Bq77915 application question

Hi Dustin,
1. When bq77915 detects OV and balancing is enabled it will balance the cell down so that the time at high voltage is reduced.
2. The D1 TVS helps control transients on the EVM. With high balance current the input filter resistors are smaller and the input filter may be less effective for transients. A component like D1 may be helpful in a design, or a different technique. The need for transient suppression will vary in a system with the cells, construction, load characteristics and the mentioned filtering. Select an appropriate design.
3. Yes, the S8264 data sheet functional diagram shows a resistor divider between the inputs and a relatively high input bias current. That part design will not allow a low current open wire detection. To detect open wire the secondary protector would need to be high impedance or use an aggressive test method in the battery circuit design to overcome the bias by the secondary protector.

Hi Dustin,
Q2: D1 is the designer's choice. The requirement is to prevent damaging voltages from reaching the bq77915 or other electronic part in the design. The designer may not have significant transients in their system or may have other techniques to adequately suppress them.
Q3: 2nd protectors with a resistor divider are intended to prevent open wire detection. Without a resistor divider, if a cell connection wire is open during battery assembly the 2nd protector may see the a high cell voltage and activate the attached fuse. The resistor divider provides a normal voltage when there is no connection to prevent the fuse blow. Some designers find this desirable. This helps during pack assembly, but is still present in the product after assembly. With properly connected cells the low impedance of the cell easily overcomes the resistor divider so that the 2nd protector can see an over voltage cell. Some designers find this acceptable. If a cell connection wire separates, the resistor divider will bias the input to a normal condition even though the BMS has no knowledge of the actual individual cell voltage. Some designers find this unacceptable. The designer may select a 2nd protector with high impedance or design a strong open wire detection mechanism into the battery. The designer and assembly staff must be careful during battery assembly that a permanent fault such as the fuse is not activated, it may require some assembly or test sequence or operation.
TI 2nd protectors do not have resistor dividers internally. See the functional diagram in the data sheet for bq7718, bq7716, bq2947, bq2961 or other protector of interest. While the bq77915 is expected to provide an open wire detection when used with a part such as the bq7718, the cautious designer will notice that the IIN tolerance of the 2nd protector is at the nominal open wire current of the bq77915 and may choose to specify a higher open wire current for the bq77915 in their design. Knowing their design has an open wire capability the battery and assembly process designers will need to pay attention to assembly instructions to avoid triggering non-reversible protections during manufacturing.