BQ7718: Debalancing

Hi Guenter,
There may be uncertainty with the term "debalancing".
In our BMS documents we think of cells as "balanced" when voltages or capacities are aligned, usually at full charge. A pack is considered to have imbalanced cells or to be unbalanced or perhaps be debalanced when the voltages or capacities are not equal. The operation of matching the cells is called "balancing" or the action "to balance". Not all BMS parts have balancing capability. BMS parts which can balance cells may be:
A gauge which knows individual cell capacities,
A gauge which knows only voltage,
A monitor or AFE which has a mechanism to allow the connected MCU to implement a balancing algorithm, or
A protector device which has some voltage based balancing algorithm.

Q1:
The BQ7718 is an over voltage protector which does not have a mechanism to accomplish balancing. Its function is to indicate when one or more cells have been above a threshold for a fixed time. Its operation is voltage only and is independent of load current. It does not have a mechanism to cause cells to balance, or to indicate which cells have exceeded the threshold when that occurs.
Q2:
In measuring the cells to determine their voltage condition, there are connections which will have some sample current and by their presence may have some leakage current. Both of these currents are included in the data sheet specification IIN. As you can see from the +/- 100 nA limit the target would be 0 so that the BQ7718 would not induce any imbalance. If the part is not ideal, some imbalance may develop over time.

The BQ7718 is commonly called a secondary protector since it is often used in a system with another BMS device providing additional protection features. The BQ77905 protector has more features and is often called a primary protector. It includes an option for open wire detection where each input is loaded with a current. The bottom cell input is loaded with the OW current, the second cell input is loaded with the same nominal current, and this continues up the cells. The bottom cell sees the system current plus all the OW feature currents while the top cell sees the supply current plus only the top cell OW current. This feature will create an imbalance in the cell voltages over time, the system designer should be aware and plan for this in their design. In our common terminology an aged pack would be debalanced or have imbalanced cells. The BQ77905 does not contain a balancing mechanism. The BQ7715 is a similar protector with additional features including balancing capability. It also has an open wire feature which can result in a cell imbalance over time. When an imbalance develops over time and the conditions for balancing are met the cells will be balanced according to the algorithm.

If this does not cover the questions please clarify.