I have been using the car charger for my wifes ally, and also a universal charger, but it can't keep the phone charged up under load. I can use the google navigation + bluetooth, and it just cant keep up. Would the original charger for the droid keep up, or is this just the case for all car chargers.
Unfortunately, most car chargers provide the regular 500mA that USB ports provide, instead of the ~850mA that wall chargers provide. In a situation like that, they just can't keep up. Your best bet is to use setCPU or similar to make a profile that underclocks your phone while charging. This will keep the processor/battery temps lower, which means your battery discharges slower AND accepts a charge faster. It also obviously lowers power drain from the processor, so it discharges slower. Depending on what you're running in your car, your phone should run fine at lower speeds. I run my D2G at 600MHz max a good bit, and barely notice a slowdown in every day use.
I bought a 1A car charger from monoprice.com: For only $0.99 each when QTY 50+ purchased - Car Charger (Cigarette Lighter) to USB Female Converter - White | Car Charger Accessories (white) For only $1.00 each when QTY 50+ purchased - Car Charger (Cigarette Lighter) to USB Female Converter - Black | Car Charger Accessories (black) they do a better job b/c they can provide the full current that the DROID requires for charging. However, with Nav + Pandora + BT (running audio through a Motorola T505 BT to FM transceiver) it would barely keep the thing at he same charge, and over a very log period of time the phone would still eventually drain out. HTH
nav really kills your battery... i use my lg dare car charger still... that phone was a pos but at least it had a microusb port lol