Sudden hibernation under El Capitan

My mid-2012 15" macbook pro retina is shutting down into hibernate with 25-35% of the battery charge remaining. The only fix seems to be to plug it in (even if momentarily) then hitting the power button. This started with El Capitan. Ugh. Not great for a flight when I'm expecting to get 2-3 more hours of use and don't have a plug to "reset" it.

I've reset the SMC (which seemed to help overall battery use under El Capitan, but didn't stop the hibernation.) Battery life checks out "normal" with 617 charge cycles. Total battery capacity is down about 12% since the machine was bought new.

Any ideas?