There's documentation here for the 2mm charger; I don't think 14V from a car battery is likely to harm the N800.

BTW, if you make your own charger, and know that you won't use an accessory between charger and N800, check the accessory output spec (Figure 11) -- a 9V battery with an 8.2 Ohm resistor will work, with battery voltage up to 9.3 (fully charged), down to about 7.2 (each cell drained to 1.2 V). (This is a violation of the charger spec, but is guaranteed to work with any phone, N8x0, etc. that follows its end of the spec, as long as you don't use an accessory.)

http://www.forum.nokia.com/main/reso...interface.html