Car batteries produce hydrogen gas. Not a lot, but enough. And if you create a spark right next to that gas — which is exactly what happens when you connect jumper cables — you can ignite it. That’s not a maybe. That’s chemistry.
The good news is there’s a specific order that keeps the dangerous spark away from the battery, and it takes about 30 extra seconds to learn.
AAA recommends this exact connection order:
1. Red clamp → positive (+) terminal on the DEAD battery
2. Red clamp → positive (+) terminal on the GOOD battery
3. Black clamp → negative (–) terminal on the GOOD battery
4. Black clamp → unpainted metal on the ENGINE BLOCK of the dead car — NOT the dead battery itself
That last step is the one most people get wrong. You’re grounding the final connection on bare metal somewhere away from the battery — a bolt, a bracket, something solid and unpainted. That way, when the inevitable small spark happens as you complete the circuit, it happens nowhere near the hydrogen gas the battery is giving off.
Disconnect in reverse order when you’re done. Last cable on is first cable off.
Beyond the order, a few other things matter. Meineke points out that corrosion on battery terminals can cause problems too — if you see that chalky white or bluish buildup on the terminals, wipe it off before connecting anything. And AAA says to wear eye protection and gloves if you have them. A battery explosion is rare, but the acid and debris it throws are not things you want near your face.
Park the working car close enough that the cables reach easily but make sure the two cars aren’t actually touching each other. Turn off electronics in both vehicles before you start. Once everything’s connected, start the working car first and let it run for a couple minutes before trying to start the dead one.
It really does take less than five minutes once you know what you’re doing. And knowing the order — positive dead, positive good, negative good, metal on engine block — is the difference between getting a friend home and making a very bad afternoon much worse.