Mount the aux battery in the cargo area, behind the driver-side rear wheel well, or under-hood passenger side with a custom tray. XJ engine bay is tight - rear relocation is common.
The XJ engine bay has almost no room for a second battery once you have a 4.0L and an AC compressor. Common solutions: (1) Build a custom passenger-side tray near the airbox — fits a Group 34 in some configurations; (2) Relocate to the cargo area behind the driver-side wheel well using a sealed Optima or AGM (never a flooded battery in the cabin without a vented box); (3) Mount on the rear bumper or swing-out tire carrier in a NOCO Genius or Power Bear battery box.
Key rules: AGM only if inside the cabin (or a vented sealed enclosure). Use 2 AWG or larger for the long run forward — at 15 feet round trip, 4 AWG drops noticeable voltage at winch loads. Fuse within 12 inches of EACH battery terminal. Strap down with steel hold-downs rated for offroad — a loose battery in a rollover is a fire and a missile.
Good XJ-specific solution: behind the driver's side rear seat back, in a Group 24 AGM with a steel surround. Run 1/0 AWG up the driver's rocker under the carpet to a relay/isolator in the engine bay.
| Part | Vendor | Est. price |
|---|---|---|
| NOCO HM426 battery box | NOCO | ~$50 |
| Steel battery hold-down kit | Genesis Offroad | ~$80 |
| 1/0 AWG cable 20ft | WindyNation | ~$70 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.