do both cars need to be on to jump

[DelVivienne]

Yes, the car providing the jump (the donor car) must be running, but the car with the dead battery should be completely off. Starting the donor car before connecting the jumper cables is a critical safety step. It allows the donor car's alternator to actively supply power, stabilizing the electrical system and providing the necessary amperage for the jump-start. This prevents the good battery from draining and protects both vehicles' sensitive electronics from a damaging voltage spike.

Connecting cables to a running donor car is safer. A running engine means the battery is being charged, which reduces the risk of a dangerous spark when you make the final connection to a grounded metal surface on the dead car. The correct order is: connect positive to positive on both batteries, then connect the negative cable to the donor car's negative terminal, and finally attach the other negative clamp to an unpainted metal bolt or bracket on the dead car's engine block, away from the battery.

Once everything is securely connected, let the donor car run for a few minutes to transfer some charge to the dead battery. Then, attempt to start the disabled vehicle. If it starts, leave both cars running while you carefully disconnect the cables in the reverse order of connection. This allows the newly started car's alternator to begin recharging its battery. Driving the jumped car for at least 20-30 minutes is recommended to ensure the battery recovers enough charge to start again later.

Key Consideration	Why It Matters
Donor Car Engine Status	Must be ON to supply stable voltage and amperage via the alternator.
Dead Car Engine Status	Must be OFF to prevent electrical surges and ensure a safe connection.
Recommended Idle Time	Let donor car run for 3-5 minutes before attempting to start the dead car.
Post-Start Procedure	Keep both cars running for several minutes after a successful jump.
Minimum Drive Time	Drive the jumped car for 20-30 minutes to recharge the battery sufficiently.

[AsherLee]

From my years in a garage, the rule is simple: the donor car runs, the dead car stays off. A dead battery can create a huge power drain. If the dead car is on, it can try to pull power the instant you connect, potentially stalling the donor car or frying electronics. A running donor car has its alternator working, which acts like a buffer, handling the massive current draw needed to crank the other engine. It's all about managing that initial surge safely.

[AlinaDella]

I learned this the hard way. I once connected the cables with both cars off, and when I started the working truck, there was a loud pop from the dead sedan. My mechanic friend explained that the dead car's electronics "woke up" when power hit and created a surge. Now I always start the good car first. Its running system is designed to handle loads smoothly, which protects the computers in both vehicles. It’s a small step that prevents very expensive repairs.

[OSienna]

Think of it like this: the running car is the active power source, and the dead car is the empty container. You need the source to be actively pumping energy into the container. If the dead car is "on" in any accessory mode, it's like the container has a leak; it's using what little power it's getting instead of saving it all for the engine crank. Keeping everything off on the dead car ensures all the donated juice goes straight to the starter motor where it's needed most.

[SanSabrina]

Modern cars make this even more important. With all their computers, a simple jump-start can cause havoc if done wrong. The donor car must be running to ensure a stable voltage supply. A fluctuating voltage from just a battery can corrupt the software in the dead car's control modules. I keep the dead car completely off—no lights, no radio, nothing. This minimizes the electrical load and gives the battery the best chance to accept enough charge to start. It’s a non-negotiable safety protocol.