From what I see using a D2, and yes I understand that's a different phone, the amount of battery it uses when navigating is not as significant as you might think. I'd say a 45 minute navigation brings my battery down about 10%, that's all and that's with the display on.
Now the Inc 2 may be a whole different ball game, but I don't see how it could possibly lose more charge when connected to a charger.
After two separate 6+ hour trips (one Sunday, one Monday), it charges with the charger I was using, but very slowly. I estimate that I was probably around 50% charge when I began my trip on Sunday, and after 6+ hours of navigating, it was only around 80% charged.
Same thing with Monday's, although the percentages were different.
I will note, I'm using the car charger that I got with my Droid 2 Global with the window mount for the DINC2.
EDIT: Also, when making this similar trip in the past with the Droid 2 Global, I never had any issues with it not charging up to 100% while navigating...and this was probably from a low of 30-40%.
Thanks, My OG Droid coming out of my Pioneer radio USB port or a Belkin Mini USB Cig adapter always gained ground. The phone charged when tethered too.
I read a test on the big Evo 4G that said the internals of the phone limited the incoming charge enough to cause a drain during navigation even when plugged in. I am hoping it is not the same for the Dinc2.
No drain, but definitely not charging as fast as I would have liked. Waiting for a response back from my Verizon rep on something I had inquired with him earlier on, going to talk to him about this as well.