Car Batteries Explained: 12-Volt, Hybrid, EV and Auxiliary

Every car has a 12-volt battery that runs the lights, locks and electronics. Hybrids and electric cars add a much larger high-voltage battery that drives the wheels, and some cars carry a second, smaller auxiliary battery as well. Each one does a different job and needs looking after differently.

What does the 12-volt battery do?

The 12-volt battery is the one most drivers know about. In a petrol or diesel car it:

• powers the starter motor to turn the engine over

• runs lights, wipers, the radio and screens when the engine is off

• keeps the alarm, central locking and computer memory alive while the car is parked

Once the engine is running, the alternator recharges it. Most are lead-acid or AGM (absorbent glass mat) types. Cars with stop-start usually need an AGM or EFB battery that copes with frequent restarts, so always replace like for like.

How long does a 12-volt battery last?

Typically around three to five years, though short journeys, cold weather and cars left standing can shorten that. Signs it is getting tired include slow cranking, a clicking noise when you try to start, dim lights and stop-start that no longer works.

What batteries does a hybrid have?

A hybrid has two:

1. A high-voltage traction battery, usually nickel-metal hydride or lithium-ion. It stores energy recovered when you slow down and uses it to help or replace the petrol engine at low speeds.

2. A 12-volt battery, which runs the normal electrics and wakes the hybrid system up when you press the start button. It is charged by the hybrid system rather than a conventional alternator.

A flat 12-volt battery can stop a hybrid from starting even when the main battery is full, which surprises many owners.

What batteries does an electric car have?

An electric car has a large lithium-ion battery pack that powers the motor and is charged from the mains. It also has a 12-volt battery, or in some newer models a small lithium unit, that runs the locks, screens, lights and safety systems and switches the high-voltage system on.

As with a hybrid, if the 12-volt battery goes flat the car may not wake up, even with plenty of range showing. Our EV hub has more on owning an electric car.

Why do some cars have an auxiliary battery?

Some larger and more heavily equipped cars, including a number of Jaguar, Land Rover and other premium models, carry a second small battery alongside the main 12-volt one. Its job varies by model, but it commonly supports stop-start, keeps key systems powered while the engine restarts, or provides back-up for safety-critical electronics.

An auxiliary battery fault often shows up as a warning message about stop-start or a battery fault, rather than a failure to start. It is worth having it checked properly, as the main and auxiliary batteries are tested and replaced separately.

How do I look after my car's batteries?

• Drive the car regularly, including a longer run now and then, so the 12-volt battery can recharge fully.

• If a car will be parked for weeks, use a suitable battery maintainer.

• Keep terminals clean and tight.

• Do not ignore battery or charging warning lights.

• For hybrids and EVs, keep up with scheduled servicing, which includes checks on the high-voltage system.

Common questions

Can I jump-start a hybrid or electric car?

Many can have the 12-volt battery jump-started using a dedicated jump point under the bonnet, but the method varies. Always follow the owner's handbook, and never try to touch the high-voltage system.

Does the 12-volt battery in an EV need replacing?

Yes, eventually. It wears out in a similar way to a battery in a petrol car, although it is often used more gently.

Is a battery covered by warranty?

High-voltage batteries in hybrids and EVs often have a separate, longer manufacturer warranty than the rest of the car. Check the terms for your model, as cover usually depends on age, mileage and the battery keeping a minimum capacity.

Battery trouble?

If your car is slow to start or showing a battery warning, our service centre in Barnsley can test the battery and charging system and tell you what it needs. Call 01226 574321 to book.

Updated October 2026.