1. Pure “random”, a rare occasion. The cell can come from any batch.
2. The cell comes from a batch with a high proportion of faulty cells due to a quirk in the manufacturing process.
3. A regression in the manufacturing process
4. The cells were abused after manufacturing: during shipment, or during battery assembly, or during battery shipment/shelf life. Either mechanical or temperature/humidity abuse. TOFO is this possible that physical causes of degradation change due to these things?
5. The battery is used in an extreme environment, or too frequently cycled at high currents or to 0% or 100% SoC. See Charging at low temperature leads to Lithium plating, Charging Li-ion cells at higher voltage accelerates degradation, Storing a cell at a high temperature and state-of-charge accelerates capacity fade.
6. A problem with sensors/wiring in the battery, perhaps leading to consistent overcurrent or consistently over-charging or over-discharging the cell. TOFO is this possible, e. g. due to voltage sensor bias?
7. A miscalculation of safe limits for charging/discharging, e. g. due to a bug in a new version of BMS software.

TOFO are there any other reasons?

Analysing OCV won’t help us with 1. and 2. because the best OCV we can have is an average of OCVs of 28 cells, where a contribution of a single cell is lost. Note that we already talk about rather subtle differences in OCV curves to discern the physical causes of degradation. (Cf. Cell size tradeoff.)

By the way, this problem applies not just to the OCV function but to all parameter estimations (see above), perhaps with an exception of Cell self-discharge rate and Coulombic efficiency because normally these parameters should be very small, but in a misbehaving cell, they can jump 10 or 100 times. (See Alert on unusually high self-discharge rate or unusually low Coulombic efficiency of cellgroups.)

Quality control/EOL testing in the factory must catch 3.

Analysing OCV could help with 4.-7., however, in all these cases we will see the remaining battery capacity reducing quicker than normal. So just tracking the remaining battery capacity will be a sufficient indicator that something is wrong with the battery. Also, I really doubt 4. is even possible, 5. should be obvious anyway (if we are cycling a battery at 50 °C … it will degrade quickly), and there are much more proper ways for catching 6. (system/Poland EOL tests) and 7. (software tests) than discerning physical causes of degradation which is an indirect and a very delayed way.

Part of ‣ and Cell degradation.

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