All iPads iPad not charging while playing X-com EW

I started playing X-com enemy within with battery at 100% and ipad plugged to 12w original charger.
Now 7 hours later the ipad is at 12% battery! 88% battery drain in 7 hours?

How can the battery drain while ipad is in use when its also plugged into 12w charger? Usually it just charges very slow when playing but not draining of battery?!

I have played x com enemy unknown many times and never experianced this battery drain before on any app or any game.