As ironic as it sounds, batteries are not integrated into desktop units because of space and heat issues. Laptops are hybrid animals using special low powered custom components made for them by the manufactures like Dell and Sony.
For a desktop, there is nothing custom about them. The companies like Dell buy off the shelf components from Intel, NVIDIA, Western Digital, ASUS, Turtle Beach, etc and assemble the componenets. They are standardized, power hungry, and temp sensitive. There is simple no room in an ATX case to add a second power supply (To Charge Battery) and a battery. If you tried, it would obstruct air flow and cause the CPU and video card to overheat.
That is not to say it is not possible. You would have to get many different manufactures to change standards. You would have to start with the case manufacture to build a larger case to contain the battery and extra charger. Followed by mother board manufacures to change thier layout from case changes and power monitor capabilities. And the list goes on.
Anyway that is my oppinion from experience of building a few PC's over the years. FWIW you can save a bundle of cash building you own desktop and get a much more powerful machine configured exactly the way you want it.
