they don't always fit.I wanted a set of 4 2300 mAh NiMh rectargable batteries + a good charger for a while already, and i suddenly found one in a shop, at a fair price, so i bought it. I charged them (which goes in 3 hours, which is as fast as i wanted), and tried them in my gameboy (the classic grey brick kind), and it worked, but then it randomly "reset". and this happened a few times. i wondered if the GB was broken, and then i found the cause: the tip of the battery can't properly touch the metal contact of the GB.
The batteries are of an unknown brand with a dodgy website, and probably rebranded. So i wondered if the batteries are conformant to the specs for AA batteries, and looked it up.
The tips on these batteries is unusually short, but it's more than 1 milimeter, so, strictly, the batteries are correct. So the problem with the gameboy:
The plastic in the battery compartiment makes a battery with a short tip unable to press against the metal contact. On the left, a normal battery. On the right the problem. Strictly, the gameboy is not conformant to international standards, it's wrong. But i think someone who makes batteries should know that this kind of things are the reality, and should try to be as conservative as possible, looking at the average of established brands/kinds of batteries etc. So i blame the batteries.