I was jumping another car with a 12 V car battery and it jumped the other car. However, it seemed to kill the 12 V car battery in my car and then I couldn't start mine. I used a multimeter and found out that the battery only had about 9 V left in it. Can someone explain what caused this to happen? Could I have caused a short or something?
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1\$\begingroup\$ How old is the battery? Has it had problems starting your car? \$\endgroup\$– JustmeCommented Jul 22, 2021 at 9:35
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\$\begingroup\$ @Justme - Which car? My car that was alive, had the OEM 12 V battery, that is roughly 11 years old. My other car that was dead had a brand new bosch battery. It was dead because I hadn't started it in awhile. \$\endgroup\$– AngryParticleCommented Jul 22, 2021 at 9:37
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\$\begingroup\$ No wonder it died - typical car battery life is about 5 years, so after 11 years it could not handle being connected in parallel to a new but discharged battery. \$\endgroup\$– JustmeCommented Jul 22, 2021 at 10:11
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1 Answer
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You normally leave your car running while jump starting another car.
"Jump starting" transfers energy from the battery in your car to the battery in the other car as well as providing current to the starter motor. Charging the battery in the other car lowers the charge of your battery. The starter motor draws a lot of current, further reducing the charge in your battery.
What you have there is not unexpected if you jump a car without the motor running.
You'll need to charge the battery of your car with a regular charger - or find someone to jump start your car. Just remember that the car providing the boost should keep its motor running.
When you leave the motor on your car running while jump starting another car, you get several advantages:
- You can rev your motor and charge the other car from your alternator instead of from your battery - your battery doesn't discharge as much.
- You can put more charge into the jumped car battery because you can let your motor run for a while to charge it. If you just use your battery, then the two batteries will equalize at some point lower than a full charge. Say the "dead" battery only has half a charge and yours is full. If you charge the dead one from your battery, then both batteries end up with something less than 75% charge.
- Your battery doesn't have to crank up your motor when you are done. The motor is already running so once you disconnect the jumper cables it goes straight back to recharging your own battery.
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\$\begingroup\$ I did have my car on and reved my car to charge the other car's dead battery. Although after I turned on the dead car, I turned off my car and then removed the jumper cables then removed it from the dead cars battery. \$\endgroup\$ Commented Jul 22, 2021 at 9:35
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4\$\begingroup\$ There's the problem. You shut off your engine before it had time to recharge your battery. \$\endgroup\$– JRECommented Jul 22, 2021 at 9:35
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\$\begingroup\$ Shoot. I should've asked this question earlier. I ended up buying a new battery.. Is there away I could still salvage the old 12 V battery by recharging it? I haven't disposed the battery yet. \$\endgroup\$ Commented Jul 22, 2021 at 9:40
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\$\begingroup\$ Probably. It ist probably just discharged. Charging it with a regular battery charger will probably fix it right up. Some places that sell car batteries can also test one if you bring it in. \$\endgroup\$– JRECommented Jul 22, 2021 at 9:41
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2\$\begingroup\$ Nothing will fix a 11 year old car battery as they last about 5 years (depending on use). When they fail, they fail, and the best thing to do is to recycle them, a battery with 9V means there are 1 or 2 dead lead-acid cells so it will not hold a charge any more. \$\endgroup\$– JustmeCommented Jul 22, 2021 at 10:13