External FETs are suited for applications with typical 4.2V lithium ion cells since balancing is most commonly done at higher voltages during charge. For applications that need higher balancing current than the internal balancing can provide but also need to balance at lower cell voltages, external BJTs may be considered. The balancing current for an external BJT can be controlled by selecting the appropriate balance resistor (R_bal) and base resistor (R_bn). In Figure 2-5, as the internal FET is turned on inside the device, the current flowing through R_bn puts the NPN transistor into saturation.

Figure 2-5 Balancing Circuit Using External BJTs

A Zener diode is also used in this circuit to protect from pack transients similar to the FET circuits. A standard diode is also suitable to use instead of a Zener when using BJTs because it is the forward voltage of the Zener that protects the transistor. The base-emitter diode (or emitter-base diode for a PNP) will conduct in the reverse direction which will prevent the Zener from conducting.

In Figure 2-6, the circuit was designed with an R_n of 100Ω and R_gn of 240Ω. The R_bal resistor is set to 50Ω for a balance current of approximately 74mA through the BJT at 4V. At this cell voltage, an additional approximately 15mA of current flows through the internal FET of the device for a total balancing current of close to 89mA. An NPN transistor was selected with hFE of 30 at IC = 100 mA.

Figure 2-6 BQ76907 Cell Balancing With NPN BJT, Cell 7 (VC7) Rbal_voltage Approximately = 3.68V