How to Jump Start a Car Safely (and Avoid Damaging the Electronics)

• Connect the red lead to the flat battery’s positive terminal first, then to the good battery’s positive terminal, before the black lead goes to the good battery’s negative terminal and finally to an unpainted metal point on the stalled car, never the flat battery’s own negative terminal.
• Never jump start a hybrid or electric car from another hybrid or electric car, and never touch anything wrapped in orange casing under the bonnet. Use only a petrol or diesel donor vehicle or a dedicated jump starter pack, and only at the marked 12V jump point.
• A portable lithium jump starter costs roughly £50 to £90 (about $67 to $121) and pays for itself after a single roadside call-out, which can run to £75 to £150 (about $100 to $201) if you have no breakdown cover.

How to Jump Start a Car Safely, Lead by Lead, Without Frying the Electronics

A flat battery on a cold morning is one of the most common reasons drivers call a mechanic or a breakdown provider, and it is also one of the easiest problems to fix yourself with the right leads and the right order. Get the sequence wrong, though, and you risk a shower of sparks, a blown fuse, or a damaged control unit on a modern car packed with sensitive electronics. The method below follows the sequence used by RAC-trained patrols, and it works whether the flat car is parked on a driveway or stuck on a side street.

The Right Order for Connecting Jump Leads

Before touching any leads, switch off the ignition, lights, radio, heated seats and anything else drawing power in both cars, and put both in Park or Neutral with the handbrake applied. Take off rings, watches and any other metal jewellery. A jump lead carries enough current to cause a serious burn if it brushes against a stray earring or bracelet while you work near the terminals. Stop entirely if the flat battery looks cracked, is leaking, has swollen casing, is heavily corroded, or appears to have frozen, and call a professional instead. Attempting to jump start a frozen or damaged battery risks it rupturing.

With both bonnets open and the cars close enough for the leads to reach without stretching, connect the leads in this exact order. First, clip the red (positive) lead to the positive terminal of the flat battery. Second, clip the other end of the red lead to the positive terminal of the good battery. Third, clip one end of the black (negative) lead to the negative terminal of the good battery. Fourth, clip the final end of the black lead to a solid, unpainted piece of metal on the engine of the stalled car, such as a bolt head or bracket, rather than the flat battery’s negative terminal itself. This earths the circuit away from the battery and reduces the risk of a spark igniting any hydrogen gas that a struggling battery can give off.

Starting the Engine Without Damaging Either Car

Start the donor vehicle first and let it idle for a minute or two before attempting the flat car. This gives the donor’s alternator a chance to build up a steady charge rather than dumping a sudden surge through the leads. Then try starting the flat car. If it does not fire immediately, avoid holding the key or button down for long stretches. A widely used rule from breakdown patrols is to crank for no more than around five seconds at a time, then pause for thirty seconds before trying again. Prolonged cranking on a very low battery can overheat the starter motor. If the car has not started after three short attempts, stop and treat it as a fault beyond a simple flat battery.

Once the flat car starts, leave both engines running for a few minutes before disconnecting the leads in the exact reverse order: black lead off the earthing point first, then black lead off the donor’s negative terminal, then red lead off the donor’s positive terminal, and finally red lead off the previously flat battery’s positive terminal. Removing them in reverse order keeps the circuit safely earthed for as long as possible and avoids a spark at the final connection point on either battery.

Hybrid and Electric Cars Need a Different Approach

Every hybrid and electric car still carries a conventional 12V battery to run the lights, infotainment and onboard computers, separate from the large high-voltage battery that drives the wheels, and it is only this small 12V battery that can ever be jump started. The high-voltage battery itself must never be connected to jump leads under any circumstances, and the orange-cased cables running from it are a clear visual warning to keep well away.

Do not jump start a hybrid or electric car from another hybrid or electric car. The safest donor is always a petrol or diesel vehicle, or a dedicated portable jump starter pack designed for the job. Locating the 12V jump point can also differ from a conventional car. Some hybrids place it in the engine bay near the fuse box rather than at the battery itself, so check the owner’s handbook first rather than guessing, and never proceed if you cannot positively identify the correct 12V terminals. Both vehicles should be completely switched off, not just in Park with the engine running, before any cable goes near a hybrid’s terminals. A live connection at the moment of contact is far more likely to damage onboard electronics that manage the battery and motor.

What a Jump Starter Pack Costs and When It Pays for Itself

Portable lithium jump starter packs have made roadside jump leads and a willing stranger less necessary than they once were. Entry-level packs start under £50 (about $67), with capable mid-range units such as well-reviewed 12V-only packs sitting around £90 (about $121), and heavier-duty trade-grade packs with jump-start capability for larger engines reaching up to £260 (about $349) at the top end. Most petrol and diesel family cars are well served by a pack in the £50 to £90 (about $67 to $121) range, and many double as a portable phone charger and torch.

Compare that against the cost of getting stuck without one. A call-out for a flat battery without breakdown cover typically runs to £75 to £150 (about $100 to $201) depending on location and time of day, and can be higher again in central London or late at night. Breakdown cover itself, which also covers a flat battery among many other faults, costs from roughly £25 to £60 (about $34 to $81) a year as an add-on to existing car insurance, or £49 to £109 (about $66 to $146) or more a year as a standalone policy, with some providers offering entry packages from under £10 a month. Anyone who has been caught out once by a flat battery on a cold morning tends to view a jump starter pack sitting in the boot as one of the cheaper pieces of kit they own.

What to Do After the Engine Is Running

Getting the engine started is not the end of the job. A battery that needed a jump start has been drained low enough to need a proper recharge, and idling on the driveway for ten minutes will not do it. Drive for at least thirty minutes, ideally on a route with sustained speed rather than constant stop-start traffic, to let the alternator put a real charge back into the battery. If the engine cuts out again as soon as it is switched off, or the same flat battery reappears within a few days, the battery itself is very likely failing and close to needing a replacement rather than another jump start, and a load test at a garage will confirm it either way. Ignoring repeated flat batteries in the hope the next jump start will hold rarely ends well. A battery that keeps failing to hold charge can also point to a fault in the alternator or a parasitic drain elsewhere in the electrics, both of which are worth ruling out with a proper diagnostic check rather than reaching for the leads again and again.

Sources

• RAC Drive: How to jump start a car in 10 steps
• AAA: Jump-Starting an EV, precautions to prevent damage
• Autodoc: Best car jump starters for UK drivers
• Flat Battery Call Out UK: Jump start call-out cost guide
• RAC Drive: What to do if you have a flat or dead battery