A battery, a battery charger, and a battery charging system are used for charging cylindrical batteries having a certain predetermined feature. The battery charger has at least one compartment having first and second circular recesses provided at the rear wall of the at least one compartment for engaging the cathode terminals of batteries having the predetermined feature of a first and second size respectively, wherein the recesses are of such dimensions as to receive the cathode terminals of batteries having the predetermined feature, but not the cathode terminals of conventional, prior art primary and secondary batteries not having this feature. It provides a safe, easy to use, low cost battery charging system that will not charge undesired primary or secondary batteries.