Battery life also depends on your mappings. In my case ive noticed my battery life decrease or extend after mappings with my Audi. I also assume thickness of hair and skin are a factor too. The more the processor has to work the quicker the shorter the battery life will be.
I use the N7. The compact rechargeable battery gives me anywhere from 7-8 hours. The standard can get me to around 12-13 hours
Good Luck