when jumping a car should both cars be on

[CoreyFitz]

No, both cars should NOT be running when you start the jump-start process. The correct and safe procedure is to have the donor car (the one with the good battery) running and the dead car completely off when you make the final connection and attempt the jump. Starting the dead car with the donor car already running provides the necessary stable electrical current from the donor's alternator, protecting both vehicles' sensitive electronic systems.

The primary risk of having both cars on stems from voltage spikes. A car's electrical system is designed to operate within a specific voltage range, typically around 12-14.5 volts when the engine is running. When you start an engine, especially one with a weak battery, it can create a significant and sudden power surge. If both engines are running, this surge can travel through the jumper cables and damage expensive components in the donor car, such as its Engine Control Unit (ECU), infotainment system, or other onboard computers. Modern vehicles are packed with complex electronics, making them more vulnerable to such spikes than older models.

The safest sequence is as follows:

1. Connect the jumper cables in the correct order: positive (red) to the dead battery, then positive to the donor battery, then negative (black) to the donor battery's negative terminal, and finally, the last negative clamp to an unpainted metal surface on the dead car's engine block (not the negative battery terminal).
2. Start the donor car and let it run for a few minutes to allow its alternator to charge the dead battery slightly.
3. With the donor car still running, attempt to start the dead car.
4. Once the dead car is running, you can leave both cars idling for a few more minutes before carefully disconnecting the cables in the reverse order of connection.

Adhering to this method minimizes the risk of electrical damage and ensures a safer jump-start process for both vehicles.

Safety Consideration	Why It Matters	Potential Consequence of Error
Voltage Spike Protection	A running donor car's voltage regulator stabilizes the electrical system.	Damage to ECUs, sensors, or audio systems in either car.
Battery Gas Venting	A charging battery emits flammable hydrogen gas. A spark near the battery can cause an explosion.	Connecting the final clamp to a ground point away from the battery minimizes spark risk.
Correct Cable Connection Order	Establishes a circuit safely, reducing the chance of a short circuit.	Reversing polarity can instantly destroy vehicle electronics and cause cable melting/fire.
Cable Gauge and Quality	Thick, high-quality cables (e.g., 4-gauge or lower) minimize resistance and heat buildup.	Thin, cheap cables can overheat, melt insulation, and cause a fire during the high-current draw of starting.
Donor Car Engine RPM	Revving the donor car's engine slightly (to around 1500-2000 RPM) after connections are made can provide extra charging current.	Not necessary for a successful jump, but can help in extreme cold or with a very depleted battery.

[ElliottLynn]

Turn the donor car on, but leave the dead car off until you're ready to crank it. The running car acts like a steady power supply, preventing a nasty voltage spike that could fry your car's computer when you turn the key. It’s all about protecting the expensive electronics. Once the dead car starts, then both can run together while you disconnect the cables.

[DiThomas]

From a safety standpoint, having both cars on is a significant risk. The moment you engage the starter on the dead vehicle, it creates a massive current draw. If the dead car's engine is already running, this draw is unnecessary and can cause a dangerous voltage drop or spike across both electrical systems. This instability is a leading cause of damaged control modules. The procedure is designed to manage this surge in a controlled way.

[VanJoseph]

I learned this the hard way years ago. I helped a friend jump his SUV, and we had both trucks idling when he tried to start his. There was a loud pop from his engine bay, and afterwards his power windows and radio were dead. A mechanic told us a voltage spike from the dual-running setup blew a module. It was a costly mistake. Now I always make sure only the donor car is running until the jump is complete.

[StKiara]

Think of it like this: you're giving a weak heart a jump start. You wouldn't shock a healthy heart and a failing one at the same time. The donor car running is the healthy, steady rhythm. You use that stability to gently encourage the dead car's engine to turn over. Once it's alive and idling on its own, both "hearts" can beat together. The key is that initial stability to avoid a dangerous electrical "arrhythmia" that can damage both vehicles.