I have some battery chargers for AA rechargeable batteries. They are differnt make and models by different vendors, but have the following in common: Both have four charging slots, and both can only charge in pairs, i.e., they charge well if I put batteries in slots 1&2, or 3&4, or 1&2&3&4.
What is the technical reason behind this? Why can't these devices charge only one slot or other combinations? (Also: Could these cause problems when I add a full or semi-full batterie just in order to turn a singleton into a pair and charge a single empty battery?)