Compact Macintosh Battery Adapter

The original 128K Macintosh, Macintosh 512K, 512Ke and Macintosh Plus use a rather unusual PRAM (BIOS) battery. It's a 4.5 volt 523 (AKA EN113A or V21PX or TR133A) alkaline battery. It might look like a AA, but it's not.

If you want to replace it with something of the same size and voltage, I have created this little holder for 3 x 1.5 volt LR44 (AKA A76) batteries, which are more readily available than the original batteries.

The holder is in two halves that screw together. The 3 x LR44 batteries are stacked on top of each other in the "negative" end of the holder, with the negative side of the batteries facing down. The negative part of the bottom battery will be exposed through the hole in the bottom of the holder.

The two other things you will need are a 5mm long, M3 screw. In truth it can be longer than 5mm, but should be no shorter (or it may not make contact with the spring). Screw this into the top of the positive end of the holder as far as it will go. You will also need a spring to hold the batteries in place, and to close contact between the positive end of the top battery and the screw in the top. You should use a compression spring of 6mm diameter or less and 35 to 45mm length. There's definitely some flexibility with the spring you use, but aim for wire diameter of 0.5mm, outer diameter of 5mm and length of 40mm. Insert the spring into the hole in the positive end of the holder and then screw the two halves together.