Jump-starting an automobile is an endeavor that can sometimes be fraught with difficulty—and even danger in rare scenarios. Sure, most of the time you’ll be able to bump start vehicles fitted with a manual transmission. However, if you drive an automatic, you’ll need to break out the jumper cables and phone a friend.
Sometimes if the battery or electrical system in your car is too far gone, a standard jump start won’t do the trick. This is where some mechanics suggest revving the engine of the donor car to provide the extra juice needed for it to spark back to life. As (ill) luck would have it, one of our editors has been having battery trouble with his Jeep. So we took the opportunity to put this revving theory to the test.
How To Safely Jump Start a Car
While a routine jump start may sound relatively harmless on the surface, it’s easy for newcomers to get mixed up with which jump lead needs to go where and at what time. Thankfully, this process is much easier than you think.
Before you even reach for the jumper cables, be sure to position the vehicles properly. Ensure that the front of the donor vehicle is as close as possible to the disabled one. Jumper cables are only so long, and this will ensure they reach without any risk of being accidentally yanked off the terminals.
Once the hoods of both vehicles are up and the engines turned off, begin by connecting one of the positive (red) alligator clips to the positive terminal of the donor vehicle and then to the positive terminal of the dead battery. Then proceed to connect the negative (black) alligator clip from the donor vehicle to the dead vehicle before beginning the process—some might suggest connecting the negative terminal to a solid piece of metal on the disabled vehicle, but we noticed little difference.
Does Revving Make a Difference? In Short, Barely.
Unfortunately, our first attempt at jump-starting the dead Jeep was unsuccessful regardless of whether we were revving the engine or not. Sometimes vehicles are too far gone for a jump-start to save them, and that was likely the case here. To be sure, we took a volt-meter to the “dead” battery, and it registered 4.65 volts. For some context, the electrical system in your vehicle needs roughly 12.5 volts to successfully start.
After our initial failure, we assumed that the old battery was just too far gone. We then proceeded to measure voltage from the battery terminals of the donor car, rather than the terminals of the disabled vehicle. This confirmed that, yes, revving the engine does make a slight difference in the amount of voltage supplied by the donor car. However, it was a gain of merely a few hundredths of a volt—not enough to revive a dead battery. Wade Hughes from Sun Devil Auto in Sun Lakes, Arizona, says that it’s much more productive to leave the dead battery plugged in for a little while to allow it to charge up if the vehicle doesn’t immediately start.
In our testing, the dead battery proved to be too far gone for any of these techniques to spark it back to life. It took a full-on battery swap to revive the Jeep. Therefore, to give this hypothesis a Mythbusters-type rating, we rule it’s plausible.