An auxiliary car battery supports your vehicle’s electrical needs alongside the main starter battery. It’s often used in vehicles with start‑stop systems and helps keep electronics steady and responsive.

• Supports start‑stop systems by powering accessories when the engine pauses.
• Stabilizes voltage for infotainment, safety, and control modules.
• Backs up electronics so settings and features remain reliable under load.

Many vehicles use compact 12V AGM auxiliary batteries in sizes like AUX12 or AUX14 (also listed as BCI Group 401 or 400). Exact requirements vary by model, so it’s a good idea to confirm your specifications in the owner’s manual or with a qualified technician.

A weak or failed auxiliary battery can lead to noticeable quirks in your vehicle’s electronics. Depending on the model, you might see:

• Dashboard alerts related to the battery or charging system.
• Start‑stop temporarily disabled to protect the electrical system.
• Clock or radio presets resetting in some cases.
• Intermittent power to accessories like interior lighting or infotainment.

The engine may still start using the main battery, but electrical behavior can be inconsistent. Testing the system and replacing the auxiliary unit when needed can help restore normal function. Vehicle designs differ, so checking your owner’s manual or consulting a trusted service professional is a practical next step.

If the auxiliary battery is failing, you can take a few precautionary steps while arranging service:

• Reduce electrical loads (heated seats, high fan speeds, rear defrost) to ease the demand.
• Turn off auto start‑stop if your vehicle allows it.
• Avoid frequent short trips that stress the system.
• Schedule a diagnostic and replacement soon to prevent additional warnings.

Some vehicles continue operating on the main battery, but behavior varies. If warnings escalate or multiple systems act erratically, it’s prudent to stop in a safe place and seek assistance. Always follow the guidance in your owner’s manual for your specific vehicle.

Finding the right auxiliary battery is easiest when you match your vehicle’s specs:

• Check the owner’s manual for the required group size and chemistry.
• Read the existing battery label to match BCI group (e.g., AUX12/Group 401 or AUX14/Group 400), 12V, and terminal layout.
• Confirm chemistry: Many start‑stop vehicles specify AGM for durability.
• Verify dimensions and orientation so it fits the tray and cables.
• Use site filters and compatibility tools to narrow options.

If anything’s unclear, a qualified technician can confirm fitment and advise on any programming or resets that may be needed after installation.

Replacement costs vary because several factors influence the total:

• Battery type and size: Compact AGM auxiliary units can be priced differently than standard designs.
• Access and location: Batteries under seats, in trunks, or behind panels may take longer to reach.
• Service requirements: Some vehicles need electronic resets or registration after replacement.
• Local labor rates: Costs differ by shop and region.

You can order the battery online for pickup or delivery and have it installed by a trusted professional. For a location‑specific estimate, contact your local auto service provider or a Walmart Auto Care Center; availability and services may vary by store.