3800mAh Battery for iPod 5th - 7th Generation Video Classic, High Capacity 616-0232 Battery with iPod 5th 30GB 6th 80GB 6.5th 120GB 7th 160GB with Repair Tool Kits

Still works with Zunes, is plug-and-play, and super simple to install. I have this battery in two of my Zunes now and works great lol. If you want a larger battery that requires a little more modding, plenty of link/posts that go into how, you can get this battery (and yes I know it says iPod but works with your Zune as well)