SLUAB35 September 2025 BQ76905 , BQ76907

5 Cell Balance Mode

BQ76907 integrates a balancing circuit, which uses a passive balancing method and supports a maximum internal balancing current of 50mA. However, the host must send a command to enable balancing. This does not only support simultaneous balancing of non-adjacent cells, but also adjacent cells.

When balancing is turned on, the pin voltages of the corresponding cell and the adjacent cells can change. Figure 5-1 shows the equivalent circuit before and after the balancing of the Cell_n cell is turned on.

Figure 5-1 BQ76907 Internal Balance Circuit

By Figure 5-1, the corresponding VCx input pin voltage change as follows:

Table 5-1 VCx Pin Voltage Changes Before and After Cell Balance Is Turned On

Voltage	Balance OFF	Balance ON
$V_{(c n + 1)} - V_{c n}$	$V_{c e l l_n}$	$\frac{V_{c e l l_n}}{R_{n} + R_{n + 1} + R_{_d s o n}} \times R_{_d s o n}$
$V_{(c n)} - V_{(c n - 1)}$	$V_{c e l l_n}$	$V_{c e l l_n - 1} + \frac{V_{c e l l_n}}{R_{n} + R_{n + 1} + R_{_d s o n}} \times R_{_d s o n}$
$V_{(c n + 2)} - V_{(c n + 1)}$	$V_{c e l l_n + 1}$	$V_{c e l l_n + 1} + \frac{V_{c e l l_n}}{R_{n} + R_{n + 1} + R_{_d s o n}} \times R_{_d s o n}$

To prevent incorrect sampling voltage due to battery balancing, balancing must be disabled during sampling. To balance the balancing capability (balancing duty cycle) and sampling rate, the BQ76907 can add several IDLE Slots after the normal sampling ADSCAN LOOP

Figure 5-2 Voltage Measurement Loop in Cell Balance Mode (7S version)

Equation 2.

m = (2^{C B_L o o p_S L O W [1 : 0] + 1} - 1) \times 8

The more IDLE Slots are inserted, the greater the balancing duty cycle is, and the stronger the balancing ability of the battery pack is, but the sampling refresh rate can be reduced. Therefore, users need to configure the CB_LOOP_SLOW[1:0] register according to the actual usage scenario.

In addition, when the cell balancing is turned off, due to the existence of the RC network, the voltage recovery takes a certain amount of time, which can cause a large error in several voltage samplings after the cell balancing is turned off. To avoid such errors, BQ76907 opens a delayed sampling register: Settings: Cell Balancing: Balancing Configuration[CBDLY2:0]. Through this register, we can insert a 0-64ms delay between turning off Cell Balance and valid voltage sampling, as shown in Figure 5-3.

Figure 5-3 Voltage Measurement Loop in Cell Balance Mode (7S version)