# Auxiliary Batteries > 12V AGM auxiliary/backup batteries for BMW, Mercedes, Audi, VW, Porsche. 5-15 Ah, genuine warranty, 24h delivery. Find yours by plate or model. **Category:** [Start](https://baterias.com/gb/beginning) → [Batteries](https://baterias.com/gb/batteries) → [Starter Batteries](https://baterias.com/gb/starter-batteries) - **Canonical URL**: https://baterias.com/gb/auxiliary-batteries - **Language**: en-GB ## Description **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. ## 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 - [Varta Silver Dynamic Aux](/gb/batteries-varta) - [Tudor Auxiliary](/gb/batteries-tudor) - [Exide AGM Auxiliary](/gb/batteries-exide) ### 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. Catalogue curated by our technical team. **10,544 Trustpilot reviews · 4.4/5**. 24h mainland delivery, free 14-day returns. ## Products in this category | Product | Brand | Price | |---|---|---| | [Car battery Varta AUX14 12V 13Ah 200A AGM](https://baterias.com/gb/battery-varta-aux14-12v-13ah-200a) | VARTA | 69,54 € | | [Car battery Exide EK143 12V 14Ah 80A AGM](https://baterias.com/gb/battery-exide-ek143-12v-14ah-80a) | EXIDE | 40,54 € | | [Car battery Exide EK151 12V 15Ah 200A AGM](https://baterias.com/gb/battery-exide-ek151-12v-15ah-200a) | EXIDE | 53,99 € | | [Car battery Varta AUX1 12V 35Ah 520A](https://baterias.com/gb/battery-varta-aux1-12v-35ah-520a) | VARTA | 146,80 € | | [Car battery Exide EK111 12V 11Ah 150A AGM](https://baterias.com/gb/battery-exide-ek111-12v-11ah-150a) | EXIDE | 32,94 € | | [Car battery Exide EK131 12V 12Ah 200A AGM](https://baterias.com/gb/battery-exide-ek131-12v-12ah-200a) | EXIDE | 61,54 € | | [Car battery Tudor TK143 12V 14Ah 80A AGM](https://baterias.com/gb/battery-tudor-tk143-12v-14ah-80a) | TUDOR | 52,48 € | | [Car battery Tudor TK111 12V 11Ah 150A AGM](https://baterias.com/gb/battery-tudor-tk111-11ah-150a) | TUDOR | 169,00 € | | [Car battery Varta AUX9 12V 9Ah 130A](https://baterias.com/gb/battery-varta-aux9-12v-9ah-130a) | VARTA | 58,42 € | | [Car battery Tudor TK151 12V 15Ah 200A AGM](https://baterias.com/gb/battery-tudor-tk151-15ah-200a) | TUDOR | 64,30 € | | [Car battery Tudor TK131 12V 13Ah 200A AGM](https://baterias.com/gb/battery-tudor-tk131-12v-13ah-200a) | TUDOR | 62,99 € | | [Car battery Exide EK091 12V 9Ah 120A AGM](https://baterias.com/gb/battery-exide-ek091-12v-9ah-120a) | EXIDE | 54,72 € | | [Car battery Tudor TK091 12V 9Ah 120A AGM](https://baterias.com/gb/battery-tudor-tk091-9ah-120a) | TUDOR | 61,07 € | ## Technical team (editorial authorship) The FAQs, guides and comparisons on this site are reviewed by the baterias.com technical team — over 25 years of experience validating automotive batteries. → https://baterias.com/gb/technical-team --- *baterias.com — Automotive batteries · 2026-07-08*