Such a design has potential safety shortcomings. Since in such a reconfigured pack many cells are now in parallel, should a cell go bad (internal short-circuit, thermal runaway...) it would start experiencing a very high current, drawing from the capacity of its parallel neighboring cells. I.e. the neighboring cells would dump energy at a very high rate into that cell.
In the worst case of a cell failing with a short-circuit like condition, the current would be limited only by the internal resistance of the neighboring cells, resulting into 100s of Amps pouring into the failing cell. This would lead to catastrophic failure, fire, and a not too happy home owner...
How high a risk is that ? Statistically extremely low in the short term. The cells in automotive packs all have very similar characteristics, lowering the risk of having a cell straying fast and becoming much weaker and more overworked than the others.
However, on the long term, in long life applications (think 20 years!) like cars or house storage, it is guaranteed that sooner or later some cells will age non uniformally respectively to each other as they approach their end of useful life. That's one area a BMS (Battery Management System) is critical for. Its role is to monitor each cell, ensure that none is ever over or under the safe working and charging voltage levels, and to balance all cells.
But high safety and long life applications require thinking hard about failure points, like the BMS, and complementary fail safe design techniques. That's one of the reasons why Tesla in their battery packs, in addition to their BMS(s), connect each cell to its busbar via a wire that acts as a fuse if the current ever gets too high. As can be seen in this picture of a Tesla P100D model:
For more background on designing safe battery packs and management systems, this book is an excellent resource: A Systems Approach to Lithium-Ion Battery Management by Phillip Weicker. It opened my eyes to real life failing conditions I would have never anticipated.
So, since my pack has 16 cells in parallel in each group, I decided to design Busbars with integrated Fuse wires. Here is the finished setup:
Whole pack:
To hold the busbars, and keep them safely away from the chassis and battery posts, I designed and 3D printed Holders that slide onto the mechanical stress relief bars described in the previous post. The picture above shows an old version of those, that I used to build the 1st subpack. The 2nd one is under construction and will use the V2 revision:
Now that the principles for interconnecting the cells have been laid out, the next couple of posts will describe the sizing and implementation of the "fuses". Boop !
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