# Auxiliary batteries (back-up and in-vehicle service) > Over 40 auxiliary and back-up AGM batteries from 8 to 50 Ah: Varta AUX, Exide, Tudor, Optima and more. Search by VIN or model, with VAT included. **Category:** [Beginning](https://bateriasonline.es/en/) → [Battery types: starter, deep cycle, traction and stationary](https://bateriasonline.es/en/batteries) → [Starter batteries](https://bateriasonline.es/en/starter-batteries) - **Canonical URL**: https://bateriasonline.es/en/auxiliary-backup-batteries - **Language**: en ## Description An auxiliary battery keeps the electronics of many modern cars running; buy yours by reference, such as Varta AUX14 or AUX9. Many modern cars carry a second, smaller battery besides the starter battery. It keeps electronic systems alive during engine restarts, supports the electric gear selector or brake, or powers equipment while the main battery is protected. Match the reference, dimensions and technology of the original unit, since the vehicle is designed around them. ## What an auxiliary battery does An auxiliary or back-up battery is not meant to start the engine. Its job is to stabilise voltage or provide independent power to specific circuits. Typical uses include: - Supporting the onboard network during engine restarts in cars with [start-stop systems](https://bateriasonline.es/en/start-stop-batteries-agm-efb), so the radio and displays do not reset. - Ensuring that electronic parking brakes, gear selectors or emergency call systems keep working if the main supply drops. - Running accessories and campers' equipment in some vans, separated from the starting circuit. ## How to choose a replacement Auxiliary batteries are usually small sealed AGM units between roughly 8 Ah and 20 Ah. Check the **exact dimensions**, the **terminal type and position** and the **capacity** printed on the original. Vehicle manufacturers often place them under seats, in the boot or under the dashboard, where space is tight and ventilation limited; that is why sealed AGM is the norm. Brands such as [OPTIMA](https://bateriasonline.es/en/optima-batteries) and other AGM specialists supply compact sealed units for secondary circuits. The main starter batteries are under [starter batteries](https://bateriasonline.es/en/starter-batteries). ## Symptoms of a failing back-up battery Warning messages about the auxiliary or back-up battery, start-stop being disabled, the infotainment rebooting when the engine restarts or unexpected faults from electronic brakes. Often the main battery is fine and only the auxiliary needs changing. A diagnostic scan usually shows which battery the fault code refers to, which saves replacing the wrong one. ## Frequently asked questions ### Do I need to replace both batteries at the same time? Not necessarily. Each ages at its own pace. Test both and replace only the one that fails, unless the manufacturer recommends otherwise. ### Can I fit a larger auxiliary battery? Stick to the original format. The compartment and the charging circuit are designed for that size, and a different capacity can confuse the vehicle's monitoring. If the original model is no longer made, choose an equivalent with the same size, terminals and technology. ## Choose by brand The same auxiliary batteries (back-up and in-vehicle service), filtered by manufacturer. Each brand has its own page with all its references and a buying guide. - [Exide batteries](https://bateriasonline.es/en/exide-batteries) (8) - [Tudor batteries](https://bateriasonline.es/en/tudor-batteries) (8) - [Fulmen batteries](https://bateriasonline.es/en/fulmen-batteries) (5) - [Deta batteries](https://bateriasonline.es/en/deta-batteries) (5) - [Varta batteries](https://bateriasonline.es/en/varta-batteries) (4) - [Fulbat batteries](https://bateriasonline.es/en/fulbat-batteries) (4) - [Optima batteries](https://bateriasonline.es/en/optima-batteries) (4) - [Kronobat batteries](https://bateriasonline.es/en/kronobat-batteries) (3) - [Fullriver batteries](https://bateriasonline.es/en/fullriver-batteries) (1) ## Products in this category | Product | Brand | Price | |---|---|---| | [Exide EK091 · Auxiliary battery 12V 9Ah 120A](https://bateriasonline.es/en/exide-ek091) | Exide | 54,10 € | | [Exide EK111 · Auxiliary battery 12V 11Ah 150A](https://bateriasonline.es/en/exide-ek111) | Exide | 32,74 € | | [Exide EK131 · Auxiliary battery 12V 13Ah 200A](https://bateriasonline.es/en/exide-ek131) | Exide | 60,76 € | | [Exide EK143 · Auxiliary battery 12V 14Ah 80A](https://bateriasonline.es/en/exide-ek143) | Exide | 40,18 € | | [Exide EK151 · Auxiliary battery 12V 15Ah 200A](https://bateriasonline.es/en/exide-ek151) | Exide | 53,32 € | | [Exide EK454 · Auxiliary battery 12V 45Ah 380A](https://bateriasonline.es/en/exide-ek454) | Exide | 207,87 € | | [Exide EK457 · Auxiliary battery 12V 45Ah 380A](https://bateriasonline.es/en/exide-ek457) | Exide | 208,15 € | | [Exide EK500 · Auxiliary battery 12V 50Ah 570A](https://bateriasonline.es/en/exide-ek500) | Exide | 106,43 € | | [Varta AUX1 · Auxiliary battery 12V 35Ah 520A](https://bateriasonline.es/en/varta-aux1) | Varta | 144,55 € | | [Varta AUX14 · Auxiliary battery 12V 13Ah 200A](https://bateriasonline.es/en/varta-aux14) | Varta | 68,60 € | | [Varta AUX9 · Auxiliary battery 12V 9Ah 130A](https://bateriasonline.es/en/varta-aux9) | Varta | 57,73 € | | [Varta A10 · Auxiliary battery 12V 40Ah 420A](https://bateriasonline.es/en/varta-a10) | Varta | 103,21 € | | [Fullriver FT438-U1R · Auxiliary battery 12V 35Ah 438A](https://bateriasonline.es/en/fullriver-ft438-u1r) | Fullriver | 170,19 € | | [Tudor TK091 · Auxiliary battery 12V 9Ah 120A](https://bateriasonline.es/en/tudor-tk091) | Tudor | 60,27 € | | [Tudor TK111 · Auxiliary battery 12V 11Ah 150A](https://bateriasonline.es/en/tudor-tk111) | Tudor | 166,11 € | --- *bateriasonline.es — Batteries and accessories · 2026-11-10*