Car How to jump-start a flat battery
Car · Beginner
How to jump-start a flat battery
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A flat 12-volt battery can often be started with jump leads and a second vehicle, or a booster pack, if you follow the handbook and a careful clamp order. This is not for high-voltage hybrid or electric drivetrains, not for a cracked or frozen battery, and not for a car that smells of gas or is badly damaged. If you are unsure, get roadside assistance.
Your call
Go or no?
Fancy giving it a go, or would you rather call a garage? There’s no wrong answer.
Right then. Here’s the kit, then the steps.
No problem. We’ll show you how to find a local garage.
At a glance
Shop the full kit
You can pick up the items below as you need them. Prices vary by shop and country, so we have not invented a figure here — check the live price before you buy. Shop links go to a product search on a local store for your country.
All-in-one kit
There is not usually a single boxed kit for this job. Buy the items below as you need them, and skip anything you already have.
Before you start
Safety
Stop and get a pro: Do not jump-start a hybrid or electric vehicle unless the maker’s handbook clearly allows a 12-volt jump and you follow that procedure only — never touch orange high-voltage cables. Do not jump a battery that is cracked, leaking, or frozen. Do not lean over the battery while someone connects a clamp. If you smell fuel, see sparks you cannot explain, or the donor car’s electrics behave oddly, stop and call for help.
- Read both handbooks first. Some cars specify special jump points, not the battery posts, and some must not be jump-started.
- Park both cars so the leads reach, but the vehicles do not touch. Apply both parking brakes, put both in park or neutral, and switch both ignitions off before you connect anything.
- Wear eye protection if you have it. Batteries can vent hydrogen. No smoking, no naked flames.
- Connect and disconnect in the right order, and keep the red and black clamps from touching each other or bodywork while they are live.
- Remove loose metal jewellery. Do not let a clamp dangle against the body while the other end is connected.
Prep that is worth doing
- Confirm it is a 12-volt battery problem: dim lights, a slow or silent starter, and no other obvious fault. If the car was in a crash, or the battery case is damaged, do not jump it.
- If you are using a second car, it should also be 12-volt. Do not use a large commercial vehicle as a donor unless you know that is safe.
- If you have a jump pack, charge it in advance and read its instructions — the clamp order may differ slightly from two-car leads.
Tools and materials
Check the live price at a shop you trust. Typical prices on this site, where they appear, were checked in the UK and may be out of date. Do not invent or assume a figure from this table. Shop links go to a product search on a local store for your country.
| Item | Why you need it | Typical price | Shop link |
|---|---|---|---|
| Quality jump leads, long enough and thick enough for a car | Thin, short leads struggle in the cold and can get hot. Match the pack’s rating to a typical passenger car. | Typically £15–£40 at a UK shop. Check the live price. | |
| Or a 12-volt jump starter pack | Useful if you often park alone. Follow the pack’s instructions over any generic advice if they differ. | Typically £40–£90 at a UK shop. Check the live price. | |
| Safety glasses and gloves | Splashes and a spark are unlikely if you are careful, but a battery can vent. | Typically £5–£12 at a UK shop. Check the live price. | |
| A torch | Battery bays are often dark and tight. | Typically £8–£18 at a UK shop. Check the live price. |
Shop this list:
Step-by-step
Use the handbook’s jump points if they are shown. The sequence below is the common safe order for two 12-volt cars. If your handbook or your jump pack says otherwise, follow that.
Prepare both vehicles
Park so the leads reach without the cars touching. Parking brakes on, ignitions off, accessories off. Open the bonnet and find the official jump points or the battery terminals. If you cannot see them clearly, stop.
Connect the red (positive) lead first
Clamp one red end to the positive terminal of the flat battery, or the official positive jump point (often marked +). Clamp the other red end to the positive terminal or official jump point on the donor car. Make sure the clamps are firm and not touching anything else.
Connect the black (negative) lead
Clamp one black end to the negative terminal or official negative point on the donor car. Clamp the other black end to a solid, unpainted metal earth on the car with the flat battery — a designated earthing point or a substantial engine-block bracket — not onto a moving part, not onto a fuel line, and preferably not onto the flat battery’s negative post unless the handbook says to. This keeps any spark away from the battery.
Start the donor, then the flat car
Start the donor car and let it run for a minute or two. Then try to start the car with the flat battery. If it does not start within a few seconds, stop cranking, wait, and check the clamps. Do not crank continuously. If it still will not start, disconnect and call for help rather than forcing it.
Disconnect in reverse order
Once the flat car is running, take the leads off in reverse: black from the recovered car’s earth, black from the donor, red from the donor, red from the recovered car. Do not let the clamps touch as you remove them. Keep the recovered car running, then drive for a reasonable spell or use a charger so the battery can recover. A short start around the block may not be enough.
If it dies again, or the battery is old and repeatedly flat
The battery or the charging system may have failed. A garage can test both. Repeated jump-starts are a warning, not a plan.
General guidance only. Check our disclaimer.
Common mistakes
- Connecting the wrong terminals, or letting two clamps touch. That can wreck the electrics on both cars, or cause a spark at the battery. If you are not sure which post is which, do not guess.
- Jumping a cracked, leaking or frozen battery. It can burst or spray acid. Have the car recovered.
- Trying a high-voltage hybrid or EV the same way you would a petrol car. Only follow the maker’s 12-volt procedure, if they give one. Leave orange cables alone.
- Disconnecting the leads in a hurry while they are still touching bodywork. Undo them in reverse order, one at a time, and keep the metal ends apart.
- Assuming a successful start means the battery is fine. If it goes flat again overnight, get it tested. You may have a faulty cell or an alternator problem.
FAQ
Can I use a jump pack instead of a second car?
Yes, often, if it is rated for your engine and fully charged. Follow the pack’s clamp order. Some packs want the vehicle off, some want you to wait before starting. Read the card in the case.
The donor car is a small petrol and the flat car is a large diesel. Is that all right?
A much larger engine can demand more than a small donor or thin leads can give. A quality jump pack, or roadside assistance, is safer than stressing a small donor.
Should I let the donor charge the flat battery for 10 minutes first?
A minute or two of the donor running is common. A long charge through jump leads is a slow way to charge a badly flat battery. If the car will not start after a couple of tries, stop.
The battery is in the boot or under a seat.
Use the official jump points under the bonnet if they are provided. Do not start pulling interior trim off to reach a battery you cannot see clearly.
Would rather not DIY? Find a local garage
If the battery is damaged, the car is a hybrid or EV, or a jump-start does not work, roadside assistance or a garage can test the battery and the charging system and, if needed, replace the battery.
We may earn from shop links, at no extra cost to you. Disclosure