Another example is the bq20zxx fuel-gauge family, which uses the Impedance Track balancing strategy. Instead of trying to minimize the effects of misleading voltage divergence, this fuel gauge calculates the needed charge (QNEED) to reach a fully charged state for each cell (Figure 5). The balancing algorithm turns on the cell balancing FETs during charging to supply the required QNEED. This type of fuel gauge carries out this QNEED-based cell-balancing scheme with ease because the total capacity and SOC are readily available from the gauging function. Since cell balancing does not distort differences in cell impedance it, therefore, operates independently of the battery charge, discharge, or even idle states. More importantly, it achieves the best balancing accuracy.

Figure 5: QNEED-based cell balancing

Since passive cell balancing with an integrated-FET solution has limited balancing capabilities, the rate of cell divergence or imbalance may overwhelm the cell balancing. Also, because of low bypass current, it may take several cycles to make corrections for a typical imbalance. Designing external bypass circuits with existing components can strengthen cell balancing (Figures 6 " 7). In Figure 6, the internal balancing MOSFET is first turned on when a balancing decision is made for a particular cell. This creates a low-current path through the external filter resistors connected to the cell terminals (Cell 1 and Cell 2) and IC pins. When the internal-FET gate-to-source voltage is established across the resistor, the external MOSFET is turned on. The drawback is that the adjacent cells cannot be balanced quickly and simultaneously. For example, if adjacent internal FETs are turned on, Q2 cannot be turned on because there is no current flow through R2.

Figure 6: Passive cell balancing with external FETs

Figure 7: New topology for passive cell balancing with external FETs

Figure 7 shows the latest example of passive cell balancing. It is a low-cost, single chip fuel-gauge solution. Unlike the fuel-gauge solution previously mentioned, this IC does not have internal cell balancing, but needs a similar external bypass circuit to perform balancing. However, because the enabling circuit for balancing is an open-drain inside the IC, it can simultaneously balance several cells including adjacent cells. This balancing circuit uses an improved voltage-based algorithm, just like the circuit shown in Figure 6. However, the external FET drivers in Figure 7 provide more effective cell balancing.