Cell Failure in a Battery Pack Notes

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We will likely never get a root cause report but it is still a curiousity of what happened and why.

i have looked at the BMS boards and there is no device or mechanism to send current around a cell to prevent it from charging or discharging, i.e. no current shunting.

The Linear LTC6802 chip has an onboard multiplexed ADC for reading cell voltages.  It has an onchip temperature sensor and in addition can read 2 thermistors for a total of 3 temperature measurements to report back for each LEV50-4 module.  With an 88 cell pack there are 22 modules/boards for a total of 66 temperature measurements.

Notes on cell failure:

Sudden cell failure due to charging after over-discharging is usually due to growth of copper dendrites that puncture the plastic separator sheet.  These cells are typically shorted and read zero volts.

Some notes from the battery university website:

Further studies revealed that the lithium ions responsible to shuttle electric charge between the electrodes had diminished on the cathode and had permanently settled on the anode. This results in the cathode having a lower lithium concentration than a new cell, a phenomenon that is irreversible.

During charge, lithium gravitates to the graphite anode (negative electrode) and the voltage potential changes. Removing the lithium again during discharge does not reset the battery fully. A film consisting of lithium atoms forms on the surface of the anode called solid electrolyte interface (SEI). Composed of lithium oxide and lithium carbonate, the SEI layer grows as the battery cycles. The film gets thicker and eventually forms a barrier that obstructs interaction with graphite.

SEI is an electric insulation that has sufficient ionic conductivity to allow the battery to function normally. While the SEI layer lowers the capacity, it also protects the battery, without which Li-ion might not get the accustomed longevity.

The cathode (positive electrode) develops a similar restrictive layer known as electrolyte oxidation. Dr. Dahn stresses that a voltage above 4.10V/cell at elevated temperature causes this, a demise that can be more harmful than cycling. The longer the battery stays in the high voltage, the faster the degradation occurs. The build-up can result in a sudden capacity loss that is difficult to predict by cycling alone. This phenomenon had been known for some years and measuring the coulombic efficiency can verify these effects in a more scientific and systematic manner.

This causes a permanent capacity loss and increases the internal resistance.  Keeping Li-ion at a voltage above 4.10V/cell while at an elevated temperature promotes electrolyte oxidation.  No remedy exists to remove the layer once there but electrolyte additives in modern Li-ion minimize the effect of forming it.  Field use revealed that the combination of heat and high voltage can cause more stress to Li-ion than harsh cycling.

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So what is the mechanism in which a cell passes full current in discharge and charge, yet doesn’t itself recharge?  Electrons are moving but the lithium ions are being blocked–the growth of the solid electrolyte interface (SEI) layer at the anode (negative) terminal, or the electrolyte oxidation layer at the cathode (positive) terminal.

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CT scan of a cell:

cell_scan

 

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