Before the cables come out
A battery gives off hydrogen gas, which is why the order of the clamps matters. If the case of the dead battery is cracked, leaking or bulging, or if the battery may be frozen solid, call for a tow instead of jumping it. Check both owner's manuals as well. Many newer cars, and most hybrids, hide the battery in the trunk or under a seat and provide marked jump-start terminals under the hood.
Park the two cars close enough for the cables to reach but not touching. Put both in park or neutral, set both parking brakes, and switch off both ignitions along with the lights and the heater fans.
The full sequence
- Clamp one red lead to the positive terminal of the dead battery, which is marked with a plus sign.
- Clamp the other red lead to the positive terminal of the good battery.
- Clamp one black lead to the negative terminal of the good battery.
- Clamp the last black lead to bare, unpainted metal on the engine or frame of the dead car, well away from the battery and from any belts or fans.
- Start the working car and let it run for three to five minutes.
- Try to start the dead car, and if it fails, wait another five minutes before trying again.
- Once it runs, remove the clamps in exactly the reverse order, starting with the black clamp on the bare metal, and never let the clamps touch each other while any of them is connected.
After it starts
Keep the revived car running, and drive it for at least twenty to thirty minutes so the alternator can put back some charge. Then find out why the battery died. A dome light left on overnight is an accident, but a battery that is four or five years old is simply finished, and most auto parts stores will test it free.
When it will not start
If the engine cranks strongly but will not catch, the battery was never the problem. If you hear one click or nothing at all after several attempts, clean the corroded terminals and check that the clamps are biting into metal. After that, the suspects are the starter or a battery too far gone to accept a jump.
Tip A lithium jump pack the size of a paperback costs well under a hundred dollars and removes the need for a second car. Recharge it every three months, or it will be as dead as the battery.
