How To Safely Jump Start A Car With A Dead Battery

By Eric Peters, Automotive Columnist

Almost everyone will, at some point in their lives, have to deal with a car that won’t start because of a weak or dead battery. Jump-starting a car is simple but a few cautions should be observed:

1) Look under the hoods of both cars; pull the car with the good battery as close to the car with the dead battery as possible without the cars physically touching.

2) Put both vehicles in Park, depress the parking brake, shut off the engine (jumper car), turn the ignition switch to “off” for both cars and disconnect any plug-in accessories such as cell phones, iPods or radar detectors. The reason for this is to prevent current draw to accessories, which will make the car harder to start – and to protect accessories from possible power spikes through the system during the jump-starting process.

3) Inspect both batteries for signs of physical damage, in particular, cracks in the case and/or leaking fluid. If you see either, forget about jumping and call a tow truck. Attempting to jump-start a leaking battery could result in a catastrophic explosion resulting from sparks igniting volatile gasses escaping from the battery.

4) Locate the positive (+) and negative (-) terminals on each battery. Each terminal is usually marked with a stamped-in “+” or “-” sign, or a red plastic cap (positive) and black plastic cap (negative). As a further identification aid, the positive terminal usually has a red cable leading to it while the negative cable is usually black. Be 100 percent sure which terminal is positive and which terminal is negative before proceeding. If you have any doubt at all, do not connect jumper cables. Call AAA or another towing service. Better to risk a little inconvenience than a possible explosion/fire or severe damage to your car’s electrical system.

5) Use a rag or paper towel or whatever is handy to clean off the exposed surfaces of the terminals. This will help assure a good connection and a successful jump.

6) Get your jumper cables. These should be in good physical shape, with the positive and negative clamps on each end clearly marked (red for positive, black for negative). Check for damaged insulation/frayed wires before proceeding. If the cables are damaged, find a better set or call AAA. Once again, you don’t want to risk a major problem over a relatively trivial inconvenience.

7) Assuming the cables check out ok, have one person hold one end – making sure to keep the positive and negative clamps from touching.

8) First, connect one red/positive clamp of the jumper cables to the red/positive terminal of the dead battery. Next, connect the other end of the red/positive jumper cable clamp to the red/positive terminal of the good battery.

9) Second, connect one end of the jumper cable’s black/negative clamps to the negative terminal of the good battery.

10) Finally, connect the remaining black/negative jumper cable clamp to a “ground” on the car to be jump-started. A “ground” is a metallic/unpainted part of the engine or frame/chassis. An accessory bracket or large bolt usually works well. The reason for connecting the negative cable to ground rather than to the negative terminal of the dead battery is to minimize the chances of a spark near the battery, where there may be potentially explosive gasses. If the dome/interior light comes on in the car to be jumped, it’s a good sign the cables are connected correctly.

11) Start the engine of the car with the good battery and raise the engine rpm to a fast idle (about 1,500 rpm, if the car has a tachometer). Run the engine for about five minutes, then shut if off. Disconnect the cables and attempt to start the car with the bad battery. For the first attempt, do not try to start the car with the dead battery while the jumper car’s engine is running and the cables are still connected. Reason? It’s possible the strain on the jumper car’s alternator could cause damage or excess wear. If the car to be jumped is slow to crank or doesn’t want to start, don’t keep cranking the starter; this will only further weaken the battery.

12) If the car won’t start using the above method, reconnect the cables as described, start the car with the good battery (being sure to turn off the headlights, which will draw power you need for maximum boost) and let it idle again for a couple of minutes. Now try to start the car with the dead battery. You may need to rev the engine of the jumper car a bit, especially if the other car’s battery is really dead and it’s really cold outside.

13 Once the car starts, remove the cables in the reverse order you hooked them up, beginning with the black ground cable.
Be super careful that the clamps don’t accidentally touch as you do this.

14) Do not shut off the engine of the car that was jump-started for at least 10-15 minutes (which will give the car’s alternator time to recharge the battery) and ideally, not until you get home or to a safe place. If the battery is weak, it won’t be able to hold a charge and the car may not restart even after you drive it or 10-15 minutes. If your headlights/dash lights seem dimmer than usual, you may have a bad alternator, too. Get home – or to a repair shop – as quickly as possible.

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7 Responses to “How To Safely Jump Start A Car With A Dead Battery”

  1. Randy says:

    There is no safe way for any reader of this site to jumpstart a car. Call AAA or have it towed to a mechanic or you will hurt yourself.

    • Randy says:

      No reader of this site can ever hope to follow through to the 14th step. On second thought, don't even bother to call AAA; call AA instead and have ignition interlocks installed or you will hurt yourselves and innocent bystanders.

    • Hubcap says:

      I thought there were only 12 steps.

  2. Stuart Somers says:

    A battery low or dead will freeze in cold weather. NEVER try to jump start a frozen battery !! It will blow up!
    Bring it in the house and let it thaw. It will take several hours to thaw.

    • Hubcap says:

      Good point, Stuart. Over the course of my automotive career, I've dealt with several exploding batteries and it's never a pleasent experience. One of them put me in the hospital; fortunately I didn't suffer any permanent eye damage.

      Additionally, since water expands when it freezes, a frozen battery will usually have some serious damage to the plates once it thaws, so you might as well just replace it anyway.

    • Fleet Admiral says:

      And never put it near a source of heat with access to an open flame. The fumes will ignite as batteries give off toxic vapor. You don't want to inhale it either or it'll perm clean your sinuses.

      Best thing to do is have one of those portable / cig lighter jump starters. Don't rely on some carjack shy person to hopefully stop and help you.

  3. reddog4321 says:

    I have jump started cars many times, and each time try the negative to ground connection on the depleted car. It has never once worked for me after dozens of tries. I think it is either urban legend that negative to ground actually works or you need some extra long negative cable and have to crawl under your car to look for some magic hidden piece of metal that conducts. I've tried unpainted bolts on both the chassis under the hood and engine block. Nothing! Luckily I have never blown myself up going direct to the negative post.