white exec wrote: ↑
07 Mar 2019, 10:50
I still find it odd that the basic building block of the battery packs is something the size/shape of an AA. Must be a good reason for that; can't just be lack of imagination on Panasonic's part... Interconnection aside, I suppose it does give a certain amount of flexibility for vehicle battery-pack design, let alone all the other, non-automotive, applications.
An 18650 cell as used in a Tesla Model S is quite a bit bigger than an AA cell. In fact it's 18mm diameter x 65mm long.
And the 21700 cell used in the Model 3 is even larger at 21mm diameter and 70mm long.
In any case, Tesla are actually one of the few EV's using "small" cylindrical cells. Most other EV's are using either large prismatic cells (my Ion, BMW i3, others) or large pouch cells. (Hyundai Kona, Leaf, many others) The 18650 cells are only about 3.4Ah while the large prismatic and pouch cells are anywhere from about 50Ah (my Ion) to about 150Ah.
There's actually a lot of debate in the industry as to which cell format is the best in terms of cost, energy density, ability to cool them etc. Tesla are adamant that cylindrical cells are the best while other manufacturers are equally adamant about the cell types they have chosen. This is quite a good video that goes into the different cell types:
One big advantage of Tesla's approach of lots of small cells instead of fewer large cells is redundancy and fault tolerance.
In my Ion there are 88 cells in series, each one 50Ah, in a series only configuration. If a single cell goes faulty the entire battery pack stops working, at least until that cell is replaced, and a single weak cell with lower capacity limits the usable capacity of the entire pack.
However in a Tesla pack you have cells grouped together in parallel in groups of about 74 cells to give you 232Ah. Each of these groups of 74 paralleled cells is then connected in series with another group to give in total about 96 groups in series with a total of about 7104 cells. Every cell has its own individual spot welded fuse wire.
On a Tesla pack if a single cell goes shorted the individual fuse wire will burn out and isolate it from the rest, or if it goes open circuit it will likewise no longer affect the rest of the group. The end result is that parallel group only has 73/74 of the original capacity or 98.6%.
So a complete cell failure would only cause a 1.4% loss of usable range/capacity which may not even be noticed compared to an individual cell failure in my car putting it off the road...