Auxiliary Batteries

At a glance

• What it does: keeps the on-board electrical system stable when the engine stops and restarts, and backs up the main battery.
• Brands: Varta (AUX9, AUX14 and AUX1) and Tudor (TK091, TK111, TK143 and TK151).
• Sizes: compact 9 to 15 Ah formats and one 35 Ah model.
• Shipping: 24 hours to Spain and Portugal, with a 14-hour express service. We also ship to France, Italy, Germany, Belgium and the Netherlands.
• Free returns and help choosing by email.

What an auxiliary battery does

Every time Start-Stop restarts the engine, the starter motor draws a lot of current from the main battery. The auxiliary battery holds the voltage up so the radio, navigation and driver-assistance systems do not cut out or reboot. In 48 V mild hybrids and hybrids, a 12 V battery still powers the conventional on-board network. It does not replace the main battery: it is a second battery with its own compartment and terminals.

How to choose an auxiliary battery

1. Dimensions: the space is usually tight. Compare length × width × height with the table.
2. Terminals: some models have threaded terminals and others motorcycle-style ones; check the type and which side the positive is on.
3. Capacity (Ah) and cranking current (A): the same as the original or slightly higher.
4. Technology: if the original is AGM, fit another AGM. The Tudor TK091, TK111 and TK151 are AGM.

All auxiliary batteries

Our best seller is the Varta AUX14, 13 Ah and 200 A. The main battery for a Start-Stop car is under Start-Stop batteries, and the rest under car batteries.

Frequently asked questions

How do I know if my car has an auxiliary battery?

The owner's manual will say. Depending on the model, it can sit in the engine bay, under the floor or in the boot. If the label says AUX or the car shows an "auxiliary battery" warning, it has one.

What happens when it fails?

Typically a warning appears on the instrument cluster and Start-Stop stops working. On some models the warning has to be cleared with a diagnostic tool after replacement.

When will my order arrive?

Orders placed before the cut-off time leave the same working day. Delivery to Spain and Portugal takes 24 hours, and there is a 14-hour express service. You will see the exact delivery time when choosing shipping, before you pay.

What if I don't know which one I need?

Write to us via our contact page with your car's make, model and year, or a photo of the label, and we will tell you. Returns are free.