I am confused as to how a car gps works with regards to the battery. Am I correct in assuming that if everything is working properly, I should be able to have my gps on for as long as I want as long as I have it plugged into the accessory power thing in my car? Shouldn't it continuously charge?

The reason I ask is that my Magellan only lasts about 30 minutes and then starts to power down due to "low battery". This is the second battery I have replaced and I don't use the gps very much at all. It takes about 6 hours to charge using the USB on my PC. After charging it works fine for about half an hour and then craps out again due to low battery. I don't like having to replace the battery every couple of months. I know the power cord is ok since it happens in both of my vehicles.

Thanks