How to Jump-Start a Car Safely

Ninety seconds of checking prevents the two failure modes that matter: connecting to a battery that should not be jumped, and reversing polarity.

• Look at the battery case. Cracked, bulging or leaking means do not jump it. Call for a tow or a replacement instead.
• Check for ice. A frozen battery can rupture when current is applied. If the case is distorted and the car sat in deep cold, leave it alone.
• Identify the posts. Positive is marked + and usually wears a red cover. Negative is marked −. On many modern cars the battery sits in the trunk or under a seat and you use the remote jump terminals under the hood instead — those are labeled and are the correct connection point.
• Confirm both cars are off. Ignition off, parking brake set, transmission in park or neutral.
• Check the cables. Corroded clamps and thin, flimsy cables are why a jump fails. Real jumper cables are 6-gauge or thicker.

1. 1

   Red clamp to the dead battery's positive post

   Start with the dead car. Wiggle the clamp so the teeth bite through surface corrosion and reach clean lead.

2. 2

   Red clamp to the good battery's positive post

   Same cable, other end. Keep the free black clamps from touching anything metal while you do this.

3. 3

   Black clamp to the good battery's negative post

   The donor car's negative terminal, or its designated ground point if the battery is not directly accessible.

4. 4

   Final black clamp to unpainted metal on the DEAD car's engine

   A bare engine mounting bolt, a strut tower bolt, or the manufacturer's marked ground stud. Not the dead battery's negative post. Pick something solid, unpainted, and at least a foot from the battery.

5. 5

   Start the donor car and let it run

   Two to five minutes at idle, or a fast idle around 1,500 rpm. This is less about charging the dead battery than about giving the starter something to draw from.

6. 6

   Start the dead car

   Crank in bursts of no more than 10 seconds, with 30 seconds between attempts. If it has not started after three attempts, the problem is not the battery.

7. 7

   Disconnect in exact reverse order

   Ground clamp off the engine first, then black off the donor negative, then red off the donor positive, then red off the revived battery. Never let a live clamp touch the body.

8. 8

   Drive for at least 30 minutes

   Highway driving, not idling in the driveway. An alternator at idle barely covers the car's own accessory load, let alone replaces what the starter pulled out.

A lithium jump pack has largely replaced cables for solo drivers, and for good reason: no second car needed, no polarity anxiety on most modern units, and it fits in a glovebox. A pack in the $70 to $120 range with 1,000 or more peak amps will start a typical four- or six-cylinder engine.

• Buy one rated for your largest engine. V8 trucks and diesels need meaningfully more current than a Corolla.
• Charge it every three to six months. A dead jump pack in the trunk is a paperweight, and this is the single most common way they fail people.
• Good units have reverse-polarity protection and refuse to output current until they see a correct connection. Cheap ones do not.
• The same ground rule applies — clamp the negative to engine metal rather than the battery post whenever the pack allows it.

What you hear when you turn the key narrows it down fast.

Rapid clicking, no crank

Still not enough current. Reseat the clamps on clean metal, let the donor run longer, or accept the battery is beyond a jump.

One loud click, no crank

Starter or solenoid. A jump will not fix it — see starter replacement cost.

Nothing at all, no dash lights

Broken connection, blown main fuse, or badly corroded terminals. Check the battery cable ends first.

Cranks strongly but never fires

The battery is fine. You have a fuel, spark or security-system problem — see car won't start.

Starts, then dies as soon as clamps come off

The alternator is not producing. Expect the battery light to be on. Do not drive far.

If the car starts fine on the jump but dies again after sitting overnight, you have either a battery at the end of its life or a parasitic drain. A battery over four years old that needed a jump is usually just done — see battery replacement.

Every hybrid and EV still has a conventional 12-volt battery running the computers, and that is the one that strands people. The high-voltage traction pack is irrelevant here and must never be touched.

• You can jump-start a hybrid or EV from another car using its marked 12-volt jump terminals, usually under the hood even when the battery itself lives elsewhere.
• Never use a hybrid or EV as the donor vehicle unless the owner's manual explicitly permits it. Their DC-DC converters are not built to supply starter-level current.
• Start-stop gas cars usually use an AGM battery, which must be replaced with another AGM and registered to the charging system. A standard flooded battery in that slot fails early.