03/01/2025
Check this article to know more differences between active and passive balancer.
Active balancing and passive balancing are hot topics in the electric vehicle BMS industry.
A lithium battery pack without balancing is like an engine that is not maintained. A BMS without balancing function is just a data collector, and it is difficult to call it a management system. Both active balancing and passive balancing are to eliminate the inconsistency of the battery pack, but the implementation principles of the two are completely opposite.
Because some people define the balancing initiated by BMS based on algorithms as active balancing, to avoid ambiguity, all balancing that uses resistors to dissipate energy is called passive balancing, and all balancing achieved through energy transfer is called active balancing.
丨Basic battery pack design principles
Why do batteries need active and passive balancing? First, let's understand the basic battery pack design principles.
Basic battery pack design principles:
· When the first single battery is fully charged, charging must be stopped.
· When the first single battery is out of power, discharge must be terminated.
· Weak battery cells age faster than strong batteries.
·The weakest cell will eventually limit the available charge in the pack (the weakest link).
·Systematic temperature gradients in the pack weaken cells that run at higher average temperatures.
·Without balancing, the voltage difference between the weakest and strongest cells will increase with each charge and discharge cycle. Eventually, one cell will always be close to maximum voltage, while the other is close to minimum voltage → hampering the pack's ability to charge and discharge.
Since the cells will never be as matched to each other as they were when they were first used, and since our installation method will subject them to different temperature environments, I must do a good job of cell balancing.
There are two main types of mismatches in lithium-ion batteries; charge mismatch and capacity mismatch. Charge mismatch occurs when cells of the same capacity gradually contain different amounts of charge. Capacity mismatch occurs when cells with different initial capacities are used together. Since battery packs are usually assembled from cells produced at almost the same time, and the manufacturing processes of these cells are also similar, the cells are usually well matched. However, if the battery pack is assembled from single cells of unknown origin or has great differences in manufacturing processes, capacity mismatch may also occur.
丨Active balancing VS passive balancing
1. Purpose
The battery system group is composed of many batteries in series. Each battery cannot be exactly the same. When the first single cell is fully charged, charging must be stopped. When the first single cell is out of power, discharging must be terminated. The significance of balancing is to use electronic technology to keep the voltage deviation of lithium-ion battery cells within the expected range, so as to achieve overall usability and controllability, thereby ensuring that each single cell is not damaged during normal use and extending its service life.
2. Design comparison
Passive balancing generally discharges the battery with higher voltage through resistance discharge, releases electricity in the form of heat, and gains more charging time for other batteries during the charging process.
Active balancing is a complex balancing technology that redistributes the charge in the battery cell during the charging and discharging cycle, thereby shortening the charging time and extending the discharge time. The bottom balancing strategy is generally adopted in the discharge mode, and the top balancing strategy is generally adopted in the charging mode.
2. Comparison of advantages and disadvantages
丨Conclusion
The concept and products of BMS were first proposed by foreign countries. Foreign semiconductor manufacturers first designed dedicated ICs. At first, they only detected voltage and temperature. Later, after the concept of balancing was proposed, the method of resistor discharge was adopted and this function was added to the IC (because this discharge control function is easy to integrate into the chip). Now TI\MAXIM\LINER, which are widely used, have such chips in production. Some of them have the switch drive built into the chip, and some even try to build the switch into the chip.
From the passive balancing principle and schematic diagram above, we can see that if the battery pack is compared to a wooden barrel, the series-connected batteries are the boards that make up the wooden barrel. The battery with low power is the short board, and the battery with high power is the long board. The work of passive balancing is to "cut the long without making up the short". The energy in the battery with high power is dissipated as heat, and the power utilization efficiency is low. Moreover, because the electric energy is converted into heat dissipation, a dilemma is brought about. If the balancing current is large, the heat will be more, and how to dissipate the heat will become a problem; if the balancing current is small, the efficiency of the power balancing effect in a large-capacity battery pack with a large power difference is very low, and it takes a long time to achieve balance, which is like scratching an itch in the application. Weighing the pros and cons, the current of passive balancing is generally at the level of 100 mA.
Because of the limitations of passive balancing, the concept of active balancing has been proposed and developed. Active balancing is to transfer energy from high-energy batteries to low-energy batteries, which is equivalent to "cutting the long and making up the short" of a wooden board. Because unlike passive balancing, which only "cuts", the industry has fully utilized their respective advantages and imagination on the issue of how to "make up", and the active balancing scheme is colorful.
The benefits of active balancing are obvious: high efficiency, energy is transferred, and the loss is only the transformer coil loss, which accounts for a small proportion; the balancing current can be designed to be large, reaching a few amperes or even 10A, and the balancing effect is fast. Despite these benefits, active balancing also brings new problems. The first is the complex structure, especially the transformer scheme. How to design the switch matrix for dozens or even hundreds of battery strings and how to control the drive are all headaches. This is why the active balancing function has not been fully integrated into a dedicated IC. Semiconductor manufacturers have always hoped to make a unified chip, but they are really unable to do so in BMS. The same is true for BMS manufacturers. In terms of active balancing circuit structure, few manufacturers' designs can be refreshing and applauded. The second is the cost issue. Complex structures will inevitably lead to complex circuits, and rising costs and failure rates are inevitable.
Because the two balancing functions have their own advantages and disadvantages, we recommend that you can choose passive balancing if the battery consistency is good, and consider choosing active balancing if the battery consistency is relatively discrete.
