Car basics · Some practice · about 15 minutes, plus a 20 to 30 minute drive
Jump-Start a Car
When a car's battery runs down, another car's battery can help it start. Jumper cables go on in a set order that keeps sparks away from the battery. Learn the order once and you can help yourself or a neighbor.
When to teach it: Watch from about 10, help from about 13 by reading the manual and naming each clamp spot, and do it alone around the time they start driving.
You'll need
- Heavy-duty jumper cables (AAA suggests a six-gauge, 16-foot set)
- A working car with a 12-volt battery
- Both cars' owner's manuals
- Safety glasses
Before you start
- Read the jump-starting section of both owner's manuals. Some cars, including some hybrids and electric cars, have special rules or shouldn't be used to jump another car.
- Check that both cars use 12-volt systems.
- Look at the dead battery. Don't jump it if it's cracked, bulging, leaking, smells like rotten eggs, or is frozen.
- Put on safety glasses and take off rings, watches, and other metal jewelry.
- Park the cars close but not touching, both in Park with the parking brakes set and the engines off.
Step by step
Tick each step as you go if you like; ticks stay on this device.
Physics
How it works: Why a weak battery can't crank the engine
A car battery is really six small batteries, called cells, connected in a row. Each lead-acid cell makes about 2 volts, so six of them add up to 12 volts. Inside, plates of lead and lead dioxide sit in sulfuric acid, and a chemical reaction pushes electrons through any circuit you connect.
Voltage is the push. Current is how much electricity flows. The starter motor that cranks the engine needs a very high current. In a circuit, current equals voltage divided by resistance.
Every battery also has some resistance inside it, and that inside resistance tends to rise as the battery runs down or ages. With a light load, a weak battery can seem fine. Under a heavy load like the starter, its voltage drops a lot, so the starter can't get enough current to turn the engine.
Jumper cables connect the good car's battery to the dead one so they share the work. Once the good car is running, its alternator helps charge the dead battery. After your car starts, its own charging system tops the battery back up while you drive.
current = voltage ÷ resistance (I = V ÷ R)
Try it: Look inside a flashlight that takes two or more batteries. They sit end to end, like the six cells in a car battery. Read the voltage printed on each battery and add them up to get the flashlight's total voltage.
Keep learning: Science
Teaching it to your kids
- Have your child read the jump-start page of the owner's manual out loud.
- Point out the + and − marks on the battery and the red and black cable colors.
- With the cables not attached to anything, have them say where each clamp goes, in order: red-dead, red-good, black-good, black-metal.
- Show them a good unpainted metal spot on your engine, away from the battery.
Tips
- Keep jumper cables in your trunk.
- Running the good car's engine a little above idle can speed up charging.
- On the drive afterward, keep the air conditioner and rear defroster off so more power goes to recharging.
- A short drive won't fix an old battery. Have it tested.
Common mistakes
| Mistake | Instead |
|---|---|
| Putting the last black clamp on the dead battery's negative terminal | Clamp it to bare metal on the engine, away from the battery, so any spark happens away from battery gases. |
| Letting the clamps touch each other | Hold the clamps apart and connect them one at a time. |
| Leaning over the battery while connecting | Stand to the side and keep your face back. |
| Jumping a damaged or frozen battery | Don't. Replace it. |
Safe habits
- Batteries can give off hydrogen gas, which can explode. Keep sparks and flames away from the battery.
- Wear safety glasses. Car batteries contain sulfuric acid, which is corrosive.
- Use only a 12-volt source to jump a 12-volt car.
- Keep hands, hair, and cables away from belts and fans once an engine is running.
When to call a pro
- If the battery is cracked, bulging, leaking, or frozen, don't jump it. It needs to be replaced.
- If you're unsure about any step, get a trained technician or a tow service to help.
- If you hear a sharp, repeated clicking but the engine doesn't crank, the problem may be something other than the battery. Have a shop check it.
- If the battery keeps going dead or you don't know why it died, have the car checked.
Sources
Free how-to guides for families, with nothing to earn, pass or send in. They follow U.S. rules and sources; your owner's manual, product labels and state and local rules come first.