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.