How to Install a Second Battery in Your Car | Auxiliary Battery Wiring Guide, by Next Level Neo on YouTube. 27 min 17 s.
- Video length
- 27 min 17 s
- Time to do it
- 4 hours
- Difficulty
- Takes practice
- Steps
- 10
What you need
Steps
Pick the auxiliary battery to match your load. Lithium gives more usable capacity for its size, while lead acid is cheaper but heavier for the same reserve.
Work out your total current draw, then use a wire gauge calculator to choose cable size. This build uses 4 AWG for the power and ground runs.
Fit a fuse holder within about 30 cm of each battery, one at the front and one at the rear. Size the fuse just above your expected current, here 150A.
Crimp heavy-duty ring terminals onto both ends of each cable with a hydraulic crimper. Slide heat-shrink over the joint and shrink it fully.
Find an existing firewall grommet and push the power wire through it, or drill a new hole and fit a rubber grommet. Run the cable along the sill under the trim.
Choose a rear battery spot that is flat, dry and away from heat. Mark and drill the mounting holes, then bolt down the battery box.
Sleeve and clip the engine bay wiring away from moving parts and hot surfaces. Leave a little slack at the battery and fuse holder.
Disconnect the negative terminal on the main battery before you connect anything. Then join the front fuse to the main battery positive and the rear fuse to the auxiliary positive.
Bolt the ground cable from the auxiliary battery negative to bare metal on the chassis. Sand the paint back to bare steel and tighten the bolt firmly.
Set the auxiliary battery in its box and clamp it so it cannot move. Reconnect the main negative terminal last and check every connection before powering up.
Tips and common mistakes
Never skip the fuse at the rear battery. A short in a long cable run without one can start a fire.
Keep both batteries the same chemistry and similar age where possible, so they charge evenly.
Use a grommet on every hole the cable passes through, or the metal edge will wear the insulation.
Disconnect the main battery negative first and reconnect it last to avoid sparks.
Questions
Can I connect a lithium auxiliary battery to my car's standard charging system?
Only if the battery is rated for your alternator's charge voltage. Check the battery spec sheet, and use a DC-DC charger if the voltages do not match.
Do I need a fuse at both batteries?
Yes. Each battery can feed a fault in the cable, so each end needs its own fuse close to the battery terminal.
How do I stop sparks when connecting the batteries?
Connect the positives first, then the ground at the chassis last. A small spark at the final ground connection is normal.
This guide was drafted from the video's details and general task knowledge. It has not been checked against the full video. Check the video and relevant product instructions before following a procedure. Report a correction.
