How to Jump-Start a Car Safely

A dead battery is one of the most common reasons a car will not start, and a jump-start is usually a five-minute fix. Done in the wrong order, though, it can spark near a battery that vents flammable hydrogen gas, or damage the electronics in either car. Here is the safe sequence for jumper cables and for a portable jump pack, plus what to do afterward.

Jump-starting only helps if the battery is the problem. Typical signs:

• Slow, labored cranking or a rapid clicking when you turn the key or press start.
• Dim or flickering dash lights, or no lights at all.
• Interior lights or headlights left on overnight.

Signs it may be something else:

• Strong, normal cranking but no start: more likely fuel, ignition or a sensor problem. A jump will not help.
• A single loud click and nothing else, with bright dash lights: could be the starter or a connection.
• Everything works, but the car will not shift out of park or the key will not turn: check the shifter, the brake pedal and the steering lock.

• Jumper cables in good condition, ideally 4 to 6 gauge and at least 12 to 16 feet long (thicker cables, which have a lower gauge number, carry current better), or a charged portable jump starter (jump pack).
• A second vehicle with a battery of the same voltage (almost all cars and light trucks use 12 volts).
• Gloves and eye protection if you have them.
• A flashlight at night.

1. 1Park the working car close, nose to nose or side by side, without the cars touching.
2. 2Turn off both cars. Set parking brakes. Put automatics in Park, manuals in neutral.
3. 3Turn off lights, radio, climate control and accessories in both cars, and unplug phone chargers.
4. 4Open both hoods and locate the batteries or the remote jump terminals. Positive is marked + and usually has a red cover; negative is marked −.
5. 5Inspect the dead battery. Do not jump it if the case is cracked, bulging or leaking, if you smell rotten eggs, or if it may be frozen (a frozen battery can rupture or explode when charged). Get help instead.

The connection order is designed so that the final connection, which may spark, is made away from the dead battery.

1. 1Red clamp to the dead battery's positive (+) terminal.
2. 2Other red clamp to the good battery's positive (+) terminal.
3. 3Black clamp to the good battery's negative (−) terminal.
4. 4Other black clamp to an unpainted metal surface on the dead car's engine or chassis, such as a bolt or bracket on the engine block, away from the battery and moving parts. Many cars have a dedicated ground point; check the manual.
5. 5Start the working car and let it run for a few minutes to put some charge into the dead battery. A gentle raise of the idle can help.
6. 6Try to start the dead car. If it does not start, wait a few more minutes and try again. Do not crank continuously for more than about 10 to 15 seconds at a time; let the starter cool between attempts.
7. 7Once it starts, leave both running and remove the cables in reverse order: black from the ground point, black from the good battery, red from the good battery, red from the formerly dead battery. Do not let clamps touch each other or any metal as you remove them.

A jump pack is often easier and safer, since you need no second car.

1. 1Make sure the pack is charged and switched off.
2. 2Turn the dead car off, with accessories off.
3. 3Connect the red clamp to the positive (+) terminal.
4. 4Connect the black clamp to an unpainted metal ground (or the negative terminal if the pack's and car's instructions allow it).
5. 5Switch the pack on. Many packs have a light that confirms correct connection.
6. 6Start the car. If it does not start, wait about a minute between attempts as the pack's instructions direct.
7. 7Once running, switch off the pack and remove the black clamp, then the red.
8. 8Recharge the pack when you get home. Check it every few months, since lithium packs slowly self-discharge.

• Keep the engine running for at least 20 to 30 minutes, ideally by driving, so the alternator can recharge the battery. Idling recharges slowly.
• Avoid shutting the engine off until you reach somewhere you can get it started again or tested.
• Get the battery and charging system tested. Many parts stores do this free. A battery that needed a jump once from a simple mistake (lights left on) may be fine; one that keeps going flat is failing, or something is draining it.
• If your car shows a battery or charging warning light while driving, the alternator may not be charging. A scan tool may show charging-system codes; look them up in our OBD-II code database.
• Some cars need a few drive cycles to relearn idle, power-window auto-up or other settings after the battery has been very low.

Hybrids and EVs have a large high-voltage battery and a separate 12-volt battery that powers the computers and lets the car "boot up". When an EV or hybrid will not turn on, it is usually the 12-volt battery that is flat.

• Follow the owner's manual exactly. Many have dedicated jump terminals under the hood or in a fuse box.
• You can usually jump the 12-volt battery with a jump pack or another car, using the manual's procedure.
• Do not use a hybrid or EV to jump another car unless the manual says it is allowed. Their 12-volt systems are not always designed to supply starting current to another vehicle.
• Never touch orange high-voltage cables or components.

• Check the clamps. Corroded terminals or a poor ground connection are the most common reason a jump fails. Clean the terminals, reposition the black clamp, and try again.
• Give it longer. A deeply discharged battery may need 5 to 10 minutes of charging from the donor car before it will crank.
• Listen. Clicking with good voltage can point to the starter; normal cranking without starting points to fuel or ignition.
• Check fuses and the main battery cable for looseness or damage.
• Call roadside assistance if two or three attempts fail. Continued cranking can overheat the starter.

• Replace batteries at about 3 to 5 years, sooner in hot climates; see how long car batteries last.
• Test before winter and before long trips.
• Keep terminals clean and tight.
• Use a battery maintainer if the car sits for weeks.
• Check for recalls on your model; some involve batteries or electrical drains. See our recalls pages.
• Record the battery's install date in a free CarManager garage so you know when it is getting old.