maps app uses triangulation off cellphone masts... basically it's a huge battery drain as it's sending/receiving signals constantly...
It shouldn't noticeably drain your battery when it's plugged in to mains/charger though. It may be that your in-car charger isn't delivering enough power to compensate-I'm not sure what the output of in-car chargers are.. I'd start troubleshooting with the charger/USB connector; look for damaged pins/wire/dirt.