How Active and Passive Cell Balancing Work

okhxyyo

How Active and Passive Cell Balancing Work [Copy link]

In the Power System Design article, “Active and Passive Balancing of Battery Management Systems,” Stefano Zanella describes how multi-cell systems can become unbalanced. In this article, I want to explore how batteries can become unusable if they are unbalanced and expand a little on the effects of mismatched battery capacities. I will focus on automotive lithium-ion (Li-ion) batteries, but in general the principles apply to all batteries. Multi-cell batteries are typically built as arrays of cells in series or parallel. Too many cells in series will result in a higher battery pack voltage, while too many cells in parallel will result in a higher total battery capacity (expressed as an ampere-hour rating, or Ahrs). The battery capacity will then dictate the number of cells in parallel, which will be equal to the number of cells in parallel multiplied by the battery capacity required for the system to operate. Depending on the battery type, automobiles tend to use 96 Li-ion cells in series and 24 cells in parallel. For example, an electric vehicle with a range of 100 miles will require 20-30kWh of batteries, depending on the weight of the vehicle, the expected usage pattern, and the efficiency of the various systems in the vehicle. Several aspects of the system will determine the battery pack voltage, including the overall size and type of electric motors, cable size, and isolation requirements. A multi-cell battery is charged by supplying current to the positive terminal of the cell at the top of the stack. (Assume the battery consists of n cells in series). In other words, the cells are not charged individually. If you read Stefano's article, you will learn that at the end of charging, the amount of charge left in each cell is different; and that this difference increases as you charge and discharge the battery repeatedly (without balancing). Click this link to see how active and passive cell balancing works If you think of the two cells in Figure 1 as identical charging containers, then driving an electric vehicle will result in energy being extracted from the batteries, which will deplete those containers. Charging the electric vehicle injects charge into the battery, filling those containers. Not all cells are identical to one another, and they are not uniform; therefore, weaker cells will charge and discharge at slightly different rates. The voltage level of each cell will slowly rise and fall as the battery charges and discharges, respectively. Let's start with a complete battery. All the energy contained in the battery (the available energy) is available to power the car. In order not to over-discharge the battery (because over-discharge reduces battery life and can affect safety), discharge must be stopped when the first cell reaches the undervoltage threshold (plus a safety margin that is usually determined by the protector). In order not to overcharge the lithium-ion battery, charging must be stopped when the first cell reaches the overvoltage threshold. However, the lagging cell is not fully charged yet, leaving some energy in the battery that cannot be used for driving, because charging must be stopped again when the first cell is full. In other words, after the first charge/discharge cycle, some energy is stranded in the battery pack. It can never be used to power the car. As the battery is charged and discharged repeatedly, the stranded energy increases, reducing the available energy. In addition, the loss of available energy is twice the amount of the stranded energy, because the stranded energy is not available and the equivalent energy cannot be injected into another cell. After enough charge and discharge cycles, the available energy starts to approach zero. How do you avoid this problem? Balancing! You can achieve cell balancing by dissipating the excess energy into resistors, thus regaining the ability to charge the cells to full capacity. As long as all cells have the same capacity, full balancing is not necessary at the end of each charge cycle - because the effects of charge imbalance are fully reversible. I have observed a case during battery electronics development where the passive balancing portion of the cells was implemented only after many charge/discharge cycles. By the time the balancing system was ready, the available charge had dropped by more than 25%. However, after balancing all cells, the pack was fully charged with minimal loss of available energy. You should choose the amount of balancing current based on the application and thermal considerations. For example, in a 24kWh system (96 cells in series), assuming that the cells have less than 1% charge time difference at the end of their life (the difference in charge time increases over time), a 66Ah system will need to compensate for 660mAh. With a 200mA balancing current, you can balance this system in 3.3 hours, but it will take twice as long to balance a 100mA current.

Applications	#Series Cells	TIMonitoring & Protection Components
Notebooks/Tablets	2-4	bq40z50-R1, bq2947
Power Tools & Garden Tools	3-10	bq76930, bq76920, bq76925
Ebike	7-16	bq76930, bq76940, bq76Pl455A-Q1, bq78350-R1
EV/HEV/PHEV	60-96	bq76Pl455A-Q1, bq76PL536A-Q1
Micro Hybrid	4-6	bq76PL536A-Q1
Mild Hybrid	12-16	bq76PL455A-Q1
eCall	1-2	bq76PL455A-Q1, EMB1428Q, EMB1499Q
Telecom, UPS, ESS	10-16	bq76940, bq76PL455A-Q1, bq78350-R1

Table1: Monitoring and protection devices for specific applications

If you want to start balancing battery cells, the monitoring and protection devices in Table 1 may be a perfect match for your application. If your application is not included in this table, or if you have questions about your current design, you can follow the post to add or ask questions.