The JL electrical part on this page is the stop-start auxiliary battery, Mopar BBAUX101AB. It supersedes BBAUX101AA and several BBAUA-series numbers. Most JLs with engine stop-start carry this small battery next to the main one. When it weakens, stop-start drops out and the truck posts charging or stop-start-unavailable messages. It is not a second battery for a fridge or a winch. That job is a separate system.
An improper aux bypass has been tied to U0121 in owner reports. Disconnecting the battery, or pulling fuse F42, to silence a no-crank can move the problem into the network instead of out of it. On some early 2018 trucks the starter-relay logic depended on the aux battery, which is why a healthy main battery still produces silence at the starter. The dealer fix for that logic is a software bulletin, not a deleted battery.
eTorque Wranglers do not use this small 12-volt battery. They use a 48-volt pack. If the badge or the pack says eTorque, this part number is the wrong chase. The part is cataloged for JL Wranglers with stop-start. A Gladiator has the same kind of ESS battery, a small one for restart support, but confirm the JT number by VIN instead of assuming this JL listing.
The lookup lists the battery, not the Z-case fuse array, the power distribution center, or the starter relay. Those show up in the no-crank guides linked below. Interstate MTX-AUX14-ES is the aftermarket AUX14 battery listed for the 2018 JL. Later letter suffixes and older Mopar numbers are on the part page.
What this category covers
- Auxiliary stop-start battery BBAUX101AB and the numbers it supersedes
- What a weak aux battery does to stop-start, and on early trucks, to cranking
- Why an eTorque 48-volt pack, and a Gladiator accessory battery, are different parts
Parts in this category
Each row links to the child part page for that SKU: the current Mopar number, what it supersedes, aftermarket cross-references, failure symptoms, and whether to rebuild or replace. Every number on those pages cites a source.
| Part | Mopar # | Fits |
|---|---|---|
| Auxiliary Battery (Stop-Start) | BBAUX101AB | JL Wranglers with engine stop-start, per Mopar dealer catalogs. BBAUX101AB supersedes BBAUX101AA and several... |
When to replace
Load-test the aux battery before you replace it. A weak main battery can disable stop-start on its own. Replace BBAUX101AB when the aux battery fails that test, or the truck repeats ESS and charging messages with a known-good main battery. Owners who replace only the main battery often see the same no-crank come back, because the two batteries drag each other down. Do not bypass the aux battery as a permanent fix. The U0121 guide covers a 2019 case that traced to fuse F42 and a disconnected aux negative. A correct replacement is the lower-risk path. Out of warranty, the software bulletin for early cranking logic is still a dealer visit, not a driveway delete.
Safety
This is high-current stop-start hardware sitting next to the power distribution center. A wrench across a main or aux terminal, or across the Z-case fuse array on top of the panel, can weld the tool and take the truck offline. Disconnect the negative and confirm the circuit is dead before you pull a cable. Stop if you are not sure which battery is which, or whether the truck is eTorque. ESS logic, airbag circuits, and the high-current side of the panel are dealer or qualified-tech work. A TSB for the cranking logic does not make the fuse panel safer to open.
Related guides
- JL No-Crank: Z-Case Fuse, Aux Battery, and Starter Circuit
- No Power to the JL Starter Relay
- Gladiator Dual Battery Setup: The ESS Battery Is Not for Accessories
- JL Code U0121
Other systems
- JK Brakes & ABS
- JK Cooling
- JK Engine
- JK Steering & Suspension
- JK Heating & A/C
- JK Electrical
- JK Body & Doors
- JK Fluids & Filters
- JL Brakes & ABS
- JL Cooling
- JL Engine
- JL Steering & Suspension
- JL Fluids & Filters
Part numbers change as Mopar supersedes them. Match the number on your old part, or run the VIN at a Mopar counter, before you order. Back to the parts lookup.