SLUSE14A December 2020 – February 2021 BQ76942
The BQ76942 device supports passive cell balancing by bypassing the current of a selected cell 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 autonomously balance cells 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 configuration setting. For host-controlled balancing, adjacent as well as non-adjacent cells can be balanced. Host-controlled balancing can be controlled using specific subcommands sent by the host. The device also returns status information regarding how long cells have been balanced through subcommands.
When host-controlled balancing is initiated using subcommands, the device starts a timer and will continue balancing until the timer reaches a programmed value, 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, which are described in detail in the BQ76942 Technical Reference Manual.
Due to the current that flows into the cell input pins on the BQ76942 device while balancing is active, the measurement of cell voltages and evaluation of cell voltage protections by the device is modified during balancing. Balancing is temporarily disabled during the regular measurement loop while the actively balanced cell is being measured by the ADC, as well as when the cells immediately adjacent to the active cell are being measured. Similarly, balancing on the top cell is disabled while the stack voltage measurement is underway. This occurs on every measurement loop, and so can result in significant reduction in the average balancing current that flows. In order to help alleviate this, additional configuration bits are provided which cause the device to slow the measurement loop speed when cell balancing is active. The BQ76942 device will insert current-only measurements after each voltage and temperature scan loop to slow down voltage measurements and thereby increase the average balancing current.
The device includes an internal die temperature check, to disable balancing if the die temperature exceeds a programmable threshold. However, the customer should still carefully analyze the thermal effect of the balancing on the device in system. Based on the planned ambient temperature of the device during operation and the thermal properties of the package, the maximum power should be calculated that can be dissipated within the device and still ensure operation remains within the recommended operating temperature range. The cell balancing configuration can then be determined such that the device power remains below this level by limiting the maximum number of cells that can be balanced simultaneously, or by reducing the balancing current of each cell by appropriate selection of the external resistance in series with each cell.