I would go down the lines of getting an external battery pack that does usb but itself can be charged through the car charger. The voltage would be much smoother out of the battery pack and there would be much less likely hood of damaging the charging circuit of the DR800. It would be less expensive to replace the external pack than to replace the circuit board of the 800.
And you can carry the pack around with you if you want or leave it in the car.