it to the positive post of your extra battery.

Step 6

Ground the middle post of the battery isolator directly to the frame of your car by attaching a wire connecting the post to a bolt or screw passing through the frame.

Run a wire from the negative post of your extra battery to the floor pan of your vehicle. Make sure the connection is directly to the metal of the pan and not to any plastic body parts. Reconnect the negative cable to your main battery.


  • Crimp ring connectors onto your wires where practical (grounding to frame bolts or posts). This will help form a more reliable connection than simply coiling bare wire around the post.


  • Make sure that the posts on your extra battery are covered and protected from any contact with the vehicle frame or any cargo that you may carry. Accidental contact could short the battery and cause an explosion.

Items you will need

  • Battery tie-downs
  • Battery isolator (diode type)
  • Drill (if needed)
  • Sheet metal screw (if needed)
  • 14-gauge wire
  • Ring connectors
  • Electrical pliers (Crimp-and-strip type)

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