The most common reason a car won't start is a flat 12V battery. Before you call breakdown cover, it's worth trying a jump start — it costs nothing and takes about 10 minutes.
EVs and PHEVs: Jump starting a full electric vehicle is different — do not connect jump leads to the traction battery. The 12V battery in an EV can be jump started normally, but check your handbook first. PHEVs vary by model.
Signs it's a flat battery (not something else)
- The car clicks rapidly when you turn the key but doesn't turn over
- The engine turns over very slowly and dies
- Nothing happens at all — no lights, no dash, completely dead
- The interior lights work but dimly
If the engine cranks normally but doesn't fire, the battery is probably fine — you may have a fuel, ignition, or sensor issue instead.
How to jump start correctly
Order matters. Get it wrong and you risk damaging electronics or causing a spark near the battery.
- Position the donor vehicle close enough for leads to reach — engines don't need to be touching.
- Red lead: dead battery positive (+) first, then donor battery positive (+).
- Black lead: donor battery negative (–), then an unpainted metal part of the dead car's engine block — not the negative battery terminal itself. This reduces spark risk near the battery.
- Start the donor vehicle and let it run for 2–3 minutes.
- Try starting the dead vehicle. If it doesn't start first time, wait another 2 minutes and try again.
- Once running, disconnect in reverse order: black from engine block, black from donor, red from donor, red from your battery.
- Drive for at least 20–30 minutes — ideally motorway speed — to let the alternator recharge the battery properly.
Battery packs: A portable jump starter pack (around £30–£60) is worth keeping in the boot. They work the same way but without needing a second vehicle.
When a jump start won't fix it
If the battery won't hold a charge and goes flat again within a day or two, one of three things is happening:
- The battery is old and needs replacing (most batteries last 4–6 years)
- The alternator isn't charging the battery while you drive
- Something is drawing power when the car is parked — a parasitic drain
PAD 12V battery test: A professional load test tells you whether your battery still has capacity or needs replacing — not just whether it has enough charge to start. £25, mobile across Pontypridd and RCT.
12V battery check →
Battery not holding charge?
Find out if it needs replacing.
A professional load test tells you the battery's actual cold cranking amps and state of health — not just whether it has enough charge to start. £25, mobile across Pontypridd and RCT.