The Avometer 8 multimeter has lots of useful ranges, including a special high-resistance range which can measure resistances of up to 20 megohms. This is handy, but it needs a special 15 volt battery. The battery it’s designed for is a BLR121, which was once fairly common but is now dying out. The BLR121 is just about still available but it’s expensive, and since the meter is likely to last a long time I wanted a battery which would also last, and be easy to replace when necessary.
An old solder reel, a bit of copper pipe, and five common-or-garden lithium coin cells is all it took. The coin cells are CR2032, which are 20mm in diameter, and they fit just neatly inside the solder reel. I cut down the reel to form a tube about 35mm long. The stack of five cells is about 16mm long, so I filled the remaining space with a bit of copper pipe cut to about 22mm long. This is what the assembly looked like:
It fitted just neatly into the battery compartment of the meter. The spring contacts are very convenient because you can fit more or less any shape between them!
It works perfectly, and it’s cheap. The CR2032 cells are available for less than 50p each if you shop around, and they have a capacity of around 200mAh. The BLR121 replacements I’ve seen have a capacity of only 40mAh, so the lithium replacement should last about five times longer. Not bad.
Here’s a gratuitous picture of the Avo in use checking the power supply of a BBC Micro.