can you use 2 batteries to jump start a car

[VanCooper]

Yes, you can use two batteries to jump-start a car, but it must be done correctly to avoid serious damage to the vehicle's electrical system. The safest and most common method is to connect the two batteries in parallel using jumper cables. This means connecting positive to positive and negative to negative, which combines their amperage without increasing voltage, providing the necessary cranking power to start the dead engine.

The Correct Procedure: Parallel Connection

1. Position the Cars: Ensure both vehicles are off and in Park or Neutral with the parking brakes engaged.
2. Identify Terminals: Locate the positive (+) and negative (-) terminals on both batteries. Clean off any corrosion.
3. Connect Positive Cables: Attach one end of the red positive cable to the dead battery's positive terminal. Connect the other end to the positive terminal of the good battery.
4. Connect Negative Cables: Attach one end of the black negative cable to the good battery's negative terminal. Crucially, connect the other end to an unpainted metal surface on the dead car's engine block or chassis, not to the dead battery's negative terminal. This provides a ground and minimizes the risk of sparking near battery gases.
5. Start the Donor Car: Start the engine of the car with the good battery and let it run for a few minutes.
6. Attempt to Start: Try to start the car with the dead battery. If it starts, leave it running.
7. Disconnect in Reverse Order: Carefully remove the cables in the exact reverse order: black negative from the ground on the jumped car, black negative from the donor battery, red positive from the donor battery, then red positive from the jumped car.

Risks and Important Considerations

Attempting to connect the batteries in series (positive to negative) will create a 24-volt system, which can instantly destroy your car's 12-volt electronics, including the Engine Control Unit (ECU). Modern vehicles with complex electronics are particularly vulnerable. Using a jump starter pack is a safer, more convenient alternative that eliminates the need for a second vehicle.

Scenario	Recommended Action	Key Risk
Standard Jump-Start	Use one good car battery via jumper cables.	Incorrect cable connection.
Two 12V Batteries (No Car)	Connect in parallel only.	Creating 24V in series destroys electronics.
Dead Battery in Hybrid/Electric Car	Consult owner's manual immediately.	High-voltage systems are extremely dangerous.
Battery is Old or Physically Damaged	Replace the battery; jumping may be ineffective or unsafe.	Internal short circuit.
Jump-Start Fails Repeatedly	Problem may be the starter, alternator, or a blown fuse.	Requires professional mechanic diagnosis.

[Colin]

I learned this the hard way. My truck's battery was completely dead, and a friend had a spare marine battery in his garage. We hooked them up just like you would with another car: red to red, black to black. It worked, but we were super careful. The key is making that last black cable connection to a bare metal bolt on the engine, not the dead battery itself. It sparked a little, but the truck started right up. It felt like a real MacGyver moment.