You turn the key, or press the start button, and nothing happens — or the engine turns over slowly and then gives up. A flat battery is one of the most common reasons a car will not start in the UK, and it often shows itself on a cold morning, after the car has sat unused, or after a run of short journeys.
A jump start can get you going again, but only if it is done in the right order, on the right car, in a safe place. Done wrong, it can damage the car’s electrics or cause sparks near the battery. This guide covers how to tell whether the battery is the problem, when not to jump start at all, and when to stop trying and call for recovery instead.
If you are in danger, or broken down in a live lane, call 999 first — do not work on the car in moving traffic.
1. First, check it really is the battery
A flat battery usually shows itself before you even try to start the engine:
- the engine fails to start, or cranks slowly and labours
- you hear a clicking sound when you try to start it
- headlights look dim, and the radio, windows or central locking are weak or dead
- dashboard warning lights are dim, or do not come on properly
The RAC points out that a battery can still have enough charge for dim warning lights but not enough to turn the engine over, so dim lights on their own do not rule a flat battery out. Before you assume the worst, try two quick checks: try a spare key fob if the central locking will not work, and if the steering lock has engaged because the wheel is under tension against the kerb, gently rock the steering wheel from side to side as you turn the key.
If the electrics work normally, the engine cranks strongly, or there is a warning light, a smell of fuel, smoke, or a noise you do not recognise, the fault may not be the battery at all. Do not keep cranking the engine. Note what happened and call for help instead.
2. Where you are decides what to do next
At home or in a car park, with room to work safely, a careful jump start may be an option — if the checks in section 3 pass.
At the roadside, move the car out of traffic if you can do so safely, switch on your hazard lights, and work only where you are well away from moving vehicles.
On a motorway, do not attempt a jump start or any repair at the roadside. Follow the motorway breakdown steps in our earlier guide — get left, get everyone out on the side away from traffic, wait behind the barrier, and call 999 if you are in a live lane — then call for recovery. The guide is here: What to Do If Your Car Breaks Down on the Motorway in the UK.
3. When NOT to jump start
Do not jump start the car if any of these apply:
- the battery is cracked, leaking, swollen, badly corroded, or frozen — never jump start a damaged battery
- you can smell fuel, or there is smoke or a burning smell
- your vehicle handbook says not to jump start it, or gives a different procedure for your model
- either vehicle is not a standard 12V system, or you are not sure the two systems match
- the donor vehicle is a hybrid or an electric car and its manufacturer does not specifically permit using it as a donor — the RAC warns against this unless the manufacturer permits it
- you do not have sound, properly insulated jump leads, or you are not confident about the lead order
Some modern cars do not have the battery under the bonnet — it may be in the boot — and some have remote jump-starting points under the bonnet instead of direct battery terminals. Always check your handbook for your model before you connect anything. If you are unsure, call for recovery rather than risk damaging the electrics.
A portable jump starter is an alternative to a donor car, but only if it is compatible with your car and you follow both its instructions and your handbook.
4. How to jump start safely, in the right order
You need sound jump leads, a donor car with a well-charged battery, and the confidence to do the job. Park the donor car close enough for the leads to reach, but do not let the two cars touch. Apply both handbrakes, put automatics in Park (manuals in neutral), switch both ignitions off, and switch off lights, radio, heating, sat nav and anything plugged in. Remove rings and metal jewellery, and keep loose clothing and hair away from fans and belts. Gloves and eye protection are sensible.
Connect the leads in this order:
- Red lead to the positive (+) terminal of the flat battery.
- The other end of the red lead to the positive (+) terminal of the donor battery.
- Black lead to the negative (−) terminal of the donor battery.
- The other end of the black lead to a solid, unpainted metal part of the engine or chassis of the car with the flat battery — well away from the battery itself, and away from fuel and brake lines. Do not connect it to the flat battery’s negative terminal.
Keep the red and black clamps apart at all times. Start the donor car and let it run for a few minutes. Then try to start the car with the flat battery. If it does not start after a couple of tries, stop — repeated attempts can cause more damage, and the fault may be the battery, starter or charging system rather than a simple flat.
When the car has started, remove the leads in the reverse order: black lead from the engine metal first, then black from the donor battery, then red from the donor battery, then red from the flat battery. Take care not to let the clamps touch each other or any metalwork while you do this. If a lead gets hot, stop and let it cool.
5. After a jump start: do not just switch it off
A jump start is a restart, not a repair. A short trip may not recharge the battery, especially after repeated short journeys. Take a longer drive if the car is running normally, and get the battery and charging system tested promptly — a garage or breakdown technician can test whether the battery is holding charge, whether the alternator is charging it, and whether something is draining it.
Car batteries typically last three to five years, according to the RAC, and extreme heat and cold, corrosion, leaving electrics on, and repeated short trips all shorten battery life. If the battery keeps going flat, it may be nearing the end of its life, or there may be a fault elsewhere. Get it tested rather than relying on another jump start.
6. If it still will not start, call for recovery
Stop trying if the car will not start after a careful jump start, if the battery is damaged, if you are on a motorway or in an unsafe place, or if you are unsure at any point. Have this information ready when you call:
- where you are, including the road name and any landmark or postcode
- your vehicle make, model, colour and registration
- what happened, and what you have already tried
- how many passengers you have, and whether anyone is vulnerable
Do not try to push-start the car, and do not ask passers-by to tow it with a rope unless you know that is safe and legal for your car — most modern cars, and all automatics, should not be push-started or towed that way.
Need recovery now?
UK Breakdown Recovery is a London-based, 24/7 vehicle recovery service for cars, vans, motorbikes and more. If your car will not start, call or WhatsApp +44 7588 282876 for jump-start / battery help or recovery, and tell us where you are and what has happened.
If you are in a live lane, or anyone is hurt, call 999 first.
FAQ
How do I know if my car battery is flat?
The usual signs are an engine that will not start or cranks slowly, a clicking sound, dim headlights, and weak electrics such as the radio, windows or central locking. Dim dashboard lights can also point to a flat battery, even if some electrics still work.
Can I jump start an electric or hybrid car?
Check your handbook first — procedures vary by model, and some cars have remote jump-starting points or a battery in the boot. Do not use a hybrid or electric car as the donor vehicle unless its manufacturer specifically permits it. If you are unsure, call for recovery.
How long should I drive after a jump start?
Take a longer drive rather than a short trip, and get the battery and charging system tested promptly. A jump start does not prove the battery is healthy — if it keeps going flat, the battery or charging system needs testing.
How long does a car battery last?
Typically three to five years, according to the RAC. Cold weather, age, corrosion, leaving lights or devices on, and repeated short journeys can all shorten its life.
