Please note: I'm not taking any responsibility for this modification of medical equipment - you do it at your own risk.
This converter enables use of a standard 3.7V LiPo in a c-cell WelchAllyn handset and dramatically improves the quality of the light.
Total cost of battery, charger, and parts should be less than £10.
One battery lasts in excess of 6 weeks per charge in a standard GP practice. We have been running two converted handsets for a few years. I blew one bulb after taking a battery directly from a charger and putting the brightness immediately to full. It wasn't a new bulb, but I now leave the batteries to rest after charging, and use it at less than full for the first couple of examinations.
You might think WelchAllyn make highway-men and rapists look like saints with the cost of their rechargeable handsets, especially given that the C-battery versions have a dull yellow light which makes examination difficult. However I hear the cost is justifiable as the certification of new medical equipment is apparently a horrifically expensive process...
Step 1: Parts + Assembly
Cut a 32mm long piece from 6mm diameter metal rod
3d print the holder, or
take a 50mm long piece of 22mm diameter dowel. Drill a 19mm hole into one end to a depth of 10mm. Then drill a 6mm hole the full length of the dowel
Put the rod in the hole of the dowel. Drop it into the handle where the battery goes as per the pic (you don't need to take the handle apart to do this it was just easier to photograph)
Replace the battery the same way you would have replaced your c-cells.
Step 2: Printable Insert
This is the OpenSCAD code if you want to edit the object, or you can download and print the stl
translate([0,0,40]) cylinder(10.1,9.6,9.6, $fn=100);
translate([0,0,-0.5]) cylinder(50,3,3, $fn=100);