I have a MacBook Pro from 2011 (17", 16 GB RAM) that I use to run Civ 5 and Beyond Earth. I've run them in both Mac OS and Windows, via Boot Camp. (I've also tried them in Windows via Parallels, which ... If you had to ... Is possible.) So it sounds like we're on similar setups.
The Civilization games are extremely CPU- and GPU-intensive on this setup. I've played Civ 5 on this machine, on battery, successfully, but it was never pleasant -- and I was never able to get more than about 45 minutes out of the game before I felt I had to exit the game and put the machine to sleep.
In order to even get that much, I typically find myself turning the brightness on the display as far down as I can, while still being able to see it. That's always been my #1 battery-saving tip for anyone: Drop the display brightness. You'd be surprised how much THAT eats.
Aside from the brightness, generally speaking, anything that improves overall game performance is likely to improve battery performance while playing. So, some other things you can try include:
- Switch to strategic view. It's ugly, but in my experience (particularly late game), it performs much better than the pretty 3D rendered view. It may improve battery life as well, by being considerably less taxing to the GPU.
- Turn off movement and combat animation. Again, the less work the GPU is doing, the better your battery life will be. This will be less relevant (maybe irrelevant?) if you switch to strategic view.
- Use smaller maps. I'll occasionally play a Large map, but I usually won't go larger than that. Less data, fewer civs, fewer city-states, fewer megacities (for my civs, anyway), better all-around performance. But my preferred size is Standard, which doesn't (usually) seem to start taxing my fan until mid-game. And if you plan on trying to play a game entirely on battery (good luck), maybe even go Tiny or Duel.
Also, since you're running in Boot Camp, if you bought your copy via Steam or some other multi-platform option, consider running it on the Mac side. I did feel like I got better battery performance there than under Windows 8 in Boot Camp -- although I'll admit that I haven't benchmarked or tested that properly, it's just how it "feels." You can still install and use (most) mods, although it will be a bit trickier, and mods that change the DLL (expanded diplomacy, etc) won't work.
But ultimately, Civ 5 and Beyond Earth really want your laptop to be plugged in (and not on your lap, due to the heat it will generate) if you're going to play for a while. This isn't light word processing and e-mail here; it's a lot of computation and data processing, plus pushing out high-end graphics and sound. All that work is causing the CPU and GPU to run hot, which is kicking on your fan, which is further draining the battery. Basically, Civ games (and probably most games, to be honest) are inherently going to drain your battery very quickly. Plug in when possible.
Update for Windows 10: I can't speak to battery life per se, but performance of Civ V in Windows 10 seems to be considerably better for me (and uses the fan considerably less and less intensely) than it was in 8.1, on this exact same MacBook. (I've just switched it to be a primarily Win10 machine, in fact.) I partially attribute this to being able to use the DirectX 11 mode, and I partially attribute it to possibly-updated drivers as part of Win10's installation, and I partially attribute it to Win10 itself. Again, I can't speak to battery life in this world yet, but it seems like upgrading to 10 may also help.