can you jump a car without the other car running

[LaOlivia]

No, you cannot safely jump-start a car without the other vehicle's engine running. The running engine in the donor car is critical because it allows the alternator to generate the necessary electrical current. Attempting a jump-start with the donor car off can damage both vehicles' sensitive electronic systems and will likely not provide enough sustained power to start the dead battery.

The primary role of the donor car is to act as a portable power source. When its engine is off, it's relying solely on the charge of its own battery. A healthy car battery is designed to deliver a very high amperage burst for a few seconds to start its own engine, not to simultaneously charge a completely dead battery and start another car. The dead battery acts as a massive power sink, rapidly draining the donor battery. This can lead to both batteries being depleted, leaving you with two stranded cars.

Furthermore, modern vehicles are equipped with complex ECUs (Engine Control Units), infotainment systems, and various sensors. A jump-start attempts to create a stable electrical circuit between the two cars. Without the alternator from the running donor car regulating the voltage, you risk dangerous voltage spikes or fluctuations that can fry these expensive components. The alternator ensures a steady flow of power, making the process safer and more effective.

A quick comparison of the power source:

Power Source	Voltage Output (Approx.)	Can it reliably jump-start?	Risk to Electronics
Donor Car's Battery (Engine OFF)	12.4V - 12.7V (surface charge)	No, voltage drops rapidly under load	High risk of voltage drops/spikes
Donor Car's Alternator (Engine RUNNING)	13.5V - 14.5V (steady charge)	Yes, provides a continuous, stable flow	Low risk when properly connected

The correct and safe procedure is always to have the donor car running at a moderate RPM (around 1500-2000 RPM) before connecting the jumper cables and attempting to start the disabled vehicle. This ensures the alternator is generating ample power to charge the dead battery and support the electrical load, protecting both vehicles.

[LaCayden]

No way, don't try it. You'll just end up with two dead batteries. The good car needs to be running so its alternator can do the heavy lifting. Otherwise, your good battery is trying to fill a dead battery by itself, and it'll lose that fight every time. Just start the working car, let it run for a minute, then hook up the cables. It's the only method that actually works without causing more problems.

[JesseLee]

I learned this the hard way helping a neighbor. His SUV was dead, and I figured my truck's big battery could handle it without me starting the engine. We hooked up the cables, and my truck's dashboard lights dimmed dramatically when he tried to crank. Nothing happened except my truck now struggled to start. A mechanic friend later told me I was lucky I didn't fry my computer. The running engine is what creates the stable power you need; the battery alone isn't designed for that task.

[William]

Absolutely not. The core reason is electrical safety and system protection. A car's battery provides the initial cranking power, but the alternator—which only runs when the engine is on—maintains stable voltage. Jump-starting without the donor car running forces its battery to attempt a massive charge cycle it's not designed for. This causes a severe voltage drop, which can corrupt data in the recipient car's electronic control modules. The potential repair costs for damaged electronics far outweigh the minute of fuel used to idle the donor car.