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How do you jump start a flat car battery?

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WARNING
Failure to follow these instructions carefully, especially in the + / - polarity matching of the hookups, could result in damage to both batteries and/or burns to your person. Please be safe.

1 Open the hoods on both cars. Locate the batteries.

2 Close the hood on the running car, and move it into position so that both cars' batteries are as close as possible. (Reopen the hood.)

3 Attach a red jumper cable clamp to each battery's positive (+) terminal.

4 Locate a large, unpainted metal surface on the engine of each car. A bracket near the alternator works best. (The alternator is a grapefruit-sized motor-like device on the front of the engine.)

5 Attach a black jumper cable clamp to each location described in step 4.

NOTE: The last connection you make could make a small spark, and the engine on the running car should slow slightly. If this does not happen, check all 4 clamp connections by wiggling them.

6 Let the dead battery charge for about 5 minutes before attempting to start the car.

More details below
To jump a car, get some jumper cables and another car, and line the other car's front up with the dead one so that the batteries of each car can be as close enough together so the cables will reach. Leave the car with the good battery running. Now you're ready to connect the cables.

Start by connecting the negative cable to each battery first. Then connect your positive cables. Be sure to "manage" your clamps so you don't short anything out. Triple check your battery polarities before making cable connections - do not blow it connecting the units up incorrectly. Negative to negative, and positive to positive.

Have the one car that's fine running before making the last connection. What will happen when the last connection is made is that the "good" battery will start to charge the "dead" one, and this will drag down the "good" one. You'll want to have the vehicle with the "good" battery running so that the charging system can help transfer energy to the "dead" battery and minimize the impact of the discharge on your "good" battery. Wait a few minutes while the other battery charges off of the good one and the charging system of the operating vehicle.

The car should start after that. If it sounded like it was going to start and it didn't, wait even longer and try again. Each time you try to start the car with the "dead" battery, you'll be draining the dead battery more, so make sure and let it charge a bit.

Another answer
Jump starting a car battery is best accomplished with a pair of jump cables, normally sold in a "jump start pack", to jump start from the good battery of another car.

It's a good idea to read your owner's manual on the subject. Certain manufacturers recommend different techniques. For example BMW suggests turning on the headlights and blower fan. The technique is to connect the positive cable from the jump pack between the positive terminal on your battery and the positive terminal on the other car's good battery, and the negative cable from the jump pack to the negative terminal on your battery, or - better - to a grounding point on your vehicle, to the other car battery's negative terminal.

Don't ever get the jump cables reversed as doing that will cause the them to heat up so much that their insulation will melt and may even catch on fire and you may also cause damage to both cars' electrical systems.

Make sure you know the voltage on your car, most are 12 volt systems but some are 6 volt. If jumping from another car make sure they are both the same voltage.

Start the car to be jumped from and connect the positive cable ends to the positives terminals of both batteries and the negative cable to the negative terminals on both batteries. Allow the jumper car to run for a while before trying to start the jumpee car.

Another way to start a car that has a dead battery
If the vehicle with the dead battery has a manual transmission and it's parked on a hill, or you can push it to one, you can try another method: with the car in first gear and the ignition on, push in the clutch and let it roll down hill. When it gets going about 10 miles an hour or so, release the clutch quickly and it will snag the flywheel and force your engine to start turning, and that should start the car. If you aren't going fast enough, the engine will stall. Push in the clutch quickly after it starts and rev the engine a bit to insure it is going to keep running. Wait a few minutes before driving off to insure the battery gets a bit of a charge from the charging system, too. This can also be done in reverse, but it is much more dangerous, and it is inadvisable unless you are good at it and actually have a clear spot to roll backwards on.
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