With the rapid development of new energy and electric vehicles, lithium batteries with higher energy density have been used more. In the process of using lithium batteries in series, in order to ensure the consistency of battery voltage, BMS will inevitably be used to improve battery performance. performance and service life.
Shanghai Hangxin general MCU ACM32F0 series is supported by its low power consumption + 1 channel CAN + 100,000 times of erasing and writing 128K on-chip Flash + 125 degrees high temperature; ACM32F4 series is supported by its 180MHz M33 core + Flash acceleration + 100,000 times of erasing and writing 512K on-chip Flash +2 channels CAN + 125 degree high temperature support, which is widely used in BMS scenarios. The main functions of BMS include: power management, voltage detection, battery balancing, etc.
Overview of Cell Balancing
Battery balancing is a technology that prolongs battery life by maximizing the capacity of multiple series-connected batteries to ensure that the energy of each battery cell is available. Cell balancing refers to the use of differential current for different cells in a series of battery packs. A battery equalizer is a functional component in a battery management system that performs battery balancing commonly found in Li-ion battery electric vehicles and ESS applications.
Typically, the individual cells of a battery pack have different capacities and are at different SOC levels (SoC refers to the remaining capacity of an individual battery relative to its maximum capacity as it charges and discharges). Without redistribution, discharge must stop when the lowest-capacity cell is vented, even if the other cells remain unvented, limiting the battery pack’s ability to deliver energy. A balanced cell, on the other hand, means that each cell in a battery pack has the same state of charge (SoC).
During equalization, the higher-capacity battery undergoes a full charge/discharge cycle. Without cell balancing, the lowest-capacity cell is a weak point, and the entire battery pack can only be charged after its weakest cell is fully discharged, even if the other cells still have plenty of charge left. Therefore, balancing the individual cells maximizes the capacity of the battery pack and ensures that all the energy in it is available, thereby increasing battery life. In addition to maximizing battery capacity, cell balancing prevents overcharging and overdischarging of cells, ensuring safe battery operation. Cell balancing is one of the core functions of a BMS, along with temperature monitoring, charging, and other features that help extend battery pack life.
The need for cell balancing
Cell balancing is required when you need to combine multiple cells to power a device. Because battery cells are fragile, they can die or be damaged if charged or discharged too much. Taking batteries with different SoCs and starting to use them, their voltages start to drop until the battery with the least energy stored in it reaches the battery’s discharge cutoff voltage. At that point, if energy continues to flow through the battery, it can be irreparably damaged. If you try to charge this set of cells to the correct combined voltage, the healthy cells will overcharge and thus become damaged, as they will be absorbing energy that the damaged cells can no longer store. Unbalanced Lithium batteries can be damaged on the first try, which is why battery balancing is required. Our portable power station uses LiFePO4 batteries, and we strictly control the battery balance to maximize the stability of the product.
Other reasons for cell balancing include:
Thermal runaway
Batteries, especially lithium batteries, are very sensitive to overcharge and overdischarge. Thermal runaway occurs when internal heat is generated faster than it is dissipated. The increase in temperature can cause the lithium battery to change its structure and form a surface film on the electrodes, making the lithium battery decay faster. Also, excess heat buildup may cause damage to the cell balancing switches and resistors. By using cell equalization, each non-defective cell in the battery pack should be equalized to the same relative capacity as the other non-defective cells. Since heat is one of the main factors leading to thermal runaway, a cooling system can be used in addition to the battery equalizer to keep the battery pack at room temperature to minimize heat retention.
Battery aging
When a lithium battery is overcharged, even slightly above its recommended value, the battery’s energy capacity, efficiency, and life cycle are reduced. Battery aging is mainly caused by the following reasons:
Mechanical degradation or stack pressure loss of electrodes in pouch cells.
Growth of the solid electrolyte interface (SEI) on the anode. The SEI is considered to be the cause of the capacity loss of most graphite-based lithium batteries when the charging voltage remains below 3.92 v/cell.
Electrolyte oxide (EO) is formed at the positive electrode, which can cause a sudden loss of capacity.
Lithium plating of anode surfaces resulting from high charge rates.
Incomplete charging of the battery pack
The battery is charged at a constant current rate of 0.5 to 1.0 times, and the battery voltage rises as the charge progresses, peaks when fully charged, and then drops. Consider three batteries with 77Ah, 77Ah and 76Ah with 100% SoC, then all batteries are discharged and the SoC drops. It quickly became apparent that the AAA battery would be drained first, as it had the lowest capacity.
When the battery pack is energized and the same current flows through the battery, battery 3 lags again during charging and can be considered fully charged since the other two batteries are fully charged. This means that the coulombic efficiency (CE) of cell 3 is lower due to cell unbalance due to self-heating of the cell.
Incomplete use of battery pack energy
Consuming more current than the battery is designed for or shorting the battery is most likely to cause premature battery failure. When discharging the battery pack, weaker cells discharge faster than healthy cells, and they reach their minimum voltage faster than other cells. During battery operation, regular rest periods are provided to allow chemical conversions in the battery to maintain demand for current.
Types of Cell Balancing
Active equalization
Active cell balancing typically transfers energy from one cell to another. That is, moving from a high voltage/high SoC battery to a low SoC battery. The purpose of active balancing is that if you have a bank of lower capacity cells, you can extend the life of the battery pack or SoC by diverting energy from one of the higher energy cells in the pack than the other.
Active cell balancing efficiently transfers energy from high-voltage cells to low-voltage cells through a micro-converter circuit, avoiding energy loss due to heat. There are two different classes of active cell balancing methods: charge transfer and energy conversion. Charge transfer is used to actively transfer charge from one cell to another to achieve equal cell voltages, and energy conversion is the use of transformers and inductors to move energy between cells in a battery pack.
Other active cell balancing circuits are often based on capacitors, inductors or transformers and power electronics interfaces, which require:
Capacitor-based
• Single capacitor, this method is simple because it uses a single capacitor regardless of the number of cells connected in the battery. However, this approach requires a large number of switches and intelligent control of the switches.
• Multiple capacitors, this method connects multiple capacitors to each battery and transfers unequal battery energy through multiple capacitors, it does not require voltage sensors or closed loop control.
Inductor or transformer based
• Single/multi-inductor, single-inductor battery balancing circuit is small in size and low in cost, while multi-inductor balancing speed is fast and battery balancing efficiency is high.
• Single transformer, this method has fast equalization and low magnetic loss.
• Multi-transformer, this battery equalizer has fast equalization speed, however, it requires an expensive and complex circuit to prevent the transformer from being flooded.
Based on Power Electronics Interface
• Flyback/Forward converter, the energy of the high voltage battery is stored in the transformer, the battery equalizer has high reliability.
• Full bridge converter, this cell equalizer has fast equalization speed and high efficiency.
Active equalizers are capable of pushing large amounts of current from one cell to another.
Advantages of Active Equalization:
• It improves capacity usage and works well when there are different battery capacities in a series.
• It improves energy efficiency, it saves energy by transferring excess energy to lower energy batteries instead of burning excess energy in batteries.
• Extended life, it increases the life expectancy of the battery.
• Fast equalization.
Disadvantages of Active Equalization:
• When energy is transferred from one battery to another, approximately 10-20% of energy is lost.
• Charge can only be transferred from the upper battery to the lower battery.
• Although active cell equalizers have high energy efficiency, their control algorithms can be complex and expensive to produce because each cell should interface with additional power electronics.
Passive equalization
Energy-consuming balance is usually defined as passive balance, passive balance uses resistance, consumes the energy of high-voltage or high-charge cells to achieve the purpose of reducing the gap between different cells, is a kind of energy-consuming balance . If cells are connected in series, and some cells have higher energy than others with lower energy, the cells can be equalized by connecting a resistor across the cells to equalize the burning energy of the top cell, which releases the energy into heat Group energy.
Passive balancing makes all batteries appear to have the same capacity. There are two different classes of passive cell balancing methods: fixed shunt resistors and switched shunt resistors.
A fixed shunt resistor circuit is usually connected to a fixed shunt to prevent it from being overcharged. With the help of resistors, passive equalization circuits can control the limit value of each cell’s voltage without damaging the cells. The energy dissipated by these resistors to equalize the battery may cause heat loss in the BMS. Therefore, this proves that the fixed shunt resistor method is an inefficient cell balancing circuit.
Switching shunt resistor battery balancing circuit is the most commonly used method in battery balancing at present. The method has continuous mode and induction mode, in continuous mode, all switches are controlled to open or close at the same time. In inductive mode, each battery requires a real-time voltage sensor. This cell balancing circuit dissipates high energy through balancing resistors. This cell balancing circuit is suitable for battery systems that require low current when charging or discharging.
Advantages of passive equalization:
• Does not have to actively balance the battery pack and still works perfectly.
• The battery cells do not wear out when there is no charge, and once the battery is fully charged, it will only balance when it has enough power.
• It makes all battery cells have the same SoC.
• It provides a low-cost method of cell balancing.
• It corrects long-term battery-to-battery self-discharge current mismatches.
Disadvantages of passive equalization:
• Poor thermal management.
• They do not equalize when the SoC is full. It is only balanced at around 95% at the top of each unit, as the batteries are forced to burn off excess energy when they have different capacities.
• Its energy transfer efficiency is generally low. The electrical energy is dissipated as heat in the resistor, and the circuit also contributes to switching losses, in other words, a passive equalization circuit causes a large amount of energy loss.
• It does not improve the runtime of battery powered systems.
Example provided by Wangbin Zhao of Shanghai Institute of Space Power Research
The active equalization circuit of the multi-winding transformer is divided into a power module and a control module. The power module consists of battery cells, equalizing transformers and switching transistors (MOSFETs). At the same time, the module can also be expanded according to actual needs. Each battery is connected in series with the battery pack through a MOSFET, and a periodic signal with a fixed duty cycle is used to control the discharge of the battery with a higher voltage. The control module includes an FPGA control unit and an AD sampling unit. Each battery voltage signal enters AD sampling through a first-order low-pass filter. The AD sampled signals of all battery voltages are processed and sent to the FPGA, and the balance control of the battery pack is realized by using the balance algorithm inside the FPGA. The relationship between the switching period of the MOSFET and the peak current of the balancing transformer is as follows:
TS – switching period;
TON – turn-on time of the MOSFET;
TOFF – turn-off time of the MOSFET;
Lpri – primary magnetizing inductance;
Ipri-peak – primary peak current;
Ubat – single battery voltage;
Lsec – the second magnetizing inductance;
Isec – peak-sub-peak current;
UOFF – total battery pack voltage;
The design of the equalizing transformer is related to the working performance of the equalizing circuit. Therefore, the transformer parameters must be properly designed. During the charging process of the battery pack, once the active equalization circuit detects that the voltage of a certain cell is too high, it will start the corresponding equalization switch to discharge the cell. The average discharge current on the primary side of the equalizing transformer is:
In the same way, the average charging current of the secondary battery of the equalizing transformer can be obtained as:
N – the number of batteries in series;
k – the turns ratio of the primary and secondary of the transformer;
Analyzing equations (1) to (3), it is concluded that under the fixed duty cycle control method, the balanced average current is only related to the turns ratio of the transformer primary and secondary windings, the number of batteries and the current peak value.
What is the equalizing current required for the battery pack?
Balancing cells means that on some SoCs, all cells are on exactly the same SoC. The current required to equalize the cells depends on the cause of the cell imbalance. It is divided into two categories: total equilibrium and maintenance equilibrium.
Total equilibrium
If the battery pack is not manufactured or serviced with the individual cell’s initial SoC in mind, the balancer may be expected to do the overall balancing job. In this case, the maximum length of time required to balance the battery pack depends on the size of the battery pack and the balancing current. The required equalization current is proportional to the size of the battery pack and inversely proportional to the required equalization time:
Equalization Current [A] = Package Size [Ah] / Total Equalization Time [hours]
For a 100Ah empty and full battery pack, a BMS with an equalization current of 1A takes nearly a week to equalize. And a BMS with a balancing current of 10 mA cannot balance a 1000 Ah battery pack over its lifetime. In other words, if you want the BMS to balance a large capacity and extremely unbalanced battery pack in a reasonable time, it needs to provide a relatively high balance current.
Maintain balance
If a battery pack starts out balanced, it will be easier to keep it balanced. If all cells have the same self-discharge leakage, then equalization is not needed; the cells’ SoC slows down exactly the same, so the pack is balanced. If there is one cell in the battery pack that has a self-discharge leakage current of 1mA or more and the other cells have the same leakage current, the BMS takes 1mA averagely from all other cells or just adds 1mA to that cell, which is considered average equalization current.
In many applications, the BMS cannot achieve infinite balance except for continuous leakage and discharge. Therefore, the equalization current must be higher, inversely proportional to the time available for the BMS to equalize the battery pack.
E.g:
If the BMS can balance continuously, the balance current can be 1mA, and if the BMS can only balance for 1 hour per day, the balance current should be 24mA to achieve an average value of 1mA.
More importantly, if the BMS can run more equalization current than the required minimum, the BMS can:
• Keep equalization always on, but lower its value to match the battery self-discharge leakage increment.
• By switching the equalization on and off with the duty cycle, on average, the current matches the battery’s leakage current increment.
The required equalization current is proportional to the difference between the leakage current and the percentage of time available for equalization:
Equalization Current [A] = (Maximum Leakage Current [A] – Minimum Leakage Current [A]) / (Daily Equalization Time [hours] / 24 [hours])
Balancing current is the amount of current that the equalizer shunts a fully charged battery so that it can continue to allow the same current to flow into a non-full battery at the same time. The correct amount depends on how quickly you want to end the equalization.
In conclusion
Equalization compensates the SoC of a single cell, not capacity imbalance. The benefit of pack balancing is that if the pack is balanced at the factory, the BMS only needs to handle the balancing current. This makes more sense for building an already balanced battery pack without having to use a BMS that can perform total balancing.
To minimize the effects of cell voltage drift, the imbalance must be properly adjusted. The goal of any equalization scheme is to keep the battery pack operating at the desired level of performance and to extend its useful capacity. Passive balancing is the best option for customers looking to minimize cost and correct long-term mismatches in self-discharge current between cells.