Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register

Effect of a cell failure on the whole battery pack

This site may earn commission on affiliate links.

Tuyuq

New Member
Aug 3, 2017
2
0
Jo
Hi

I want to ask what would happen if one or more cells in the same "in-parallel" module fails, how would this affect the other cells in the same module under high current demand like insane mode etc.



Thanks!
 
Hi

I want to ask what would happen if one or more cells in the same "in-parallel" module fails, how would this affect the other cells in the same module under high current demand like insane mode etc.



Thanks!

based on a wk057 tear down of a 85kWh 400V pack, the cells are set up in groups of ~72 cells in parallel. The BMS is is going to limit pack discharge/charge based on the lowest/ highest voltage sub-module. With less cell, the failed sub module will be the fastest charging/ discharging portion of the pack.

If one cell fails open, or shorts and causes the fuse wire to open, the usable pack capacity will drop about 1.4%, assuming no active recharge from the battery management system (BMS).

If the cell fails as a partial short and pulls more current than the BMS can balance, the pack will ultimately fail.
 
[QUOTE="mongo, post: 2233152, member: 60387"

If one cell fails open, or shorts and causes the fuse wire to open, the usable pack capacity will drop about 1.4%, assuming no active recharge from the battery management system (BMS).

If the cell fails as a partial short and pulls more current than the BMS can balance, the pack will ultimately fail.[/QUOTE]


I find that very interesting,
If more than 1 cell fail in the same module, will it decrease the amperage of the whole pack or will the car use the same amperage during Ludicrous Mode , or will the BMS limit the available amperage and high power usage would be limited?
I don't know what amp rating of the 18650 used in the Tesla, but I assumed its 10A per cell so 720A max per module
 
[QUOTE="mongo, post: 2233152, member: 60387"

If one cell fails open, or shorts and causes the fuse wire to open, the usable pack capacity will drop about 1.4%, assuming no active recharge from the battery management system (BMS).

If the cell fails as a partial short and pulls more current than the BMS can balance, the pack will ultimately fail.


I find that very interesting,
If more than 1 cell fail in the same module, will it decrease the amperage of the whole pack or will the car use the same amperage during Ludicrous Mode , or will the BMS limit the available amperage and high power usage would be limited?
I don't know what amp rating of the 18650 used in the Tesla, but I assumed its 10A per cell so 720A max per module[/QUOTE]


Interesting question.

I see two possibilities: The first is that the BMS detects the failed cell by the increased rate of change of voltage of the sub pack compared to the rest of the module. It would then communicate the new current limit to the inverter.

The second option is that the BMS does not detect the missing cell. Then the rest of the cells in the sub-module are subjected to a proportionately higher current draw and the sub-module voltage droop is higher and limits the peak current output (for lower states of charge).