In the automotive and transportation marketplace, large battery stacks provide high output power without producing harmful emissions (i.e. carbon monoxide and hydrocarbons) associated with gasoline-powered combustion engines. Ideally, each individual battery in the stack equally contributes to the system. However, when it comes to batteries, all batteries are not created equally. Even batteries of the same chemistry with the same physical size and shape can have different total capacities, different internal resistances, different self-discharge rates, etc. In addition, they can age differently, adding another variable in the battery life equation.
A battery stack is limited in performance by the lowest capacity cell in the stack; once the weakest cell is depleted, the entire stack is effectively depleted. The health of each individual battery cell in the stack is determined based on its state of charge (SoC) measurement , which measures the ratio of its remaining charge to its cell capacity. SoC uses battery measurements such as voltage, integrated charge and discharge currents and temperature to determine the charge remaining in the battery. Precision single-chip and multi-chip battery management systems (BMS) combine battery monitoring (including SoC measurements) with passive or active cell balancing to improve battery stack performance. These measurements result in:
1. Healthy battery state of charge independent of the cell capacity
2. Minimized cell-to-cell state of charge mismatch
3. Minimized effects of cell aging (aging results in lost capacity)
Passive and active cell balancing offer different advantages to the battery stack and Linear Technology offers solutions in our battery management product portfolio for both methods. Let's first examine passive balancing.
Passive Balancing Allows All Cells to Appear to Have the Same Capacity
Initially, a battery stack may have fairly matched cells. But over time, the cell matching degrades due to charge/discharge cycles, elevated temperature and general aging. A weak battery cell will charge and discharge faster than stronger or higher capacity cells and thus it becomes the limiting factor in the run-time of a system. Passive balancing allows the stack to look like every cell has the same capacity as the weakest cell. Using a relatively low current, it drains a small amount of energy from high SoC cells during the charging cycle so that all cells charge to their maximum SoC. This is accomplished by using a switch and bleed resistor in parallel with each battery cell.
Figure 1. Passive Cell Balancer with Bleed Resistor
The high SoC cell is bled off (power is dissipated in the resistor) so that charging can continue until all cells are fully charged.
Passive balancing allows all batteries to have the same SoC, but it does NOT improve the run-time of a battery-powered system. It provides a fairly low cost method for balancing the cells, but it wastes energy in the process due to the discharge resistor. Passive balancing can also correct for long term mismatch in self discharge current from cell to cell.
Multi-Cell Battery Monitors with Passive Balancing
Linear Technology has a family of multi-cell battery monitors that include passive cell balancing. These devices feature a stackable architecture, allowing 100s of cells to be monitored. Each device measures up to 12 series connected battery cells with a total measurement error of less than 1.2mV. The 0V to 5V per cell measurement range makes them suitable for most battery chemistries. The LTC6804 is shown in figure 2 below.
Figure 2. LTC6804 Application Circuit with External Passive Balancing
The LTC6804 features internal passive balancing (figure 3) and can also be configured with external MOSFETs if desired (figure 4), as shown below. It also has an optional programmable passive balancing discharge timer that allows the user more system configuration flexibility.
Figure 3. Passive Balancing with Internal Discharge Switch
Figure 4. Passive Balancing with External Discharage Switch
For customers that wish to maximize system run-time and charge more efficiently, active balancing is the best option. With active cell balancing, energy is not wasted, but rather redistributed to other cells in the stack while both charging and discharging. When discharging, the weaker cells are replenished by the stronger cells, extending the time for a cell to reach its fully depleted state. For more on active balancing, see the continuation blog titled "Active Battery Cell Balancing."