Jump-starting a car with jumper cables is a straightforward procedure, but doing it incorrectly can damage both vehicles or cause injury. This guide walks you through the safe, step-by-step process recommended by manufacturers and roadside safety organizations. Before you begin, always consult your vehicle owner’s manual for specific instructions, especially if you own a hybrid, electric vehicle, or a car with a battery located in an unusual place.
Before You Start: When Not to Jump-Start
Jump-starting is not always safe. Do not attempt to jump-start a battery if you notice any of the following conditions:
- Cracks or leaks on the battery case. Battery acid can cause burns or damage.
- Frozen battery. A frozen battery can explode when you connect cables. If the electrolyte is frozen (the battery feels solid or bulging), do not jump-start it.
- Corroded or loose terminals that cannot be cleaned or tightened safely. Loose connections can cause arcing.
- Strong smell of rotten eggs (sulfur). This indicates a leaking or overcharged battery that may be unsafe.
- Visible damage to the battery or cables.
If any of these conditions exist, replace the battery or call a professional. Some manufacturers recommend charging a deeply discharged battery with a battery charger instead of jump-starting it, so check the owner’s manual.
What You Need
To jump-start a car, you need a few basic items:
- Jumper cables of adequate gauge and length. See the section below on choosing cables.
- A second vehicle with a healthy battery of the same voltage (12 volts for most cars).
- Work gloves to protect your hands from acid or dirt. Inexpensive gloves are a good addition to your roadside kit.
- Safety glasses (optional but recommended).
- Owner’s manual for both vehicles, especially to locate remote jump posts if the battery is not easily accessible.
If you do not have a second vehicle available, consider a portable jump starter, which is a self-contained battery pack that does not require another car.
Connecting the Cables in Order
The correct connection order is critical to prevent sparks near the battery and to avoid damaging the electrical systems. Follow these steps exactly:
- Park both cars safely. Position the working car close enough to the dead car so the cables reach easily, but do not let the vehicles touch. Turn off both engines, remove the keys, and engage the parking brakes. Turn off all lights, accessories, and climate controls.
- Identify the battery terminals. On each battery, locate the positive (+) terminal (usually red and larger) and the negative (-) terminal (usually black and smaller). Clean any corrosion with a brush if safe to do so.
- Connect the red (positive) cable to the dead battery’s positive terminal. Clamp the red clamp firmly onto the positive terminal of the dead car’s battery. Ensure a good metal-to-metal connection.
- Connect the other red clamp to the good battery’s positive terminal. Attach the opposite red clamp to the positive terminal of the working car’s battery.
- Connect the black (negative) cable to the good battery’s negative terminal. Attach one black clamp to the negative terminal of the working car’s battery.
- Connect the last black clamp to an unpainted metal surface on the dead car, away from the battery. This is the most important safety step. Do not connect the final black clamp to the dead battery’s negative terminal. Instead, find a solid, unpainted metal engine bracket, a bolt, or the car’s frame, well away from the battery. This reduces the risk of sparks near the battery, which could ignite hydrogen gas. If you cannot find a suitable ground, consult the owner’s manual for a designated jump-start ground point.
Double-check that all clamps are secure and that the cables are not touching any moving engine parts (belts, fans, pulleys).
Starting the Cars
- Start the working car. Let it idle for a few minutes. Revving the engine slightly (to about 1500-2000 rpm) can help charge the dead battery faster, but do not race the engine.
- Wait 5-10 minutes. Allow the dead battery to receive some charge. If the dead car has a completely discharged battery, it may need more time.
- Try to start the dead car. Turn the key to the start position. If it cranks slowly or does not start, wait another few minutes and try again. Do not crank for more than 10 seconds at a time to avoid overheating the starter.
- If the dead car starts, let both engines run for a few minutes with the cables still connected. This helps stabilize the charging system.
Disconnecting in Reverse Order
Once the dead car is running, remove the cables in the exact reverse order of connection. This prevents sparks and protects the electrical systems.
- Remove the black clamp from the dead car’s ground (unpainted metal).
- Remove the black clamp from the good battery’s negative terminal.
- Remove the red clamp from the good battery’s positive terminal.
- Remove the red clamp from the dead battery’s positive terminal.
Be careful not to let the clamps touch each other or any metal surface while they are still connected to a battery. After disconnecting, stow the cables safely.
If the Car Still Will Not Start
If the dead car does not start after a few attempts, several issues could be at play:
- Poor connection. Check that all clamps are firmly attached to clean metal. Corrosion or loose clamps prevent current flow.
- Dead battery beyond recovery. A battery that is completely discharged may not respond to a jump-start right away. Try letting the working car run for 15-20 minutes with the cables connected, then attempt again.
- Faulty alternator or starter. If the dead car starts but dies immediately after removing the cables, the alternator may not be charging the battery. If it does not crank at all, the starter or a major electrical fault could be the problem.
- Hybrid or electric vehicle. Some hybrids and EVs have a separate 12-volt battery that may need a special procedure. Check the owner’s manual for the location of jump-start terminals and any specific instructions. Never connect cables to the high-voltage battery.
- Remote jump posts. Many modern cars have remote positive and negative jump-start posts under the hood, away from the actual battery. Always use these if indicated in the owner’s manual.
If the car still will not start after thorough attempts, it is time to call for professional roadside assistance. A roadside emergency kit should include a phone charger and a contact number for your service provider.
Choosing Jumper Cables
Not all jumper cables are equal. The two main factors are gauge (wire thickness) and length.
| Engine Size / Vehicle Type | Recommended Cable Gauge | Typical Length |
|---|---|---|
| Small cars (4-cylinder, compact) | 6 or 8 gauge | 10-12 feet |
| Mid-size cars, SUVs, vans (V6) | 4 or 6 gauge | 12-16 feet |
| Large trucks, diesels, heavy-duty | 2 or 4 gauge | 16-20 feet |
Gauge: Lower numbers mean thicker wire and better current flow. For most passenger cars, 4-gauge cables are a good balance of flexibility and capacity. Thinner cables (8 or 10 gauge) may work for small engines but can overheat or cause voltage drop on larger vehicles.
Length: Longer cables give you more flexibility in positioning the cars, but they also have more resistance. A 12- to 16-foot cable is sufficient for most roadside situations. If you own a truck or often help others, consider 20-foot cables.
Insulation and clamps: Look for cables with thick, flexible insulation that remains pliable in cold weather. Clamps should have strong springs and wide jaws that grip the terminal securely. Some cables include a built-in polarity indicator or surge protector, but these are not essential.
When you compare jumper cables, check both the gauge and the length on the label. Keep a set in your trunk along with your other emergency gear.
FAQ
Can I jump-start a car with a battery that is frozen or leaking?
No. A frozen battery can explode when you connect the cables. A leaking battery may spray acid. In either case, do not attempt to jump-start. Replace the battery or call a professional.
What if my car has a battery in the trunk or under a seat?
Many cars with batteries in unusual locations have remote jump-start posts under the hood. Check your owner’s manual for the exact location of the positive and negative terminals. Never try to connect cables directly to a battery that is not easily accessible.
Why should I not connect the last black clamp to the dead battery’s negative terminal?
Connecting directly to the negative terminal can cause sparks near the battery, which may ignite hydrogen gas emitted from the battery. By connecting to an unpainted metal ground away from the battery, you reduce this risk significantly.
How long should I let the dead car charge before trying to start it?
Typically, 5 to 10 minutes of idle time on the working car is enough for a moderately discharged battery. If the battery is very dead, you may need 15 to 20 minutes. If it still does not start after that, the battery may need to be replaced.
Jump-starting a car is a skill every driver should know. By following the correct connection order and safety precautions, you can get back on the road quickly without damaging your vehicle. Always keep a good set of jumper cables in your trunk, and refer to your owner’s manual for any model-specific instructions. For more information on building a complete roadside emergency kit, see our checklist.
