3AA batteries make one D-Cell

youmagine

This is the development of the part coming along nicely. The first prototype shows that with minimal material usage, we can achieve a solid foundation, but it also reveals that the bottom layer needs to be thicker when some machining is done for now, the prototype and images are taking shape, but I'll definitely make it shorter than a D-Cell (61.5 mm) as the spring in these compartments for D-Cells are usually very long. D-Cell measures 61.5 mm long, 34.2 mm diameter (some sources say 33.2), and AA-Cell is 50.5 mm long, 14.5 mm diameter (I've not found any in my huge collection of NiMh cells that exceed 14.2 mm, so I dimension towards 14.5 mm). As it stands now, there will be a (+) and a (-) part of the battery pack, maybe I still manage to make it universal. Created the PT2, but even with support, it doesn't print as I want it, as the recessed part at the bottom is not printed with support (I have no clue why...). The design has been drastically updated and will be published hopefully soon. However, there's one important issue that might make this totally useless: considering 3 cells in parallel and not all of them having absolutely identical voltage makes current flow between the cells, which could lead to very fast discharge, especially when high power low internal resistance cells are used, e.g., like NiMh. I will do some measurements with my cells when time is available. Finally, continued: PT5 parts top and bottom part are identical, and middle section using less material. Probably wall thickness could be further reduced. Parts do not need to be kept together; the spring in the D-cell compartment will do the job. Worked fine with the copper tape and copper screws this summer for bbq (I would assume one A cell would have done the job as well;-)). The circles at the bottom and top part need to be machined out, so the screw can be properly placed. Will add some images when all is assembled, but maybe you already get the point. I also measured the current flow between unevenly charged cells and it's considerable, especially when no load is applied and current does not 'know' where to go, so do not leave it in the D-cell compartment; the cells will be discharged rather fast. Use cells ideally bought together as they 'might' have very similar characteristics, so self-discharge is minimal. One bad cell can easily ruin the other two cells within a few hours. The higher the current drawn from the pack, the lower the self discharge due to 'unwanted' balancing between the cells. With ideal cells, this would all work perfect. Disclaimer: use at your own risk