What's the difference between the main and the auxiliary battery?

The main battery starts the engine and handles general loads; the auxiliary is a second, smaller 12V battery backing up critical systems when the engine is off or in electric mode.

Can I fit any 12V auxiliary battery in my car?

No. It must match the original in capacity (Ah), dimensions and technology (usually AGM), since the vehicle's energy-management system is calibrated for that exact reference.

How long does an auxiliary battery last?

Working in short, frequent cycles, it typically lasts 4-6 years, similar to a well-maintained AGM starter battery.

What happens if the auxiliary battery fails?

The car may show electrical fault warnings, disable advanced Start-Stop or lose comfort features with the engine off, even if the main battery is healthy.

How do I find the exact capacity of my original auxiliary battery?

Check the label on the fitted battery, or enter your vehicle in our finder — we'll show the compatible AGM reference by Ah, dimensions and polarity.

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Auxiliary Batteries

Auxiliary batteries (12V, or 5V lithium-ion on some hybrids) power the secondary electrical systems of modern cars such as BMW 5 Series/7 Series, Mercedes E/S-Class/AMG, Audi A6/A7/A8, VW Touareg and Porsche Panamera/Cayenne. They are compact (5-15 Ah) and almost always AGM, the sealed technology that copes best with tight spaces and any mounting position.

This backup or auxiliary car battery keeps critical systems running — onboard computer, alarm, central locking — when the engine is off and the main battery is disconnected, typical of advanced Start-Stop or mild-hybrid electric mode. Without it, the car may throw electrical fault warnings or lose comfort features whenever the engine stops.

Subcategories

FULBAT Auxiliary AGM

There are 38 products.

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VARTA

Car battery Varta AUX14 12V 13Ah 200A

12V

13Ah

200A

AGM

150 × 87 × 146 mm

5,00(1)

€70.20 VAT incl.

Auxiliary Batteries

Subcategories

FULBAT Auxiliary AGM

How to identify and choose your 12V auxiliary battery

What the auxiliary or backup battery is for

Leading auxiliary battery brands

Compatibility and how to choose the right auxiliary

Frequently asked questions about auxiliary batteries

Best sellers in Auxiliary Batteries