The auxiliary battery on JL Wrangler and JT Gladiator models is small, hidden, and easy to blame for all kinds of Stop/Start problems. Replacing it is very doable for a careful home mechanic, but the access is awkward. The two common approaches are through the passenger-side fender well, which is usually easier, or from the top by lifting the fuse box area after removing the main battery.
Before You Start
This guide is for owner education and general garage planning. Verify the procedure for your exact Jeep by year, trim, engine, and battery layout. Work on level ground, set the parking brake, wear eye protection and gloves, and disconnect the main battery negative cable before working around the battery box, fuse box, or power distribution center.
Picture Guide
Which Jeeps This Applies To
The pasted guide applies to 2018 and newer Jeep Wrangler JL/JLU models and 2020 and newer Jeep Gladiator JT models equipped with the 3.6L engine and the factory auxiliary battery used with the Stop/Start system. The battery is typically an AUX14 AGM, sometimes listed as a 400 CCA auxiliary battery, mounted under the passenger-side power distribution area.
Auxiliary Battery Specs
- Battery group: 400 / AUX14
- Type: AGM
- Voltage: 12V
- Location: Under the power distribution center / fuse box on the passenger side
Tools Needed
- 10mm socket, 10mm wrench, ratchet, and extensions
- Trim removal tool or flat screwdriver
- Gloves and safety glasses
- Bungee cord or strap if using the top-access method
Method 1: Through the Fender Well
This is the recommended path for most owners because you do not have to remove the main battery or lift the fuse box. You work from the passenger-side front fender well, remove the liner fasteners, lower the auxiliary battery box enough to access the battery, and swap it in place.
Fender Well Method Steps
Prepare the vehicle
Park on level ground, turn the ignition off, open the hood, and let the Jeep sit long enough for modules to power down. Turn the steering wheel fully to the left so you have more room in the passenger-side front wheel well.
Disconnect the main battery
Remove the negative cable from the main battery and isolate the cable so it cannot spring back onto the terminal. This keeps the 12V system from being live while you work around the auxiliary battery wiring.
Open the fender liner
Remove the plastic push pins and the 10mm bolt or bolts holding the front passenger-side fender liner. Pull the liner back carefully and fold it rearward so the auxiliary battery box is visible behind the wheel well.
Find the battery box
Look upward behind the wheel well. The auxiliary battery box is mounted to the body and commonly held by three 10mm bolts. Take a moment to study the cable routing before loosening anything.
Lower the battery box
Remove the three 10mm battery-box bolts. The box will drop down several inches, but you usually do not need to remove it completely. Support it with one hand and avoid pulling hard on the cables.
Disconnect the auxiliary battery
Disconnect the auxiliary battery negative cable first, then disconnect the positive cable. Keep metal tools away from the positive terminal and any grounded body metal.
Replace the battery
Lift the old AUX14 battery out of the box. Install the new AUX14 AGM battery in the same orientation. Reconnect the positive cable first, then the negative cable, and make sure both terminals are snug.
Reassemble the fender area
Raise the battery box back into position and reinstall the three 10mm bolts. Reinstall the fender liner, clips, and bolts, then reconnect the main battery negative cable under the hood.
Method 2: Underneath the Fuse Box
The top-access method reaches the same battery from above. It can be useful if fender-well access is not practical, but it involves removing the main battery and lifting the fuse box / power distribution center enough to see the auxiliary battery underneath. The big rule here is simple: do not strain the wiring harnesses.
Top Access Method Steps
Disconnect the main battery negative cable
Remove the negative terminal from the main battery and isolate it. This is still the first move because the work area is beside the power distribution center.
Remove the main battery
Remove the battery hold-down bracket using a 10mm socket. Disconnect the positive cable, then the negative cable if it is still attached, and lift the main battery out of the tray.
Remove battery-tray parts
Remove the plastic trim piece and support brackets around the battery tray. This exposes the power distribution center mounting bolts and gives you room to lift the fuse box assembly.
Lift and support the fuse box
Remove the fuse-box mounting bolts, then carefully lift the fuse box upward. Support it with a bungee cord or strap. Do not stretch, twist, or unplug wiring harnesses unless your factory procedure specifically calls for it.
Access the auxiliary battery
With the fuse box lifted, the auxiliary battery will be visible underneath. Confirm terminal orientation and cable routing before removing the old battery.
Disconnect the auxiliary battery
Disconnect the negative auxiliary battery cable first, then the positive cable. Keep the cable ends separated and keep tools away from nearby grounded metal.
Install the new battery
Remove the old battery and install the new AUX14 AGM battery in the same orientation. Reconnect the positive cable first, then the negative cable, and verify the hold-down and terminals are secure.
Put the top side back together
Lower the fuse box back into position and reinstall the mounting bolts. Reinstall the battery-tray trim and brackets, reinstall the main battery, connect the positive cable first, then reconnect the negative cable.
After Installation
Once everything is reassembled, start the Jeep and watch the dash for battery, charging, and Stop/Start messages. A new auxiliary battery may not make Stop/Start work instantly if the main battery is weak, the battery sensor has not updated, the Jeep has not completed its readiness checks, or another condition is preventing autostop.
- Check that both batteries are secured and both terminal clamps are tight.
- Confirm the fender liner is fully clipped and cannot rub the tire.
- Look for warning lights or messages before driving.
- Have the main battery load tested if the auxiliary battery failed after several years.
- Consider a smart maintainer if the Jeep sits or mostly takes short trips.
Best Practical Advice
If the auxiliary battery is old enough to fail, test the main battery too. A weak main battery can shorten the life of the new auxiliary battery, and a weak auxiliary battery can make the whole system act confused. Treat the two batteries as a team until testing proves otherwise.