Pardon my stupidity, but is it safe to simply use the 12v charging cable for my car all the time while traveling w/ nuvi? I can only see use for the battery when im out of the car adding favorites or car isnt on? Will the battery be ruined by not discharging fully and rechg" cycles? Will nuvi be harmed by only 12v use? Im just wanting clarity on use of 12v cable vs battery and for how long for safety of machine and battery? Also is the garmin nuvi 57Lm battery replaceable? Thank you for your assist .