A jump-start is simple — if you do the steps in the right order. Done wrong, it can damage the electronics of both cars, and on modern Start-Stop vehicles the margin for error is smaller than the old advice suggests. Here is the sequence we use professionally, written for a driveway or a car park, not a workshop.
Before you start
- You need proper jump leads (thick cable, clean clamps) and a donor car with a healthy battery of similar voltage (12 V for almost all cars).
- Park the donor close, but the cars must not touch. Both ignitions off, keys out.
- Check the flat battery: if it is cracked, swollen or leaking, stop — do not jump it. That battery needs replacing, not boosting.
- In a car park, put the hazards on and make sure both handbrakes are set.
The safe connection order
- Red lead → positive (+) of the flat battery.
- Red lead, other end → positive (+) of the donor battery.
- Black lead → negative (−) of the donor battery.
- Black lead, other end → bare metal on the engine block or chassis of the dead car — a bolt head or bracket, away from the battery and fuel lines. Not the negative post itself: the final connection can spark, and you want that spark away from the battery's vent gases.
Starting
- Start the donor car and let it run two or three minutes at a fast idle.
- Try the dead car. If it cranks weakly, wait another few minutes with the donor revving gently, then try again.
- Once it starts, disconnect in reverse order (black from block, black from donor, red from donor, red from revived car), keeping the clamps from touching anything.
- Now drive — a genuine sustained run of a good half-hour or more, not a lap of the car park. Short trips will not put back what the start took out.
Start-Stop, hybrid and 'the manual says no'
Many Start-Stop cars have the battery in the boot or under a seat, with dedicated jump posts under the bonnet — use those, never the remote battery terminals directly. Some hybrids and EVs must not be jump-started from their high-voltage side at all (the 12 V side is usually fine, but check the handbook). If in doubt, don't improvise on a €40,000 car.
When a jump-start will NOT fix it
- It restarts, then dies again within a day or two: the battery no longer holds charge (common after long parking) or the alternator isn't charging. It needs a proper test, not a third jump.
- Rapid clicking that never catches: could be the starter motor, not the battery.
- Totally dead, no lights at all: check for a blown main fuse or a disconnected terminal before blaming the battery.
No donor car, no leads, or a car park where none of this is practical? That is literally our job: an English-speaking technician comes to your car anywhere we operate in France, tests before touching anything, and either boosts it professionally or fits the right battery on the spot — see how the call-out works.