I'm curious for those using laptops as their Car PCs, what kind of times are you typically getting for how long your laptop can stay in "standby" mode on battery?

I've recently picked up a really small Medion Akoya LS laptop that I'm integrating as my CarPC and even though its got a pretty small battery I'm getting pretty amazing stanby times. (I've tested it a couple of times now - after a good 20 hours of standby on battery it still is reporting 3/4 charge)

What I'm wondering is how much that is based on this laptop (and it's battery) being brand new and how long I might expect it to continue to work like that, plus I'm wondering how much temperature conditions in the car (in both directions) are going to effect this.

Matthew