Once a Gladiator gets a fridge, camp lights, an air compressor, and maybe a winch, the factory battery is doing a job it was never sized for. Run the fridge overnight at camp and you find out the hard way. A second battery fixes that: your accessories run off the auxiliary battery, and the starting battery stays full so the truck always cranks in the morning. Here's how to set one up on a JT without frying anything.
Affiliate disclosure: This post contains Amazon affiliate links. As an Amazon Associate, It's a Jeep Thing Stupid earns from qualifying purchases.
Free Gladiator overland & bed packing list (PDF) — $0, just enter your email and it's sent instantly.
Get it →Plan the power system around what you actually pack: fridge, lights, compressor, and recovery gear.
The Short Version
| Part | Pick | Why |
|---|---|---|
| Under-hood kit | Genesis Offroad Gen 3 JT kit | Bolt-in tray and smart combiner for gas JTs |
| DC-DC charger | Victron Orion-Tr Smart 12/12-30 isolated | Charges a lithium or AGM aux battery properly |
| Charging relay | Blue Sea ML-ACR 7622 | Heavy-duty combiner for matched batteries |
| Lithium aux battery | Renogy 12V 100Ah Pro self-heating | For in-cab or bed mounting |
| Battery monitor | Victron SmartShunt 500A | Real state of charge on your phone |
| Fuse block | Blue Sea 5025 6-circuit | Fused branch circuits for accessories |
| Main fuse | Blue Sea 5005 ANL block | Protects the big cable at the battery |
The Factory "Aux Battery" Isn't for Accessories
If your JT has Engine Stop-Start (ESS), it already has a second battery: a small AUX14-size battery tucked under the fuse box, reached through the passenger-side fender liner. It isn't there for your fridge. It supports the electrical system during stop-start restarts.
Don't wire accessories to it, and know that it can fail long before the main battery does. Gladiator owners report "ESS not ready" and "Aux switches unavailable, battery charging" messages as these batteries age. A proper dual battery setup is a separate system.
Factory aux switches: if your Gladiator has the factory auxiliary switch bank, those switches are handy for turning things on and off. For anything big, use the switch to trigger a relay and feed the load from the aux battery through its own fuse. Don't pull heavy current through the factory wiring.
Isolator vs DC-DC Charger
There are two ways to charge a second battery from the alternator.
Smart isolator / combiner (voltage-sensing relay)
A heavy relay that connects the two batteries when the engine is charging and separates them when the voltage drops, so your accessories can't drain the starting battery.
- Pros: simple, no extra electronics, can handle big current, and most have a manual "combine" mode to jump-start yourself off the aux battery.
- Cons: both batteries need to be the same type, and they only charge as well as the alternator voltage allows.
The Genesis Offroad Gen 3 kit works this way. Genesis says its combiner charges the cranking battery first, then links the batteries, and separates them when voltage drops below about 12.7V for a minute. It has a boost switch to combine them on demand. For a DIY build, the Blue Sea ML-ACR is a 500A automatic charging relay with a manual control and a remote switch included.
DC-DC charger
A charger that runs off the alternator and charges the aux battery with its own multi-stage profile.
- Pros: works with different battery types (an AGM starter and a lithium aux), charges the aux battery fully, and handles the long cable runs to the bed or cab.
- Cons: costs more, and charge current is limited by the unit's rating.
The Victron Orion-Tr Smart 12/12-30 isolated is a 30A, 360W charger with Bluetooth setup and lead-acid and lithium charging profiles. Victron's manual says to mount it vertically with the terminals down, keep 10 cm clear above and below, and never mount it directly above a battery.
Simple rule: two matching batteries under the hood, use an isolator. A lithium battery in the cab or bed, use a DC-DC charger.
Where to Mount It on a JT
Under the hood. The Genesis kit replaces the factory battery box with a steel tray that holds two matching Group 25 batteries. Genesis says the factory battery won't work, the kit fits gas 3.6L Gladiators with or without eTorque (there's a separate diesel kit that mounts under the seat), and most people install it in 2 to 3 hours. Genesis also notes that because the kit removes the small ESS battery, the stop-start system will throw a service warning after a few stop-start events. That resets each time you restart the truck. It costs $799 before batteries, so check the current price.
In the cab. The space under the rear seat keeps a battery cool, dry, and secure. Lithium does best here, since engine bays get hot and LiFePO4 batteries shouldn't be charged below freezing. The Renogy 100Ah Pro has self-heating, Bluetooth, and an IP67 case. Measure your under-seat space first.
In the bed. A bed box or slide works if you're running a fridge back there anyway. It's the longest cable run, so size the wire for it, and use a DC-DC charger to make up for the voltage drop. Strap the battery down hard. A loose battery turns into a missile in a crash or a hard trail hit.
Wiring Safety: Don't Skip This
Most trail electrical fires come from homemade wiring. Do it right:
- Fuse every positive cable at the battery end, as close to the battery as practical. A cable that shorts to the frame upstream of its fuse will burn. An ANL fuse block handles the main feed.
- Size the wire to the fuse and the run length. Victron's manual calls for 60A fuses on both sides of the 12/12-30, with 10 mm² cable (about 7 AWG) for runs of 1 to 2 meters and 16 mm² (about 5 AWG) for 5 to 10 meters. Use flexible, fine-stranded copper cable, which Victron specifies because coarse strands in the screw terminals can overheat.
- Check polarity before you connect. Victron says a reverse-polarity hookup can't be repaired and isn't covered by warranty.
- Use a fuse block for small loads. The Blue Sea 5025 gives you six fused circuits plus a negative bus, rated 30A per circuit and 100A total.
- Grommet every hole, loom every wire, and keep cables away from the exhaust and anything that moves.
- Monitor it. A Victron SmartShunt shows real state of charge, not just voltage.
- Carry an extinguisher. See our roll bar fire extinguisher guide.
Expect some electronics weirdness after disconnecting the batteries. Gladiator owners report the cameras and screens acting up for a few hours while modules relearn. If you have a 2021–2025 JT, also check whether your truck has the power steering pump recall done before you start adding loads.
What About the Winch?
Many builders wire the winch to the starting battery side or to both batteries combined, since a winch pulls hundreds of amps. The Genesis bus bars have studs sized for winch cables. Whatever you do, keep the engine running during a pull. See our trail recovery kit guide for the rest of the gear.
Quick Reference
- Matching batteries under the hood: Genesis kit or Blue Sea ML-ACR
- Lithium in the cab or bed: Victron Orion 12/12-30 + Renogy 100Ah Pro
- Fuse it: ANL block at the battery, 6-circuit block for accessories
- Watch it: SmartShunt
More overland gear lives on my Amazon storefront.