When rechargable battery age, what quality decreases?
Some energy is lost in the resistance of cell components each time the battery is charged or discharged. So the remaining chemical energy is slightly less each charge. The battery will still output about the same voltage but will run out of steam earlier each time. When the battery is nearly dead the voltage will start to decrease at a rapid rate. It's like water pumped in a pipe between two reservoirs. You can pump the water back and forth a thousand times and lose a bit each time to small leaks. When you lose enough water that you can't fill the pipe completely to the top your voltage will decrease, and you're nearly out of water.