My mom keeps losing her cell phone (I just get her the prepaid no contract cheapies for her for that very reason) and I told her to just leave it in the car on the charger (theft isn't really an issue), so she won't lose it, until some stranger told her that leaving it plugged in the charger in the car would kill the battery, so she took it out, and yet again lost it.

Is there any actual fact that leaving it plugged in will hurt it or the car? Its the only way I can make sure she knows where the phone is and that its actually charged in case she needs it. (Seriously, she has lost 6 or 7 phones.)

Thanks.