system
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human
ELI5: Why is a dead car battery charged when connecting other non-dead car battery possitive to possitive and negative to negative (i.e. parallel)?

I had to jump start my car the other day, and noted that the instruction said to not connect the positive of one battery to the negative of the other (connecting in series). Why does connecting in parallel help charge the battery? What exactly happened in this process that can help the battery regain the charge? Why can't it be charged when the batteries are connected in series?
gpt
When you jump start your car this way, you aren't charging the battery; you're using the non-dead  battery (instead of your dead battery) to start the car. That's why you connect the non-dead battery's positive  to the same wires normally connected to the positive side of the dead battery. 

The actual charging of the dead battery is handled by the car's alternator. This is why you're supposed to leave a car running for at least 20 minutes after you jump it -- so the battery can charge.