An electric car can show 80% charge one evening and be completely lifeless the next morning. No dashboard, no central locking, the charge flap will not open. Almost every time, the traction battery is fine and the culprit is the small 12V battery hiding under the bonnet or in the boot.
People assume an EV runs entirely off its big battery. It does not. The 12V runs the computers, the door locks, the dashboard and, crucially, the contactors: the heavy relays that connect the traction battery to the rest of the car.
If the 12V is flat, those contactors never close. The traction battery stays disconnected and the car cannot use a single kilowatt of the charge it is holding. It is the ignition key of the whole system, and it is a small lead-acid or AGM battery no different in principle to the one in a petrol car.
The car is completely dead. No dash lights, no chime, nothing when you press the start button.
The doors will not unlock from the fob, though the mechanical key still works.
The charge flap will not release, so you cannot even plug it in to try.
A rapid clicking from under the bonnet when you press start. That is the contactors trying and failing.
A screen full of unrelated warnings the day before. Low 12V voltage throws error messages across systems that have nothing wrong with them.
EV 12V batteries work harder than people expect and are often smaller than the equivalent in a petrol car. Three things account for most failures:
Short journeys. The 12V is topped up by the DC-DC converter while the car is on. Ten minutes to the shops and back, repeatedly, never fully replaces what starting used.
Standing unused. An EV left for two or three weeks is still awake in the background, listening for the app and running telematics. That slowly drains the 12V even at full traction charge.
Age. Three to five years is typical. Many are the original from the factory, and unlike a petrol car there is no sluggish crank to warn you first. It works, then one morning it does not.
1. Find the 12V or its jump points. Some EVs put the battery in the boot or under the rear seat and provide a positive jump post under the bonnet with a red cover. Check the handbook: guessing around an EV is not worth it.
2. Use a jump pack, not another car. A lithium jump pack is the safer option. Donor-car jumping can push voltage spikes into an EV's electronics, and some manufacturers explicitly forbid it.
3. Connect positive first, then the earth point the handbook specifies. Not the negative terminal itself if an earth stud is provided.
4. Wake the car, then leave it on. Once it powers up, leave it in Ready mode for 30 to 45 minutes. The DC-DC converter will recharge the 12V from the traction battery, which is what actually fixes it.
5. Never jump from the traction battery, and never open anything orange. There is nothing user-serviceable there and the voltages are lethal.
A 12V jump gets you a dashboard. If the car powers up but refuses to move, shows a red warning triangle, or reports a high voltage system fault, that is a different problem and not one to poke at. The same goes for anything that trips the residual current device when you plug in to charge.
If the traction battery itself is the worry, that is measurable rather than guessable. A certified Aviloo test reads the pack at cell level and tells you its actual state of health, rather than relying on the car's own estimate, which can be some way out.
A 12V test takes minutes and tells you whether the battery is genuinely failing or simply flat. If it is dying, a replacement is a straightforward mobile job. If it keeps going flat with a healthy battery fitted, the answer is a parasitic drain test: measuring what is still awake and drawing current after the car has gone to sleep. That is the test that actually ends the problem rather than postponing it.
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