If you play at PC, I think you can force to close the programm during the fight and save the key that way.

Yes, simply close the app if you're on mobile. 

Thanks

Plarium please fix CB key timer. The 6 hours should start when key is used not at end of battle. some battles are 20 - 30 mins

So, if the connection is lost, is it okay to also lose the key?

I believe if combat ends unexpectedly, either by turning your phone off or lost connection, the key is not spent.

Angwil is correct. Lost connection or phone dying, etc.... none of those things lose the key. :)

Nope. That's the way it used to be and it's now fixed. The problem was if the game went down, you lost connection, or accidentally turned off your phone, the key was already spent,  It was a huge issue losing a key when your combat unexpectedly ended. Not charging the key until the battle is over IS the fix.