I've replaced the battery in Braun 5314 and 5478 shavers, hopefuly the 5502 is similar. These shavers use a single tagged AA cell. The original batteries have no markings, but I've used 2000mAh tagged NiMH cells, available from Maplins, Radio Shack etc. Don't use a lower capacity cell or it might overcharge. To replace the battery, first remove the foil and cutter, then the two tiny screws that connect the top of the swivel head to the frame, so that the swivel head comes off. Next prise off the two screw covers at the bottom of the shaver, one each side of the power connector, and remove the screws. The two halves of the shaver case can then be prised apart at the bottom. You can then slide one half of the case up (that is why you have to remove the swivel head) to remove it. The circuit board will then come out. Make a note of which end of the battery is positive (use a multimeter if necessary), unsolder the battery, and solder in the new battery, ensuring it is the right way round. Then reassemble by reversing the above instructions. Note that the swivel head has to go on the right way round so that the button to remove the foil works. When complete, check that when you recharge the shaver, the shaver does not overheat and the green charging light goes off eventually.