Mechanical Cell Connections
- Leif Blake
- Micah Black
Plastic holder
Just as found in MSXII, plastic holders fitting 18650 cells can be purchased fairly cheap online. They provide structural support to a module and allow air to move freely between cells. They do however result in an increased surface area of the modules and add some weight to the battery box.
An alternative arrangement to the rectangular layout found in MSXII is a hexagonal packing layout, similar to hexagonal closest pack (HCP) but with space between the cells. Geometrically, this design should save on module area and weight. However, a CAD mockup of this holder design (with an equal minimum wall thickness to the rectangular design) revealed that it would have roughly the same module area as the rectangular holder. Moreover, it would not be significantly lighter given that material can be cut out between cells in the rectangular design.
An HCP configuration was found online in rows of ten, but this would be inconvenient for proposed module sizes. Given that the HCP holders would have to be custom made, the rectangular holders are the superior option.
Square cell holders can be bought in individual pieces or in preset grids. Individual pieces cost about 5-7 cents per cell, whereas grids can be found cheaper at 2-5 cents per cell. This isn’t a major concern. The downside of pre-arranged formats is that they can’t be modified and the chosen cell arrangement may not be available. The 3x6 arrangements discussed previously can be bought here:
Adhering cells
If an HCP arrangement of cells is sought after to minimize the total module area and reduce weight, then it would be beneficial to adhere the cells together so as not to rely solely on the electrical cell connections for mechanical strength. Additionally, a thermally conductive layer between the cells would allow for greater heat dissipation than simply packing them one against the other. This can be achieved either through the use of a thermally conductive epoxy or thermally conductive tape.
Tape
There are a few companies which produce double-sided thermally conductive pads. 3M produces a line of thermally conductive tapes, the 8000 series, but they are very expensive.
A cheaper option is T-Global Technology’s Li98C thermal pads, which are $50 for a 640x320mm sheet. If the cells are arranged 3x6 for the smaller pack or 4x6 for the larger pack, this would allow for about 10 small modules or 6 large modules to be constructed using a single sheet, assuming that the cells are held by strip running down the centre of the module.
The intended use of thermal pads is for contact between two layers, and therefore they are of questionable suitability for this application. The shear strength of the pads is 100 N/cm2. The pads are 0.25mm thick and have a thermal conductivity of 1.8W/mK, which will provide limited thermal conductivity towards the outside of the module.
Additionally, manufacturing might be complicated using tape, as the layers will have to be built up very carefully to maintain the HCP structure. If the chosen electrical cell connections require very specific alignment of the cells, this will only complicate the procedure.
Epoxy
Several epoxies exist that provide both mechanical rigidity and heat transfer capabilities. The Lord CoolTherm series and MasterBond Supreme Supreme 12AOHT-LO are promising, but added manufacturing time (particularly for two-part epoxies) and uncertainty of suitability for this application make epoxies undesirable.
Conclusion
Square holders are the most affordable option with the least amount of uncertainty. Additionally, they will make cell manufacturing easier as compared to the other options. If the chosen cell arrangement is available for purchase as a pre-made module, it will most likely be cheaper to choose that option, and will probably save weight and manufacturing time.
Update
Since we've deviated from this significantly during our design, I feel like this page now requires an update. We went with CNC-Machined (actually 3D printed by Linamar, but the original intention was to CNC machine) plates that got attached to the cells and had holes for heat-set inserts to attach the aluminum standoffs, gusset plates, and module PCB together. We also switched to an HCP cell pattern to allow for more airflow along the rows so we can circulate the air in the battery box faster.