The 12V battery runs every computer in your EV. It's small, often poor-quality from the factory, and dies after 2–4 years — locking you out of the car even when the main HV pack is full. £80–£150 to replace.
Strange but true: the most common EV breakdown is a flat 12V battery — the same kind your old petrol car had.
Why EVs have a 12V at all
All the low-voltage systems — door locks, infotainment, lights, BMS computers — run on 12V. The HV pack only drives the motor. Without 12V, the car won't even unlock to let you plug in.
Why it dies young
Manufacturers fit cheap, undersized 12V batteries to save weight. The EV's own computers wake periodically and drain it. Park up for 3 weeks in winter and you'll come back to a dead car.
Warning signs
Slow infotainment boot, dim interior lights, warning messages on start-up, sometimes a '12V battery low' message. Sort it within a week — once it's flat you may need recovery.
What to fit
Always replace with an AGM-spec battery to the manufacturer's exact rating — not a cheap lead-acid from a parts shop. Modern EVs need the new battery coded to the BMS or warnings persist.
Can I jump-start an EV? — Yes — jump the 12V terminals only, never touch the HV system. Once running, drive it for 30 minutes to let the DC-DC converter top up the 12V.
How often should I replace it? — 3–4 years typically. If you've had two flat-battery incidents, it's done.
More detail on: Why is my EV's 12V battery dead?
The short answer above covers the usual case. Below is the extra detail a UK driver normally needs to act on it — how to narrow the fault down, what it should cost, and when it is worth paying a garage rather than carrying on. This is general guidance from a working mechanic, not a substitute for someone putting your own car on a ramp.
How to describe the problem so you get a straight answer
Mechanics diagnose faster when they get facts rather than conclusions. Give the make, model, engine size, fuel type, year and mileage; say exactly when the fault happens (cold start, motorway speed, braking, turning, first five minutes); say what changed just before it started; and list any codes, warning lights or recent work. A three-line description in that shape is worth more than a paragraph of theory.
What to do before you spend money
Run a free MOT history check to see whether the same item has been advisoried before — that history often explains a fault in one line. Check whether an open safety recall covers the component, because that repair is free at a franchised dealer. Then get the fault properly diagnosed rather than replacing parts in order of price.
What Bob would check first
Start with the cheap, common and visible: fluid levels and their condition, tyre pressures and wear pattern, battery voltage and terminal condition, and every connector and vacuum pipe in the area of the fault. A surprising share of electrical gremlins are a corroded earth or a chafed loom, and a surprising share of running faults are an air leak downstream of the sensor doing the complaining. Only after that does anything get unbolted.
When it is worth going to a main dealer
Three situations justify main-dealer money: an open safety recall (free), a live manufacturer warranty or goodwill claim, and coding or programming work that needs manufacturer-level software. Everything else — servicing, brakes, suspension, clutches, cambelts, diagnostics — an experienced independent will do to the same standard for a good deal less, and using an independent does not invalidate your warranty under UK block exemption rules as long as the correct parts and schedule are used.
Safe to keep driving?
Treat anything affecting brakes, steering, tyres, seat belts or visibility as stop-now, no matter how well the car appears to be driving. Overheating and low oil pressure are also stop-now: the damage curve on both is measured in minutes, not miles. Most other faults will let you drive gently to a garage, but if the car is in limp mode, losing fluid, or the fault changes how it steers or stops, get it recovered instead. Recovery is cheaper than a tow plus a new engine.
Get an answer on your own car
If you want this looked at properly for your exact vehicle, send the details and Bob will write back — make, model, engine, mileage, when the fault happens and any codes you have. You get a written reply from a qualified UK mechanic with 32 years on the tools, not a chatbot, and your money back if he cannot help.
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