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

Figure 1. Passive battery 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 ensures that all cells have the same SoC, but it does not improve the runtime of the battery-powered system. It provides a relatively low-cost method for 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 the self-discharge currents of different battery cells.

Analog Devices has introduced a series of multi-cell battery monitors with passive cell balancing capabilities. These devices utilize a stackable architecture and can monitor hundreds of cells. Each device can measure up to 12 series-connected cells with a total measurement error of less than 1.2 mV. The 0 V to 5 V per cell measurement range 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 features internal passive balancing (Figure 3). It can also be configured with external MOSFETs if desired (Figure 4). It also has an optional programmable passive balancing discharge timer, providing users with additional 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 maximum system uptime and higher charging efficiency. During charging and discharging, active cell balancing doesn't waste energy, but instead redistributes it to other cells in the battery pack. During discharge, stronger cells replenish energy to weaker cells, extending the time it takes for cells to reach their fully depleted state.