A dual battery system on the JT pays off fast if you run a fridge, lights, air compressor, or recovery gear from camp — the stock battery wasn't sized for the accessory loads overlanders put on it.
The Gladiator JT's factory electrical system is a 160A alternator paired with a 650 CCA AGM battery. That's adequate for driving with accessories on, but a camp night with a 12V fridge, lighting, and phone charging running for 8–10 hours will deplete a single battery to the point of unreliable starting. The answer is an auxiliary (aux) battery — a second battery dedicated to accessories, isolated from the starting circuit so the engine always has what it needs to start.
**Two approaches differ fundamentally in how they charge the aux battery:**
**Automatic Charging Relay (ACR):** Connects the two batteries when alternator voltage is detected (engine running), isolates them when voltage drops (engine off). Less expensive (~$55 for the relay), more straightforward to wire, but charges the aux battery at full alternator output — which can undercharge an AGM or lithium aux if the drive is short.
**DC-DC Charger (REDARC BCDC, Ctek D250):** A smart charger that sits between the alternator and aux battery. It conditions the charge profile for the specific battery chemistry (AGM, lithium), can accept solar input through the same device, and protects against alternator damage from direct lithium charging. More expensive (~$180–240) but the right answer for lithium aux batteries and extended overland use.
For an AGM aux battery with occasional weekend use: an ACR relay works. For a lithium battery or full-time overland rig: the DC-DC charger is required.
1. Determine aux battery location. The JT truck bed gives options the Wrangler doesn't — a bed-mounted tray behind the cab is a common install that keeps the battery accessible and doesn't consume interior space. Some owners mount under-hood in the fender well, but the JT engine bay is tight.
2. Install the battery tray in your chosen location. For bed installations, most trays bolt to the bed crossmember rails.
3. Run a positive cable from the starter battery terminal, through a fuse holder positioned within 18 inches of the battery terminal, to the DC-DC charger input. Fuse the run at 40A (or per charger specs). Use minimum 8AWG wire for runs under 10 feet; go to 6AWG for longer runs.
4. Run a negative return from the DC-DC charger back to chassis ground — a nearby body bolt or the negative terminal of the starter battery.
5. Run positive and negative cables from the DC-DC charger output to the aux battery terminals. Fuse the positive run within 18 inches of the aux battery at 40A or per manufacturer spec.
6. Connect the charger's ignition sense wire (if equipped) to an ignition-switched 12V source so the charger activates when the engine runs. Many owners tap the fuse block in the cab.
7. Ground the aux battery negative to the vehicle chassis — a dedicated ground point near the battery location. Bond it back to the chassis, not to the body.
8. Wire accessories to the aux battery through a dedicated fuse block. Size each fuse to the accessory circuit — a 12V fridge draws 3–5A, a compressor draws 15–40A. Never skip fusing individual circuits.
9. Test: start the engine and verify the DC-DC charger's charge LED activates. Measure aux battery voltage with a multimeter — it should rise toward 14.2–14.8V as the charger works. Turn the engine off and verify aux voltage holds while running accessories.
ACR relay path (Blue Sea ML-ACR + Odyssey PC2150 AGM aux): ~$350–400 for parts, plus wire, fuse holders, and tray. The lowest-cost path to a working dual battery system.
DC-DC charger path (REDARC BCDC1225D + Odyssey PC2150): ~$550–600. The right answer for overlanders who may later add solar — the REDARC has a built-in solar input.
Lithium aux (100Ah LiFePO4) + REDARC: ~$750–900 depending on lithium brand. Weight savings and deeper discharge capability worth it for serious builds.
A shop that does off-road electrical work will charge $400–600 in labor for a dual battery install, depending on complexity and routing. With good wiring and a weekend afternoon, this is within reach of an owner comfortable with basic electrical work.
| Part | Vendor | Est. price |
|---|---|---|
| Odyssey PC2150 AGM Battery (aux, 1150 CCA) | Odyssey | ~$289 |
| REDARC BCDC1225D DC-DC Battery to Battery Charger (25A) | REDARC | ~$239 |
| Ctek D250SE DC-DC Charger / Solar Combiner | Ctek | ~$189 |
| Blue Sea Systems ML-ACR Automatic Charging Relay | Blue Sea | ~$55 |
| 4WD Products JT Bed Battery Tray | 4WD Products | ~$95 |
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.