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Chris (aka-Moose)
Chris (aka-Moose), Technician
Category: Car
Satisfied Customers: 47332
Experience:  16 years of experience
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how long does it take to charge an empty car battery by driving

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how long does it take to charge an empty car battery by driving around so much that i can safely assume i can at least start the car next time i use it?
Hello I am Chris.
When you jump a dead battery and run the engine for even five minutes, you can often shut the car off and immediately restart it. This will be a surface charge, which is not sufficient. To get a deep charge enough so you can come back 4 hours later and start the car, you need the car to run at least 30 minutes. If you are going to let the car sit overnight, it needs to run at least an hour.
This is assuming the battery is good and the alternator is working correct. If you can drive with the least amount of electrical loads on, it will charge much better.
If you by chance use a charger, it will take about 6 hours on 6 amps to charge a battery fully.
Customer: replied 4 years ago.
Yes, so had to jump the battery yesterday because I left the light on. Was 100% dead, nothing happened when turning the key, not even a dash light.Drove to work, a 15 min commute, and when I tried starting the car later in the evening, it made a few minor clicking and dash lights flickered briefly but way too low of a charge to start it.So, I drove around yesterday for maybe 45-60 min and now hope that it will start tomorrow.I limited transmission to 2nd gear and put O/D on (2005 Scion xB) to get higher RPM - would that have positively affected the alternator output?
The alternator output will be enough at idle, there is no need to have the engine rpm higher to get more alternator output.
Thanks for following up
Customer: replied 4 years ago.
Cool. Just out of interest though - would higher RPM help?
Higher RPM will not really when charging only one battery. If your car was running and you were trying to jump off another car, bumping the RPM up to 1500-2000 will help but not much. At idle the alternator can put out 15 volts, which is the max voltage required.
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