Passive balancing allows all cells to have nearly the same capacity

In the automotive and transportation markets, large battery packs provide high output power without the harmful emissions (i.e. carbon monoxide and hydrocarbons) produced by gasoline-powered internal combustion engines. Ideally, each cell in the battery pack contributes the same to the system. However, when it comes to batteries, not all batteries are created equal. Even with the same chemical composition, physical size, and shape of a battery, its overall capacity, internal resistance, self-discharge rate, etc. may vary. Additionally, their aging rates can vary, which adds another variable to the battery life equation.

By Sam Nork and Kevin Scott | Analog Devices

In the automotive and transportation markets, large battery packs provide high output power without the harmful emissions (i.e. carbon monoxide and hydrocarbons) produced by gasoline-powered internal combustion engines. Ideally, each cell in the battery pack contributes the same to the system. However, when it comes to batteries, not all batteries are created equal. Even with the same chemical composition, physical size, and shape of a battery, its overall capacity, internal resistance, self-discharge rate, etc. may vary. Additionally, their aging rates can vary, which adds another variable to the battery life equation.

The performance of a battery pack is limited by the lowest capacity cells in the pack; once the weakest cell is depleted, the entire pack is completely depleted. The health of each battery cell in a battery pack is determined from its state of charge (SoC) measurements, which measure the ratio of remaining charge to battery capacity. The SoC uses battery measurements such as voltage, integrated charge and discharge current, temperature, etc. to determine the amount of 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 pack performance. These measurements yield the following results:

Healthy battery state of charge independent of single-cell capacity
Minimized state-of-charge mismatch between cells
Minimized cell aging effects (capacity loss due to aging)

For battery packs, passive and active cell balancing have different advantages, and Analog Devices’ battery management portfolio provides solutions for both approaches. Let’s look at passive balance first.

Passive balancing allows all cells to have nearly the same capacity

Initially, the cells of the battery pack may be matched fairly well. But 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, so the former becomes the limiting factor for system runtime. Passive balancing makes each cell in the 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, allowing all battery cells to charge to their maximum SoC. This is accomplished with a switch and bleeder resistor in parallel with each cell.

Passive balancing allows all cells to have nearly the same capacity
Figure 1. Passive battery equalizer with bleeder resistor

High SoC batteries are discharged (power is dissipated in resistors), so 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 runtime of battery powered systems. It provides a fairly low-cost method of cell balancing, but wastes energy in the process due to the discharge resistance. Passive balancing also corrects for long-term mismatches in self-discharge currents between different cells.

Passive balancing allows all cells to have nearly the same capacity
Figure 2. LTC6804 Application Circuit with External Passive Equalization

Multi-cell battery monitor with passive balancing

Analog Devices has introduced a family of multi-cell battery monitors that include passive cell balancing capabilities. These devices feature a stackable architecture that 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 cell makes it suitable for most battery chemistries. The LTC6804 is shown in Figure 2.

The LTC6804 has internal passive equalization (Figure 3); it can also be configured with external MOSFETs (Figure 4) if desired. It also features an optional programmable passive equalization discharge timer that provides users with more system configuration flexibility.

Passive balancing allows all cells to have nearly the same capacity
Figure 3. Passive Equalization with Internal Discharge Switch

Passive balancing allows all cells to have nearly the same capacity
Figure 4. Passive Equalization with External Discharge Switch

Active balancing is the best option for customers who want to maximize system runtime and charge more efficiently. During charging and discharging, active cell balancing does not waste energy, but instead redistributes it to other cells in the battery pack. When discharged, the stronger cells replenish the weaker cells, extending the time for the cells to reach their fully depleted state. For more information on active balancing, see the technical article “Active Cell Balancing”.

About the Author

Sam Nork has been with Analog Devices since 1988 in the Power Products business unit (formerly Linear Technology). As General Manager and Design Director, Sam leads a development team of over 120 engineers focused on battery chargers, ASSPs, PMICs and consumer electronics power products. Sam has personally designed and released numerous portable power management integrated circuits and is an inventor/co-inventor on 11 issued patents. Before joining Linear Technology, Sam worked as a product/test development engineer at Analog Devices in Wilmington, MA. He holds a Bachelor of Arts and a Bachelor of Engineering from Dartmouth College. Contact information:[email protected]

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