The BQ76942 device supports passive cell balancing by bypassing the current of selected cells during charging or at rest, using either integrated bypass switches between cells, or external bypass FET switches. The device incorporates a voltage-based balancing algorithm which can optionally balance cells autonomously without requiring any interaction with a host processor. Or if preferred, balancing can be entirely controlled manually from a host processor. For autonomous balancing, the device will only balance non-adjacent cells in use (it does not consider inputs used to measure interconnect as cells in use). In order to avoid excessive power dissipation within the BQ76942 device, the maximum number of cells allowed to balance simultaneously can be limited by setting Settings:Cell Balancing Config:Cell Balance Max Cells. For host-controlled balancing, adjacent as well as non-adjacent cells can be balanced.

Host-controlled balancing can be controlled using specific subcommands (described below) sent by the host. These subcommands are also accessible in SEALED mode, to avoid the need for the pack to be unsealed in operation in order to initiate balancing. If host-controlled balancing will not be used, access to these subcommands can be disabled by setting the Settings:Cell Balancing Config:Balancing Configuration[CB_NO_CMD] configuration bit.

Table 10-1 shows the subcommands the host uses to control cell balancing.

The device also returns status information regarding how long cells have been balanced through the subcommands described below.

When host-controlled balancing is initiated using the subcommands above, the device starts a timer and will continue balancing until the timer reaches a value of Settings:Cell Balancing Config:Cell Balance Interval, or a new balancing subcommand is issued (which resets the timer). This is included as a precaution, in case the host processor initiated balancing but then stopped communication with the BQ76942 device, so that balancing would not continue indefinitely.

The BQ76942 device can automatically balance cells using a voltage-based algorithm based on environmental and system conditions. Several settings are provided to control when balancing is allowed, as described below.

Temperature—The device disables balancing (both autonomous and host-controlled) if the cell temperature is below Settings:Cell Balancing Config:Min Cell Temp or above Settings:Cell Balancing Config:Max Cell Temp, or the internal die temperature of the device is above Settings:Cell Balancing Config:Max Internal Temp.

Charge versus Relax—Autonomous balancing can be allowed during charging by setting Settings:Cell Balancing Config:Balancing Configuration[CB_CHG], in a relaxed condition by setting Settings:Cell Balancing Config:Balancing Configuration[CB_RLX], or both. If Settings:Cell Balancing Config:Balancing Configuration[CB_CHG] is set, autonomous balancing is allowed while the CC1 Current is above Settings:Current Thresholds:Chg Current Threshold. If Settings:Cell Balancing Config:Balancing Configuration[CB_RLX] is set, autonomous balancing is allowed while the current is below Settings:Current Thresholds:Chg Current Threshold and above the negative of Settings:Current Thresholds:Dsg Current Threshold. The device evaluates the conditions for continuing balancing every Settings:Cell Balancing Config:Cell Balance Interval. For example, if the device is configured to avoid balancing during charge, and while balancing the pack begins charging, balancing will continue until the interval timer expires before it is disabled.

Cell Voltage—If autonomous balancing during charge is enabled, the device allows balancing if the minimum cell voltage is above Settings:Cell Balancing Config:Cell Balance Min Cell V (Charge) and the difference between the maximum and minimum cell voltages is greater than Settings:Cell Balancing Config:Cell Balance Min Delta (Charge). Similarly, if autonomous balancing during relax is enabled, the device allows balancing if the minimum cell voltage is above Settings:Cell Balancing Config:Cell Balance Min Cell V (Relax) and the difference between the maximum and minimum cell voltages is greater than Settings:Cell Balancing Config:Cell Balance Min Delta (Relax).

While balancing during relax, when the device reevaluates the cell status at the end of each timer interval, it ceases balancing if all cell voltages are within Settings:Cell Balancing Config:Cell Balance Stop Delta (Relax) of the minimum cell voltage. This Cell Balance Stop Delta reduces the risk of over-balancing a higher voltage cell to slightly below the minimum voltage cell, and thereby slowly draining the pack. Operation while balancing during charge is similar, instead using the Settings:Cell Balancing Config:Cell Balance Stop Delta (Charge) configuration value. The Cell Balance Stop Delta parameters should be set to a lower level than the Cell Balance Min Delta parameters, then the device will have a hysteresis that delays restarting balancing until the level of imbalance again exceeds the higher Cell Balance Min Delta level. Note that when balancing is enabled because at least one cell voltage exceeds Cell Balance Min Delta, the device continues to attempt to balance all cells that are above the Cell Balance Stop Delta.

NORMAL versus SLEEP Mode—The BQ76942 device can also be configured to avoid autonomous balancing while in SLEEP mode by clearing the Settings:Cell Balancing Config:Balancing Configuration[CB_SLEEP] configuration bit. The device can also be prevented from entering SLEEP mode while balancing if the Settings:Cell Balancing Config:Balancing Configuration[CB_NOSLEEP] bit is set. Table 10-3 shows the functionality based on these bits.

Note: balancing is disabled immediately (without waiting for the interval timer to expire) if any of the following events occur:

• The device enters CONFIG_UPDATE mode.
• An enabled protection alert from Settings:Protections:Enabled Protections A occurs.
• An enabled protection fault from Settings:Protections:Enabled Protections A occurs except for a COV fault.
• An enabled Permanent Fail fault occurs.
• The device enters DEEPSLEEP mode.
• The device enters SHUTDOWN mode.

To disable autonomous cell balancing, the Settings:Cell Balancing Config:Balancing Configuration[CB_CHG] and Settings:Cell Balancing Config:Balancing Configuration[CB_RLX] configuration bits should be cleared.

Cell voltage measurements cannot be performed while balancing is underway due to the IR drop across the external cell input resistors significantly reducing the voltage at the device VC pins. Therefore, the device autonomously interrupts balancing briefly to allow cell voltage measurements to occur. During a measurement scan, the device disables balancing on any cell being measured, as well as any cell adjacent to that cell. Balancing is also disabled on the top cell while the top-of-stack voltage is being measured.

The COV and CUV protection checks for each cell normally operate every 3.3 ms when balancing is not enabled. To avoid balancing causing a protection alert or fault, the device disables the typical COV and CUV protection schedule while balancing is active. Every 1 second while balancing is active, the device briefly stops balancing of all cells, allows all COV and CUV protection checks to occur, then restarts balancing.

The device draws current through the VC10 pin to enable the balancing switches when balancing is active, resulting in approximately 35 μA flowing into the VC10 pin times the number of cells being balanced. To avoid this current affecting the cell 10-voltage measurements while balancing is active, minimize the cell 10-input resistor to reduce the IR drop across this resistor.