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.

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.

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VARTA

Car battery Varta AUX14 12V 13Ah 200A AGM

12V

13Ah

200A

AGM

150 × 87 × 146 mm

4,79(39)

€69.54 VAT incl.

TO BUY

EXIDE

Car battery Exide EK111 12V 11Ah 150A AGM

12V

11Ah

150A

AGM

150 × 90 × 130 mm

4,76(20)

€32.94 VAT incl.

TO BUY

TUDOR

Car battery Tudor TK151 12V 15Ah 200A AGM

12V

15Ah

200A

AGM

150 × 90 × 145 mm

4,76(33)

€64.30 VAT incl.

TO BUY

TUDOR

Car battery Tudor TK111 12V 11Ah 150A AGM

12V

11Ah

150A

AGM

150 × 90 × 130 mm

4,75(22)

€169.00 VAT incl.

TO BUY

EXIDE

Car battery Exide EK143 12V 14Ah 80A AGM

12V

14Ah

80A

AGM

150 × 100 × 100 mm

4,78(39)

€40.54 VAT incl.

TO BUY

TUDOR

Car battery Tudor TK091 12V 9Ah 120A AGM

12V

9Ah

120A

AGM

150 × 90 × 105 mm

4,80(34)

€61.07 VAT incl.

TO BUY

TUDOR

Car battery Tudor TK143 12V 14Ah 80A AGM

12V

14Ah

80A

AGM

150 × 100 × 100 mm

4,80(29)

€52.48 VAT incl.

TO BUY

EXIDE

Car battery Exide EK151 12V 15Ah 200A AGM

12V

15Ah

200A

AGM

150 × 90 × 145 mm

4,78(33)

€53.99 VAT incl.

TO BUY

EXIDE

Car battery Exide EK091 12V 9Ah 120A AGM

12V

9Ah

120A

AGM

150 × 90 × 105 mm

4,81(25)

€54.72 VAT incl.

TO BUY

VARTA

Car battery Varta AUX9 12V 9Ah 130A

12V

9Ah

130A

151 × 87 × 106 mm

4,82(27)

€58.42 VAT incl.

TO BUY

Fulmen

Car battery Fulmen FK151 12V 15Ah 200A AGM

12V

15Ah

200A

AGM

150 × 90 × 145 mm

TO BUY

Fulmen

Car battery Fulmen FK143 12V 14Ah 80A AGM

12V

14Ah

80A

AGM

150 × 100 × 100 mm

TO BUY

Fulmen

Car battery Fulmen FK131 12V 13Ah 200A AGM

12V

13Ah

200A

AGM

150 × 90 × 145 mm

TO BUY

Fulmen

Car battery Fulmen FK111 12V 11Ah 150A AGM

12V

11Ah

150A

AGM

150 × 90 × 130 mm

TO BUY

Fulmen

Car battery Fulmen FK091 12V 9Ah 120A AGM

12V

9Ah

120A

AGM

150 × 90 × 105 mm

TO BUY

Deta

Car battery Deta DK151 12V 15Ah 200A AGM

12V

15Ah

200A

AGM

150 × 90 × 145 mm

TO BUY

Deta

Car battery Deta DK143 12V 14Ah 80A AGM

12V

14Ah

80A

AGM

150 × 100 × 100 mm

TO BUY

Deta

Car battery Deta DK131 12V 13Ah 200A AGM

12V

13Ah

200A

AGM

150 × 90 × 145 mm

TO BUY

Deta

Car battery Deta DK111 12V 11Ah 150A AGM

12V

11Ah

150A

AGM

150 × 90 × 130 mm

TO BUY

Deta

Car battery Deta DK091 12V 9Ah 120A AGM

12V

9Ah

120A

AGM

150 × 90 × 105 mm

TO BUY

TUDOR

Car battery Tudor TK131 12V 13Ah 200A AGM

12V

13Ah

200A

AGM

150 × 90 × 145 mm

4,83(25)

€62.99 VAT incl.

TO BUY

KRONOBAT

Car battery Kronobat AUX9 12V 9Ah 130A AGM

12V

9Ah

130A

AGM

151 × 87 × 106 mm

TO BUY

KRONOBAT

Car battery Kronobat AUX14 12V 13Ah 200A AGM

12V

13Ah

200A

AGM

150 × 87 × 146 mm

TO BUY

KRONOBAT

Car battery Kronobat AUX1 12V 35Ah 520A

12V

35Ah

520A

212 × 175 × 140 mm

TO BUY

Showing 1-24 of 26 item(s)

How to identify and choose your 12V auxiliary battery

The auxiliary battery usually sits in the boot, under the driver's seat or in the engine bay, always in its own compartment separate from the main battery. Dashboard messages like "high electrical system demand" or "backup battery fault" almost always point to a failing auxiliary. Common capacities are 5, 9, 12 and 14 Ah, nearly all AGM for their compact size and tolerance of short, frequent cycles without sulphating.

What the auxiliary or backup battery is for

Auxiliary car backup battery — keeps the alarm, onboard computer and central locking running with the engine off.
Second 12V battery for advanced Start-Stop — stops the cranking surge from disturbing sensitive electronics.
Electrical support in mild hybrids (MHEV) — takes over loads while the main engine is in electric mode.
Comfort and safety power — heated seats, infotainment and driver-assist systems without the engine running.

Leading auxiliary battery brands

Compatibility and how to choose the right auxiliary

Not every 12V auxiliary battery is interchangeable between brands: connector, dimensions and the car's energy-management software are calibrated for a specific reference. Enter your make, model, engine and year in our plate-based battery finder or use the comparison tool to check exact capacity (Ah), dimensions and polarity before buying.

Frequently asked questions about auxiliary batteries

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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