Jump-Starting Equipment & Vehicles

Jump-starting a dead battery looks easy, but a mistake can cause a battery explosion, an acid burn, a fire or damage to expensive electronics. Batteries give off explosive hydrogen gas, and sparks in the wrong place can ignite it. A few steps and the right order of cable connections keep jump-starting safe.

• Construction
• Equipment
• Vehicles

Hazards

• Explosion: lead-acid batteries release hydrogen and oxygen while charging or when stressed. A spark near the battery can ignite the gas and blow the case apart, spraying acid.
• Acid: battery acid (sulfuric acid) burns skin and eyes and ruins clothing.
• Electrical burns and shorts: a car or truck battery can deliver a huge current. A wrench or cable across the terminals can weld metal and burn skin. Jewelry can cause severe burns.
• Moving parts: in an engine bay the fan, belts and pulleys can start without warning when the engine fires.
• Fire and electronics: a wrong connection can damage control modules and wiring or start a fire.
• Hazards around equipment: traffic, uneven ground and rolling machines. See Blocking & Cribbing and Equipment Basics.

Before you start

• Read the manuals for both machines. Some vehicles, hybrids and electric vehicles have special jump-start points and warnings.
• Check the voltage. Both batteries must be the same voltage. Many cars and light trucks are 12-volt. Many heavy trucks and equipment use two 12-volt batteries wired for 24 volts. Never connect a 12-volt source to a 24-volt system, or the reverse, unless the maker says how.
• Look at the dead battery. If the case is cracked, bulging, leaking or frozen, don’t jump it. A frozen battery can explode. Replace it instead.
• Wear safety glasses or a face shield and gloves. Remove rings, watches and metal jewelry. See Eye & Face Protection.
• Park safely: put both machines in park or neutral with the parking brakes set, turn off the engines and all accessories, and make sure the machines don’t touch each other, if you use a second vehicle.
• Keep ignition sources away: no smoking, flames or sparks, and keep the area ventilated.
• Use heavy-duty jumper cables in good condition, with clean clamps, and long enough that you don’t need to stretch.
• Know where the batteries are. Some vehicles have remote jump posts under the hood or in the trunk.

Connecting jumper cables, in order

1. Connect one red (positive) clamp to the positive (+) terminal of the dead battery.
2. Connect the other red clamp to the positive (+) terminal of the good (donor) battery.
3. Connect one black (negative) clamp to the negative (−) terminal of the good battery.
4. Connect the other black clamp to bare, unpainted metal on the dead machine, such as a bolt on the engine block or a heavy bracket, away from the battery and fuel lines. This is the final connection, and any spark will happen here, away from the battery gases.
5. Make sure the cables are clear of belts, fans and moving parts and that clamps are not touching each other.
6. Start the donor machine and let it run for a few minutes, or follow the manual.
7. Try to start the dead machine. If it doesn’t start after a few tries, stop and find the cause. A repeated cranking can overheat starters and cables.

Removing the cables

Remove the cables in the reverse order:

1. Black clamp from the metal ground on the formerly dead machine.
2. Black clamp from the negative terminal of the donor battery.
3. Red clamp from the donor battery.
4. Red clamp from the formerly dead battery.

Don’t let the clamps touch each other while any end is connected. Keep the newly started machine running for a while to recharge the battery, and have the battery and charging system tested. A battery that went dead may be weak or failing.

• Don’t connect the last black clamp to the dead battery’s negative terminal. A spark near the battery can ignite gas.
• Don’t lean over the battery while making connections or starting.
• Don’t cross the cables, or connect positive to negative.
• Don’t use a welder, a charger on boost mode or another machine’s alternator as a shortcut without the maker’s instructions.
• Don’t jump-start a machine that smells of fuel, or one with a damaged battery.

Booster packs and chargers

• Portable jump starters (booster packs) are convenient and don’t need another vehicle. Use one rated for the battery voltage and engine size, follow the instructions, and make the same final ground connection away from the battery where the maker calls for it.
• Charge booster packs regularly, and store them out of heat. Lithium packs can catch fire if damaged. Don’t use a damaged pack.
• A battery charger is safer than repeated jumps for a deeply discharged battery. Charge in a ventilated area, and connect and disconnect with the charger off.
• Heavy equipment may have an external starting receptacle or slave socket for jump-starting. Use it as the manual directs. See Equipment Fueling.

Protecting electronics

• Modern vehicles and equipment have sensitive control modules. Surges and wrong connections can damage them. Follow the manual, avoid revving the donor machine excessively and don’t jump-start with the key in the on position unless instructed.
• For hybrids and electric vehicles, the 12-volt battery is often jumpable, but not the high-voltage battery, which can kill. Follow the manual, and never touch orange high-voltage cables. See Electric Vehicle Safety.

Jump-start checklist

• Both batteries are the same voltage; the dead battery isn’t cracked, leaking or frozen
• Safety glasses on; jewelry off; no smoking or sparks nearby
• Both machines off, parking brakes set, not touching
• Red to dead (+), red to donor (+), black to donor (−), black to bare metal on the dead machine away from the battery
• Cables clear of fans and belts; clamps not touching
• Removed in reverse order; battery and charging system checked afterward

Last updated: October 4, 2026