What is a 12V Battery (EV)?
Yes — EVs still have a normal 12V battery, and a flat one can stop the car "starting".
Simplified animation — not to scale.
In plain English
Even fully electric cars carry a small 12V lead-acid (or lithium) battery, just like a petrol car. It powers the computers, lights, infotainment and door locks, and — crucially — it's what wakes the car up so the high-voltage system can switch on. Without it the EV won't boot, even if the main traction battery is 100% charged.
"Think of it like the ignition key for the whole car. The huge traction battery is the fuel tank, but the little 12V battery is the hand that turns the key. Flat key, no driving — no matter how full the tank is."
How it works
When you press the start button, the 12V battery powers the car's computers and closes contactors (big switches) inside the high-voltage battery. Only then does the traction battery connect to the motor and the car becomes "ready to drive". A small DC-DC converter then tops the 12V battery up from the main battery while you drive — there's no alternator.
Signs of trouble
- ⚠Car completely dead — no dashboard, no doors unlocking from the key fob
- ⚠"12V battery low" or "Service vehicle soon" warning
- ⚠Car won't go into "Ready" mode even though the main battery is charged
- ⚠Infotainment resets or clock loses time
- ⚠Frunk/bonnet won't pop open (some EVs have an emergency 12V access point)
£90 – £260
Always get a written quote. Prices vary by car, region, and parts brand.