With an extended battery life much needed for anyone who wish to shoot images and videos in places power points are few between, you need another Nikon EN-EL3e battery in your MB-D80 battery grip.
Latest figure from Nikon claims the battery will last an average of 850 shots per charge, although using it to record video consumes far more power. Conservatively, unless you are video recording 5-minutes clip every 6, the two Nikon batteries are estimated to churn out approximately 2.5 hours of recordings (overheating may occur), plus about 500 still shots.
Now, if you are planning to do most still shots like me, medium-duration road trips lasting 7-10 days (with average daily shots of 150) can be performed without bringing the charging adapter supplied with the Nikon D90 along.