Considering the apparent popularity of Asus convertibles, I'm surprised it's taken so long for Microsoft to do this. Behold the new Surface Power Cover, which includes an auxiliary battery and is now available for pre-order on the Microsoft Store. The asking price is $199.99, and shipments kick off on March 19.
The Surface Power Cover looks a lot like the $129.99 Type Cover. It attaches to the Surface tablet magnetically, folds over the screen, and has a laptop-style keyboard as well as a touchpad. Unlike the Type Cover, though, the Power Cover includes a battery that joins forces with the Surface's built-in one to extend run times by "up to 70%"—or so Microsoft claims. The original Surface, Surface Pro, and their second-gen successors are all supported.
Oddly enough, Microsoft seems to quote the same 0.55-pound weight for both the Type Cover and the Power Cover. I'd have thought the Power Cover would be heavier, since, you know, it has a battery inside. Weird. Maybe the Microsoft Store listing is just wrong.
Now, all that's left for Microsoft to introduce is a proper keyboard dock with a hinge—you know, the kind that will let you use a Surface in your lap. Existing Surface models probably don't have the right latching mechanism for that kind of accessory, however. Oh well.