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.
Buy the battery to match a real amp-hour budget, not a round number that sounds generous. Add up what runs overnight with the engine off. A 12V compressor fridge averages 3-5A once it's cold-soaked, so 30-50Ah over a 10-hour camp night. LED interior and awning lights pull 1-2A. A vent fan, a heater blower, phone and camera charging - each adds a few amp-hours. A typical JT overland night lands around 40-70Ah of draw.
Then account for usable depth, because rated capacity is not usable capacity. A flooded or AGM battery should not drop below roughly 50% state of charge if you want it to last - so a 100Ah AGM gives you about 50Ah before you're into damaging territory. LiFePO4 (lithium) tolerates 80-100% depth of discharge, so a 100Ah lithium delivers 80-100 usable Ah at roughly one-third the weight. That difference is why serious overlanders pay the lithium premium: a 100Ah lithium replaces a 200Ah AGM bank for the same usable energy and a fraction of the mass on the payload budget.
Match the charge source to the same math. Drive two hours between camps and a 25A DC-DC charger replaces 40-50Ah - enough to break even on a moderate night. Camp stationary for days and you need solar input or a larger bank, because no amount of engine-off wiring recharges a battery. Sizing the alternator and charging system to the load is the other half of this equation, and lighting loads are the draw most owners underestimate.
The Gladiator's bed is the reason a dual-battery build is more flexible here than on a Wrangler, but the location you pick drives the wiring, not the other way around. A bed-mounted tray behind the cab keeps the aux battery accessible, cool, and out of the cramped engine bay - the tradeoff is a longer positive run from the starter battery, which means heavier cable to hold voltage drop in check and a sealed, vibration-resistant box because the bed sees weather and dust. Route the cable through an existing grommet, never over a sharp bed-panel edge, and fuse it within 18 inches of each battery terminal.
Under-hood mounting in the fender area shortens the cable run and keeps everything in the charging zone, but the JT's 3.6L engine bay is tight and hot - heat shortens AGM life and pushes lithium outside its safe charge window on desert days. Whichever you choose, a bed-mounted battery adds 40-60 lbs high and rearward, which nibbles at the same payload and tongue-weight budget your camp gear and bed storage already compete for. Weigh the build, not only the parts list.
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.
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