HP48 battery spring foam replacement

thingiverse

If you have an old HP48G, HP48GX, or the like, you may have noticed that the foamy things that are supposed to provide spring tension to the two battery compartment connectors, lose their springiness over time, especially if you have used batteries that are somehow longer than they are supposed to be (maybe this happened in my case because I had left the batteries in there for too long and they expanded). Here is a simple solution: just print new springs in a sufficiently elastic filament like TPU. It should suffice to replace only the bottom left foam. This, as well as the positive terminal spring, should provide enough spring force to push the batteries against the other connector even if its foam is completely flattened. If not, replace that one as well. This must be printed in an elastic filament. I have printed it in NinjaFlex at 0.2 mm layer height with a single 0.40 mm perimeter, 1 bottom layer and 2 top layers, with 10% gyroid infill, infill extrusion width 0.35 mm. That's right, the infill extrusion width is less than my nozzle diameter, but this works and provides for exactly the right amount of springiness. Getting the old foam off the contact is a bit of a pain because the glue is very tough. Some acetone may help to remove residue from the metal (don't use acetone on the plastic!) I have attached the new ‘foam’ to the metal piece with contact adhesive, and stuck the whole inside the calculator with a glue roller. (If you wonder why the calculator in the photos has a ‘G++’ sticker and holes in its battery compartment, it's because it has been hacked to have 256kB of RAM.)