BQ40Z50-R1: Cell 1, Cell 2 low voltage on 3 cells in series battery pack

We have 2 battery pack models that are using BQ40z50-r1 (Model A:3S2P and Model B:3S1P).

 

Regarding the failure we found, during inspection we saw that 3 batteries have almost 0V voltage on Cell 1 and Cell 2, but Cell 3 (B+) is still around 3.4V .  These battery packs have were never been used and have about 300hours of FW runtime in the extracted GG files while the rest of the good batteries have about 8months of FW runtime.

 

The 3 batteries were already in shutdown mode.

After waking up the gas gauge IC, we saw CUV and SUV after checking SBS.

 

 

After inspecting the PCB, we didn’t see any physical shorts in the circuitry or burn marks on the PCB that may

show any short and drain the cells.  The impedances of the capacitors across and resistors connected to VC1,

VC2 and VC3 seen in the table below.

We are also using BQ294700 in the PCB for secondary protection IC and impedances of capacitor and resistors

below:

 

After inspecting the parts, we also retested the PCB and found that the consumption current and function of the ICs are found

within acceptable values.

 

 

We came across this in the forums.  Although the issue looks different because we didn’t have COV,

 

Have you seen cases from other customers seeing the same issue where the cell voltages became imbalanced and eventually

become low voltage when the batteries are stored and at sleep mode?

We find that it is curious to find both cell 1 and cell 2 are drained on 3 batteries and was seen on two different models and if it

was a cell issue, we should be randomly seeing low voltages at cell1, cell2 and cell3

Thanks in advance!

Kenny