Jump-starting: how to boost a car without breaking anything
Connect in this order: red to the flat battery's positive, red to the good battery's positive, black to the good battery's negative, black to bare metal on the dead car's engine, never to its negative terminal. Then run the donor car for a few minutes before trying to start.
Key points
- The last clamp goes on bare engine metal, away from the battery.
- Reversing the polarity, even for a second, can cost modules.
- Never boost a battery that's frozen or visibly swollen.
- After a boost, drive for a good half hour or the battery won't recover.
The order, and why it's that order
Red clamp on the flat battery's positive. Red clamp on the donor's positive. Black clamp on the donor's negative. Black clamp on a bare, unpainted metal bracket or engine lifting eye on the dead car.
That last step is the one people skip. A charging battery gives off hydrogen, and the final connection is the one that sparks. Making that spark on the engine block instead of on the battery post puts it well away from the gas.
Let it sit before you crank
Start the donor car and let it run three to five minutes before you try the flat one. A deeply discharged battery needs a little surface charge before it can help at all, and cranking straight away just loads the cables.
If it doesn't start after two attempts of a few seconds each, stop and reconsider. More cranking won't fix it and heats the starter.
The mistakes that cost real money
Reversed polarity is the expensive one. Red to negative even momentarily can take out an alternator, fuses and control modules on both cars. Check the clamps twice before the last connection.
Never boost a frozen battery: a discharged one can freeze, and the case may split under charge. A swollen case means the same thing. Let it thaw, then have it tested instead.
After the boost, it isn't over
Remove the clamps in reverse order and keep the engine running. Then drive for at least half an hour. Ten minutes won't put back what starting the engine took out, and the car will be flat again tomorrow.
If it needs a boost a second time, the battery isn't the whole story: it's either finished, or the alternator isn't charging, or something is draining it overnight. All three are measurable.
Frequently asked questions
What is the correct order for jumper cables?
Red to the flat battery's positive, red to the good battery's positive, black to the good battery's negative, and the last black clamp to bare metal on the dead car's engine, not to its battery terminal.
Why does the last clamp go on the engine and not the battery?
A battery gives off hydrogen while charging and the last connection is the one that sparks. Making it on the engine block keeps the spark away from the gas.
How long should I drive after a jump start?
At least half an hour, and highway driving is better than town. Starting the engine is a big withdrawal and short trips never pay it back.
My car needed a boost twice this week. What is wrong?
Either the battery has lost capacity, the alternator is undercharging, or something is drawing current overnight. A test tells you which, and it takes a few minutes.
Want it looked at?
Pick the day and time online. It takes a minute.