I have been a supporter of the Surface Pro 2 since release and while I had no problems with initial released Power Covers, THREE of the ones I purchased late 2016 all seem to have a problem in not charging, either instantly the Surface Pro 2 recognizes the device - at zero charge then jump to listing the full capacity and says the secondary battery is not charging but plugged in.
I have searched online with as best of a context given as possible. Since the Power Covers are fitted with the same consumer electronic lithium ion batteries, I searched with varying results on how to restore lithium ion batteries and all seem to give a general answer of plugging in the battery to a USB outlet adapter (which would mean taking a Surface Pro 2 type cover connector part, and soldering the power leads). But this is also the kicker, the Power Cover won't be in a charging state if the Surface Pro 2 itself is not in a certain sleep state (meaning there has to be a certain signal to the middle ball pins that itself is just another USB arrangement - to give some sort of signaling to the Power Cover).
Finally, no where online is there any information on the "wake up" voltage needed to wake up perfectly new in box, sealed, sitting for a long time Power Covers (situation you and I are in). Information online also states that Lithium ion batteries that sat without being charged are usually at close to near empty and would require a jump start if I deduced sources right to be in a charging discharging cycle again. I tried experimenting with a DC power generator and soldering the Surface Pro 2 connector part to the leads and tried adjusting the power but I have yet to "wake" up the Power Covers I recently bought.
If someone can chime in to as what the specification of the Power Cover batteries are, that would be extremely helpful in going to attempt to restore charging capabilities to the covers.