Arlo1 wrote: ↑
Apr 21 2018 1:04pm
When 1 cell shorts in a Tesla pack it blows the fuse and the 1 single cell is no longer part of the pack.
With modern cell production failure rate of cells should be less than 1 in 1 million.
There are different kind of internal shorts in a cell. In most cases its not aa "zero Ohm" shortage, but a slower drain. In that case the cell "fuse" does nothing and the entire pack is defective.
This is far safer then a Set of bigger cells with 1 shorted making the other cells in series have to take a major overcharge.
It could be safer when one cell enters therma runaway, because large cells store more energy than small cells.
Cell overcharing will not happen. Any modern battery BMS in a quality good enough for cars will report a failure and stop charging. And overcahrging will not cause a fire in modern 18650 cells, the CID in the cell will activate (thats the thing tesla removed from the Panasnonic NCR cells). I don't know how the large pouch cells work.
As well when the lager cell format shorts or drops off it will risk being reversed. As a fuse which is not common on a large cell would have to be rated much higher and might not pop as easy allowing energy to flow into the now very dangerous cell.
For myself I have only seen open Tesla batteries and open Daimler batteries and I would take the Daimler battery if safety is my concern. But that's just out of my stomach, how those packs lock on the inside...
It's not that the mayor brands do not have their own problems with cells and battery assembling, but they did not need to rush their packs and electric cars to the market.
Its hard to say what will win out long term but for now I understand why Tesla is going the rout they are. And in the next 2-5 years you will see many others come to market with something similar.
10 years ago Panasonic had forecasted significant problems selling more 18650 cells to their main consumers, the laptop industry (because laptop have become to thin to house 18650 cells)
And there was a new car company that wanted the cheapest battery with a high density and that's how it came that Tesla uses 18650 cells. It is what was available to them.
The new 21700 cells can be produced on the same assembly lines with little modification, that why they use this format now and not something larger.
Whats better in the future, cylindrical or pouch (or prismatic?) is hard to predict imho. I wouldn't bet on either technolgy, both have advantages and disadvantages.
Its hard to compare companies who make compliance cars and their 3rd party cells to what Tesla is doing and truly think its an apples to apples comparison.
Tesla is burning week by week through investors money. What they depseratly need is that the show must go on. Part of that show is to claim, that they have some magical technological advantage.
I see no "overall advantage" in small cylidnric cells now (when large pouch cells have become much cheaper, too)
I see no "overall advantage" in using asynchronous motors
I see no "overall advantage" in using the proprietary Tesla Supercharger grid based on the MID DC connectors which already have problems at 120kW
They had been the first to produce a premium BEV for the "mass market". That's quite an achievement in itself.