...
When the energy shoots out the positive terminal, it will heat up anything in its path. If there is another cell in its path it will heat that cell up and cause it to go into thermal runaway, which causes a chain reaction if many cells are connected in this way (thermal runaway propagation). If however, we remove all the cells from the path of this thermal runaway energy, we should be able to reduce the chance of thermal runaway propagation. This was the motivation to not have any cells stacked vertically on top of each other.
Q3:
...
A3:
A few tradeoffs to consider here:
A manageable assembly size. A single module with 200 or 300 cells is pretty heavy and would need 2 hands to hold it, harder to move around and keep extra modules. At ~50g/cell, 200 cells weigh 10kg and 200 cells weigh 15kg. Smaller modules are easier to handle.
If a module fails, we need to replace the module. If we had only 4 modules in the pack, then we may only have the budget for a single spare module. Also, since there are more cells in a larger module, there is a greater chance that a cell within that module fails. Smaller modules means we can have more spare modules, and will statistically fail less often.
It is beneficial to make all modules exactly the same. Then we only have to keep 1 type of spare module, and any spare module can be swapped in for another. MSXII had 2 types of modules (and end module and a middle module) and we ended up running out of spare end modules, while still having a few spare middle modules unused.
More modules mean that we have more inter-module connections. This increases the number of failure point. Each inter-module connection also has a small resistance associated with it, so more connections means higher resistance in the pack’s current path, and more power loss (P = I^2 * R).
If you compare the weight of the cells in a module to the weight of the rest of the items (cell holders, support brackets, cables, bolts, etc.) then the weight of the extra stuff in a larger module will be smaller per cell (LM = Large Module, SM = Small Module):
(LM mass of extra items / LM # of cells) < (SM mass of extra items / SM # of cells)
The extra items can also be considered ‘overhead weight’ and would be roughly constant for all module designs. More modules = more extra weight.
The layout of the modules within the pack may also be easier when working with smaller blocks.
Q4:
...
In case the link ever gets taken down, here’s the PDF:
| View file | ||
|---|---|---|
|
A4:
Phase change material! Yes - this stuff is awesome! All Cell Technologies is where I first heard of it when looking into cooling for MSXIV: https://www.allcelltech.com/
I would absolutely love to do this if we can figure out a way. Increased safety is the biggest reason to push for this as it limits thermal runaway propagation. NASA also have a ton of presentations on this - just search ‘NASA 18650 Thermal Runaway Propagation’ on google and you should get a bunch of results.
We did look in to this a bit, and determined that it would be practically impossible to acquire some material to use (we did send an email to All Cell Technologies but they said they don’t sell the material directly, they will only sell it as built into a pack that you buy from them, which was super expensive). I did look into what it would take to make some, and determined it to be too difficult of a technical challenge. We also wouldn’t really be able to test the thermal runaway propagation technologies since the university wouldn’t really like us burning up cells on purpose. There may be a few battery labs on campus that we can reach out to about spaces to do this testing though. The main reason we did not pursue this route is that we needed something to work and be complete in a short time frame. We did not know if making a phase change material from paraffin wax stuffed with some kind of thermally conductive powder would even work, so we decided not to invest the resources into it. On this new cycle though, it would be great to reconsider!