The general rule of batteries like AA, AAA, C, D types, is that you should always replace old batteries with three NEW ones of EXACTLY the same type. The reason for this, is that if you use three batteries of different capacities, then the device usually stops working when the lowest capacity battery goes flat. But worse, the other batteries can then reverse the energy flow through the flat battery making it likely to leak and destroy whatever you have in it.
However, YOU CAN USE OLD BATTERIES. Any for anyone who has kids, who are constantly breaking things or deciding they’ve grown out of them, this can save a lot of money.
The trick, is to take the working batteries out (and you’ll need to have a cheap tester for this – because it’s worse than useless storing flat batteries), and to store them away at the front of the draw where you keep the other batteries in their own compartment (aka cardboard box) … and here’s the important part kept together with the ones they were used with, with a rubber band.
The last is important, because you then have a draw filled with various makes and types of batteries held together by a rubber band. And so long as you re-use them together (or for example, use one or two out of the three) then they are as safe as using new ones.
And being at the front of the drawer … when you just need a few odd batteries in a hurry … these are the ones you take.
But one thing to bear in mind, is that whilst re-used batteries are ideal for things like kitchen timers, more kids games … where running out is not a problem, they will not last as long – so no sticking such batteries into a GPS, going into the wilderness without any spare (or a compass) and expecting to get home.