I tried to charge a battery to 100% SoC. This battery has multiple cells in series. After the CC phase, the voltage in the CV phase is set by the charger. Cell voltages are monitored individually. During the CV phase, the data show that the voltage of one of the cell voltages (which was the lowest at the end of the CC phase by the way) decreased.
Do you have any idea what could cause this?
Thank you for your help!
It means that that cell has high internal resistance.
During the CC phase, the terminal voltage of that cell was high due to the current through that cell's internal resistance.
During the CV phase, the charging current decreases naturally and asymptotically towards zero. As the current decreases, so does the voltage drop across that cell's internal resistance. Hence, its terminal voltage decreases.
The other cells too have this effect. But, because their internal resistance is much lower, this effect is hardly noticeable.
In conclusion: that is a bad cell.