Something worth considering is buying a laptop specifically designed for long battery life. If you aren't very particular about other features you may be able to get a used one cheap on ebay..
Being able to move to an ssd would be a side benefit.
Uhh... I'm quite peculiar with my hardware preferences. A huge screen, a large keyboard, win7, and a price cap of $400 (really) were the musts dictating the choice. I'm lucky that newegg gave me this one. And I'd rather stick with it, rather than migrate to a win8+ thingie.
An alternative approach could be an affordable refurb Chromebook with top specs (such as Dell Chromebook 13 core i5 with 8GB RAM), and then use Chrome Remote Desktop to access your existing Windows laptop, if you really have to.
I've just got one of these Dell deals recently, and the battery life is incredible. It's past lunch time over here, I've already been using it today for a couple of hours, and it says I got 13 hours of battery life left.
Yesterday I used it for a full work day, and it had 3 hours left when I got back home. I also use it to access my workplace Windows systems using VMware Horizon Client, when I must use MS Word and such.
And then of course Chrome OS with i5 and 8GB RAM just flies, the fastest browsing I've ever experienced.