Do you understand “passive balancing” to make all battery cells have the same capacity?

• Healthy battery state of charge independent of single cell capacity

• Minimize state-of-charge mismatch between battery cells

• Minimize the effects of battery cell aging (capacity loss due to aging)

Initially, the cells in a battery pack may be fairly well matched. However, over time, cell matching degrades due to charge/discharge cycles, high temperatures, and general aging. Weak cells will charge and discharge faster than stronger (or higher capacity) cells, making the former the limiting factor in system run time. Passive balancing makes each cell in the battery pack appear to have the same capacity as the weakest cell. It uses relatively low current during the charge cycle, consuming a small amount of energy from the high SoC battery, so that all cells are charged to their maximum SoC. This is achieved with switches and bleed resistors in parallel with each cell.

Figure 1. Passive cell balancer with bleeder resistors.

The high SoC battery discharges (power is dissipated in the resistor), so charging can continue until all cells are fully charged.

Passive balancing makes all cells have the same SoC, but it does not improve the run time of the battery-powered system. It provides a fairly low-cost method of cell balancing, but energy is wasted in the process due to the presence of discharge resistors. Passive balancing can also correct for long-term mismatches in self-discharge current between different battery cells.

ADI has introduced a series of multi-cell battery monitors with passive battery balancing capabilities. These devices use a stackable architecture and can monitor hundreds of cells. Each device can measure up to 12 cells connected in series with a total measurement error of less than 1.2 mV. The measurement range of 0 V to 5 V per battery cell makes it suitable for most battery chemistries. The LTC6804 is shown in Figure 2.

Figure 2. LTC6804 application circuit with external passive equalization.

The LTC6804 has an internal passive balancing function (Figure 3). It can also be configured with external MOSFETs if desired (Figure 4). It also has an optional programmable passive balancing discharge timer, which provides the user with more system configuration flexibility.

Figure 3. Passive balancing with internal discharge switch.

Figure 4. Passive balancing with external discharge switch.

Active balancing is the best choice for customers who want to maximize system run time and charge more efficiently. During charging and discharging, active cell balancing does not waste energy, but redistributes energy to other cells in the battery pack. When discharging, stronger cells replenish energy to weaker cells, thereby extending the time it takes for the battery cells to reach their fully depleted state.