When you leave an act, you lose all your keys, which means that you have to find them in the new act. However, there is a way to get around that. Place a chest in your inventory. You can put whatever you want in it to save items without having to carry them all around with you. I set the chest down in the town in the battlefields. I stored several swords and armor that I wasn't able to use yet for lack of strength or agility. I also stored all my extra crystals, charms, books and all my battlefield keys there. Before going to the next act, I placed the chest back into my inventory. When I got to Act 3, all the stuff, including my battlefield keys were still in the chest. When I clicked on the keys, all the battlefield dungeons were available for Act 3.
