I was just expressing surprise at how a small battery could cost so much, and also why anyone would buy one for an old Mac (presumably someone does? I don't know - do these batteries have any other common uses?).

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The 128/512K/Plus battery was also used in some high end SLR cameras in the 1980s. The original battery design is now banned in the EU (environmental concerns) and the format was unavailable for several years. Thanks to the camera enthusiasts, a redesigned version is now available. I've not seen it anywhere in the UK but it is available online from Germany and Holland.

The 1/2 AA battery is pretty common. I've seen it used in lab equipment and in magnetic card readers.