AVO 15V B121 / BLR 121 battery substitute

The earlier marks of the AVO 8 analogue multimeter use a 15 V B121 / BLR121 battery for their high resistance range. This battery has been obsolete for decades, although commercially available substitutes exist which are based on a stack of ten button cells in a rectangular package.

The battery slot is oversized in the earlier versions of the AVO 8, so enterprising amateurs sometimes use a stack of five CR2032 cells instead, heatshrinked in series with a length of 15 mm copper pipe. I couldn't find any easily accessible STL files for an assembly to do the same job so I created this one. 

I soon discovered that some marks of the AVO 8 cannot accommodate the oversized CR2032 battery, so I developed a second design. This uses a stack of thinner CR2016 cells, which are stacked obliquely to stay within the envelope of the original battery (just!). 

Both batteries also require 2 off M3 x 8 mm stainless steel cap-head screws, which compress the stack and form the electrodes. This is an established approach so I don't claim any novelty, although I found that the screw pressure did cause slight localised deformation of the uppermost and lowermost cell cases. I don't know whether this is a problem or not, so if you use either of these battery designs you do so at your own risk.

I printed both parts in PETG with 30% infill. The M3 screws self-tap into the holes on the ends. Make sure you remove any and all support from the part (and particularly the holes) before installing the cells; this could cause electrical continuity problems as well as fitting problems. Simply fit the cells into the slot (all in the correct orientation of course!) then tighten the screws just enough until you can measure a voltage across them. With new cells I measured a terminal voltage of just over 16 V. Excessive tightening could separate the layers of the print or seriously damage the cells. You may wish to try different print orientations if layer separation is a problem.

Whichever one you print, the assembly should fit easily into the AVO 8 battery compartment and the points on the spring contacts should drop into the cap-head recesses, keeping it firmly in place. I tested both on my AVO 9sx panclimatic model. This is an AVO 8 with a metal exterior as used in the Test Set No. 1. 

Note that these are unlikely to work in an AVO 8 Mark 5 as a completely different battery is used.