SLUUCN4A January 2023 – February 2024
An alternative voltage based cell balancing is available to the BQ40Z50-R5 device. It is enabled in CHARGE mode when Settings:Balancing Configuration [CBV] = 1 and charging current is detected, or in REST mode if Settings:Balancing Configuration [CBV] = 1 and Settings:Balancing Configuration [CBV_REST] = 1. The operation balances the cells by enabling the bypass around those cells above the threshold set in Voltage Cell Balance Threshold if the maximum difference in cell voltages exceeds the value programmed in Voltage Cell Balance Min. During cell balancing, the BQ40Z50-R5 measures the cell voltages at an interval set in Voltage Cell Balance Interval.
The cell(s) to be balanced are prioritized by highest cell voltage but the BQ40Z50-R5 will not try to balance adjacent cells. If adjacent cells need to be balanced, the BQ40Z50-R5 will alternate between the highest and next-highest adjacent cells until they are balanced.
When the voltage based cell balancing is activated while the device is in CHARGE mode, the BQ40Z50-R5 either selects the appropriate cell to discharge or adjusts the cell balance threshold up by the value programmed in Voltage Cell Balance Window when the highest cell exceeds the present cell balance threshold by Voltage Cell Balance Window. While the voltage based cell balancing is active, if the min cell voltage rises above the present cell balance threshold, the same cell balance threshold will immediately be adjusted to min cell V + Voltage Cell Balance Min to allow continuous balancing while the min and max cells have not yet met the Voltage Cell Balance Min delta requirement for balancing complete detection. Figure 7-7 shows how the cell balancing operates in CHARGE mode when this feature is activated. More in-depth details and data on this voltage based cell balancing algorithm during CHARGE mode can be found in: http://www.ti.com/lit/slva155.
When cell balancing operation is activated in REST mode, the dynamic cell balance threshold will automatically be set to the voltage level which is Voltage Cell Balance Min above the lowest cell voltage, without Voltage Cell Balance Window adjustment, and continue to decrease as the cell voltages decrease over time due to leakage while remaining above the Voltage Cell Balance Threshold voltage level. This ensures the cell balancing operation continues to operate in REST if the maximum difference in cell voltages exceeds the value programmed in Voltage Cell Balance Min, without over discharging the batteries in case the voltage level programmed in Voltage Cell Balance Threshold is significantly lower than the lowest cell voltage.
The cell balancing operation completes when the maximum difference in cell voltages is less than the value programmed in Voltage Cell Balance Min. Upon completion while the lowest cell voltage is still above the voltage level programmed in Voltage Cell Balance Threshold, if a fast leaking cell causes the maximum difference in cell voltages to widen again beyond Voltage Cell Balance Min, the voltage base cell balancing at REST operation will re-activate. When the device exits REST mode or when any of the cell voltage falls below the programmed value of Voltage Cell Balance Threshold, cell balancing at REST operation will stop even before the balancing is completed. Figure 7-8 shows how the cell balancing operates in REST mode when this feature is activated.
Cell balancing occurs on non-adjacent cells at the same time. The cell balance threshold is reset to the value in Voltage Cell Balance Threshold at the start of every charge and reset cycle. The threshold is only adjusted once during any balance interval.
The configuration data flash is stored in Advanced Charging Algorithms: Cell Balancing Config. The starting and rest conditions for voltage based balancing are same as state-of-charge based cell balancing.