Parametric cellholder for Li Io-cells (18650 / 21700)

I just had to print another cellholder for a 10s4p with cells I scraped from a partially defect e-bike battery from a friend. So I decided to "streamline" my crappy OpenSCAD file a bit to make it more usable for other peoples.

This OpenSCAD file is suitable to generate simple square cellholders matching any cylindrical battery cell you come across and for which your print bed is big enough. Today you will most likely finde 21700 cells in E-Bike batteries but there are some 18650 around as well. Those five digit numbers always work the same way. The first two digits tell the diameter of the cell while the last three digits tell the height in mm.

So you guess it: 18650 means 18 mm in diameter and 650 mm in height. 21700 means 21 mm diameter and 700 mm in height. Sound easy, doesnt it ? There are also various types of cellholders available on Amazon, Ali, E-Bay, etc. which you can readily buy.

So why the hassle of printing them yourself ?

1) As usual in reality things are not that simple as you think. There are various cell producers which have different diameters. So 21 mm might be 21.4 or even 21.8 mm. This translates to issues if you buy your usual "Lego" style of cellholders which might not fit on your cells at all or leave them rattling around in the holder.

2) The usual cellholder are only top and bottom type. The middle of the cells is unprotected. If you build mostly softpacks like me that means your cells can get damaged if something hits them hard from the side. Printing a one-piece cellholder is more like a cell case rather than a "softpack". Cells are protected by the holder and the pack is much more sturdy even if cells are not welded.

3) Does not require ordering something online which might not fit and saves you from "cellholder"-Lego games. Simply adjust the parameters in the OpenSCAD file, generate the STL and your printer will do the rest of work for you.

4) Much more flexibility compared to any stock battery case. If you combine a big softpack wiht my parametric battery mount (https://cults3d.com/en/3d-model/gadget/battery-mount-parameteric-fatbike-v3-corrected) you can easily built yourself a 1kWh battery without robbing a bank ;-)

Topeak trunk bags are also a great way to store big batteries.

How to use the OpenSCAD files ?

You need to grab OpenSCAD matching your operating system from the website. Then adjust the following parameters to your needs:

// Diameter of cell in mm

cell_dia = 18.8;

// Height of cell in mm

h_cell=65;

// Distance between cells

cell_distance=cell_dia+2;

// Amount of cells in row (e.g. 10 for 36V / 13 for 48V / 14 for 54V)

cells_s=10;

// Amount of cells in parallel

cells_p=4;

I hope the description tells it all but I give you some further hints. Measure cell height and diameter with a caliper. Be carefull when measuring the height and you have a metal caliper because otherwise you can shorten your cell (which I recently did myself ;-). Then generate a smaller cellholder as sample (e.g. cells_s=1 and cells_p=3). Print with your filament of choice and print setting of choice. It might take a few adjustments. Especially the diameter can be tricky. I would modify in 0.2 steps until the cells slides easily into the cellholder sample. Using filaments such as ASA or ABS might require you to "oversize" the diameter itself since they tend to shrink a bit more than PLA for example. Personally I find PLA+ sufficient for those cellholders as I do not plan dropping my softpack to the ground frequently. With enclosure ASA / ABS might be an option. PETG works nice as well but keep in mind that any more flexible filament puts more stress on your spotwelding points due to the cells changing their position under load. This might not be an issue but could cause failing spotwelds if you have a huuuuuge pack sitting inside a bag on your bike. As for the cell_distance which also defines your outer wall of the cellholder I usually aim for multiple values of my nozzle diameter. So for a 0.8 mm nozzle (which might come handy for faster prints) I would go for 1.6 or 2.4 mm. For a 0.4 mm nozzle I would go for multiples of 4 (1.2, 1.6, 2.0 mm). As long as you do not go nuts in regard to serial and parallel cell rows those packs will most likely fit on an average 230x230 or 300x300 mm print bed. Otherwise you have to split them in a slicer and re-glue them afterwards. I did this myself for some much larger batterys which went into an integrated Bafang M600 frame and they still hold up strong. Even when being glued.

Final words of warning:

Use at your own risk. Only buy cells from thrustworthy sources and not some unknow guy from Ali or E-Bay. Buying a spotwelder only makes sense if you plan on building at least 2-4 batteries yourself. Otherwise simply buy a ready battery pack. This being said: My oldest DIY batteries where soldered with 1.5 mm² solid copper wire and are still going strong after 7+ years of usage.

Factory ready E-bikes will often not provide you with battery capacities over 800 Wh+ and if they do you pay an incredible amount of money.