How to Jump-Start a Car

It’s never a convenient time for your battery to die.

Assuming it’s not practical to pull the battery out and either have it charged or replace it — or to call for a tow truck — your only option is to jump-start your car. While there are now “jumper packs” you can buy that contain either a small lead-acid battery (similar to the larger one in your car) or a lithium-ion battery, it requires forethought and having it handy … and it being fully charged.

So, let’s figure you’re going to jump-start your car the traditional way: by using a running car.

Related: How to Change a Car Battery

Things to Consider Before Jump-Starting Your Car

There are a few things that you should remember before you try to jump-start your vehicle.

First, if your car’s engine is spinning over at what seems like normal speed when you try to start it but it doesn’t, the problem is probably not the battery — so jumping it will do no good. In that case, it’s more likely a bad fuel pump, an ignition problem or a broken timing belt. It’s best not to keep trying to start it, as you’ll only add “dead battery” to your list of woes.

Additionally, batteries on modern cars are not always under the hood. Some are in the trunk, while others may be under the rear seat or in a hard-to-reach place. In some cases, there might be remote battery terminals positioned away from the battery, largely for safety when jumping the battery. You may have to consult the owner’s manual or do a little searching to find out.

While many battery terminals are large, top-mounted posts with beefy, soft-lead cable ends, some batteries in modern cars have flat discs on one side as the terminals. These batteries, often called “side post” batteries, have a washerlike connector on the cable end attached with a small, hard-metal bolt that has a bit of a “shoulder” on it. Though they are more space-efficient, it’s much more difficult to get a good connection when using jumper cables. You may have to clean the terminal or rotate the clamp once it’s connected to get a better contact surface. (The large, soft-lead cable ends made it easy to get a lot of contact surface for the jumper cables to grab and didn’t rust, while the modern hard-metal design can.)

You should also see if you can determine why the battery went dead. If you left your lights on, that’s temporary. If it’s because your alternator went bad or its drive belt broke — which, if you’ve been driving the car, should have caused the battery warning light to illuminate on the instrument panel — jumping the car is not going to be a permanent fix. (You can visually check the drive belt, known as a serpentine belt. It winds its way over several pulleys at the front of the engine, so if you see some round, grooved pulleys with no belt around them, the belt probably broke and is laying on the ground somewhere.)

On the “dead” car, note if the engine turns over at all when you try to start it, if you just hear a “click” or if you hear nothing. If it’s the last, make sure the shift lever is in Park if the vehicle has an automatic transmission; the engine won’t crank over if you accidentally left the transmission in Drive or Reverse. What happens initially will be helpful to know later on (in No. 7 below).

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The Procedure

1. Once you’ve found the batteries on both cars, park them so that your jumper cables can reach between the batteries, but don’t let the cars touch. Place the transmissions for both vehicles either in Park (if an automatic) or in Neutral with the parking brake engaged (if a manual) with the engines off.

2. Connect one red clamp of the jumper cables to the dead battery’s positive terminal (typically marked with a plus sign) and the other red clamp to the good battery’s positive terminal. If you see any white corrosion on the terminals, don’t let it get on your skin. After connecting the clamps, move them back and forth and side to side a little to try to make a clean connection, particularly if either battery is of the side-terminal variety.

3. Connect one black clamp to the good battery’s negative terminal (typically marked with a negative sign) and the other black clamp to a nonpainted metal surface on the car with the dead battery, which might take some searching. This is because taking that clamp on and off can create a spark and the battery can give off flammable hydrogen gas, so you don’t want to have that spark occur near the battery. As mentioned, some cars even have separate terminals (perhaps only a negative one) away from the battery for just this purpose.

4. Make sure the clamps aren’t close to one another (when either engine starts, they might move a little) and that the cables don’t run near any part of either engine that may be spinning when the cars are running. Start the running car.

5. While it’s sometimes enough to let the running car idle for just a minute or two, it may be necessary to let the running car run for up to 15 minutes before being able to start the dead car — particularly if either battery is of the side-terminal variety with their smaller contact surfaces — as that lets the running car charge up the dead battery somewhat. With either type of clamp, it might be helpful to keep the running car’s engine revving at higher than idle speed in order to increase the amount of charge being transferred to the dead car.

6. Try to start the dead car. If it starts, check to make sure the battery warning light isn’t on in the instrument panel. (If it is, skip to No. 8.) If it’s not illuminated, remove the cables in the reverse order (most importantly, the black clamp that’s connected to the metal of the “dead” car first) and drive your car or leave it idling awhile to let the alternator fully recharge the battery. You also might want to get your battery tested at an auto parts store, as it’s possible it’s going bad.

7. If the vehicle doesn’t start — and the running car has been on for 10-15 minutes — note if anything different is happening than was before. For instance, if before the engine turned over very slowly or you just heard a click and now it turns over faster, you may just need to keep the running car connected longer to add more charge to the battery. If it still doesn’t turn over at all or even give a click, either you don’t have good cable connections or there is another problem, such as a bad starter or faulty ignition switch.

8. If you get your car running but the battery warning light is on in the instrument panel, your alternator may not be recharging your battery. In that case, the battery will drain down as you drive the car, and your battery will eventually end up dead again. So, it’s best to make sure any unnecessary electric accessories are turned off and use what charge you have to drive to a service facility with the running car following you (in case you need to jump yours again).

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