We can’t mix up the new AA battery with the old one. Because the old battery has low energy, less power; while the new battery owns high energy, more power; so when mix them up, the old battery would influence the service time of the new battery, which usually will shorten the service time. Besides, the new battery makes the old one over discharge and then causes leakage, and damages the electric appliance. These are the reasons why can’t mix them up.