18650 battery lithium ion 2200mah

　　Active charge balancing method for 18650 battery lithium ion 2200mah

　　The E-Cart is a drivable vehicle primarily used to demonstrate the electrical performance of hybrid vehicles. The car would be powered by a massive lithium-ion battery pack, and developers realized at the time that battery management with charge balancing was an absolute necessity. In this case, active energy transfer between cells must be used instead of the traditional simple charge balancing scheme. The active charge balancing system they developed can provide better performance at the same material cost as passive solutions (see Figure 1).

　　Battery system architecture

　　Nickel-cadmium batteries, and later nickel-metal hydride batteries, dominated the battery market for many years. Lithium-ion batteries have seen their market share grow very rapidly due to their greatly improved performance. The energy storage capacity of lithium-ion batteries is amazing, but even so, the capacity of a single battery cell is still too low in terms of voltage and current to meet the needs of a hybrid engine. Connecting multiple battery cells in parallel can increase the current provided by the battery, and connecting multiple battery cells in series can increase the voltage provided by the battery. Battery assemblers often use abbreviations to describe their battery products, such as "3p50S" which represents a battery pack with three parallel cells and 50 series cells. Modular structures are ideal when managing batteries containing multiple cells connected in series. For example, in a 3p12S battery array, every 12 battery cells form a module (block) after being connected in series. These battery cells are then managed and balanced by an electronic circuit with a microcontroller at its core. The output voltage of such a battery module depends on the number of battery cells connected in series and the voltage of each battery cell. The voltage of a lithium-ion battery cell is usually between 3.3V and 3.6V, so the voltage of a battery module is approximately between 30V and 45V.

　　The driving of hybrid vehicles requires a DC power supply voltage of approximately 450V. In order to compensate for changes in battery cell voltage depending on the state of charge, it is appropriate to connect a DC-DC converter between the battery pack and the engine. This converter also limits the current output from the battery pack. To ensure that the DC-DC converter works optimally, the battery pack voltage is required to be between 150V and 300V. Therefore, 5 to 8 battery modules need to be connected in series.

　　The need for balance

　　If the voltage exceeds the allowed range, lithium-ion battery cells are easily damaged (see Figure 2). If the voltage exceeds the upper and lower limits (taking nanophosphate lithium-ion batteries as an example, the lower limit voltage is 2V and the upper limit voltage is 3.6V), the battery may be irreversibly damaged. The result is at least a faster self-discharge of the battery. The battery output voltage is stable over a wide state of charge (SOC) range, and there is little risk of the voltage deviating from the safe range. But at both ends of the safe range, the ups and downs of the charging curve are relatively steep. Therefore, voltage must be closely monitored as a precaution.

　　Figure 2: Discharge characteristics of lithium-ion batteries (nanophosphate type).

　　If the voltage reaches a critical value, the discharging or charging process must be stopped immediately. With the help of a powerful balancing circuit, the voltage of the relevant battery cell can be returned to a safe range. But to achieve this, the circuit must be able to transfer energy between cells as soon as the voltage of any cell in the battery pack begins to differ from that of other cells.

　　Charge balancing method

　　1. Traditional passive method: In a general battery management system, each battery cell is connected to a load resistor through a switch. This passive circuit can discharge individual selected cells. But this method is only suitable for suppressing the voltage rise of the strongest battery cells in charging mode. In order to limit power consumption, such circuits generally only allow discharge with a small current of about 100mA, causing the charge balance to take up to several hours.

　　2. Active balancing method: There are many active balancing methods in relevant information, all of which require a storage element for transferring energy. If capacitors are used as storage elements, a huge switch array is required to connect them to all battery cells. A more efficient method is to store energy in a magnetic field. The key component in this circuit is a transformer. The circuit prototype was developed by Infineon's development team together with VOGT Electronic Components GmbH. Its function is:

　　a. Transfer energy between battery cells

　　b. Multiplex multiple individual cell voltages to a ground-based analog-to-digital converter (ADC) input

　　The circuit is constructed according to the flyback transformer principle. This type of transformer stores energy in a magnetic field. The air gap in its ferrite core increases the magnetic resistance and therefore prevents magnetic saturation of the core material. The circuits on both sides of this transformer are different:

　　a. The primary coil is connected to the entire battery pack

　　b. The secondary coil is connected to each battery unit

　　One practical model of this transformer supports up to 12 battery cells. The number of possible connections to the transformer limits the number of battery cells. The above prototype transformer has 28 pins.

　　The switches use OptiMOS3 series MOSFETs, which have extremely low on-resistance, so their conduction losses are negligible.

　　Each module in the picture is controlled by Infineon's 8-bit advanced microcontroller XC886CLM. This microcontroller comes with flash program memory and a 32KB data memory. In addition, it has two hardware-based CAN interfaces that support communication with the underlying processor load via the Bus Controller Area Network (CAN) bus protocol. It also contains a hardware-based multiplication and division unit that can be used to speed up the calculation process.

　　balanced approach

　　Since the transformer can work in both directions, we can take two different balancing methods depending on the situation. After performing a voltage scan on all battery cells (the details of the voltage scan will be introduced later), calculate the average value, and then check the battery cell whose voltage deviates the most from the average value. If the voltage is lower than the average, bottom-balancing is used; if the voltage is higher than the average, top-balancing is used.

　　1. Bottom balance method: The example shown in Figure 4 is the bottom balance method used. The scan found that battery cell 2 was the weakest cell and had to be strengthened.

　　Figure 4: Bottom charge balancing principle of lithium-ion batteries.

　　At this point the main switch ("prim") is closed and the battery pack begins charging the transformer. Once the main switch is open, the energy stored in the transformer can be transferred to selected battery cells. Energy transfer begins when the corresponding secondary ("sec") switch - in this case switch sec2 - is closed.

　　Each cycle consists of two active pulses and a pause. In this example, a period of 40 milliseconds translates to a frequency of 25kHz. When designing the transformer, its operating frequency band should be above 20kHz to avoid perceptible howling noise in the human hearing frequency range. This sound is caused by the magnetostriction of the transformer's ferrite core.

　　Especially when the voltage of a certain battery cell has reached the lower limit of the SoC, the bottom balancing method can help extend the working time of the entire battery pack. As long as the current supplied by the battery pack is less than the average balancing current, the vehicle can continue to operate until the last battery cell is depleted.

　　2. Top balancing method: If the voltage of a certain battery unit is higher than that of other units, then the energy needs to be exported, which is especially necessary in charging mode. Without balancing, the charging process would have to stop as soon as the first battery cell is fully charged. Balancing prevents premature cessation of charging by keeping the voltages of all battery cells equal.

　　Figure 5: Top charge balancing principle of lithium-ion batteries.

　　Figure 5 shows the energy flow in top equilibrium mode. After the voltage scan, it is found that battery cell 5 is the cell with the highest voltage in the entire battery pack. At this time, switch sec5 is closed and current flows from the battery to the transformer. Due to the presence of self-inductance, the current increases linearly with time. Since self-inductance is an inherent characteristic of the transformer, the conduction time of the switch determines the maximum current value that can be achieved. The energy transferred from the battery cells is stored in the form of magnetic fields. After switch sec5 is opened, the main switch must be closed. At this time, the transformer enters the energy output mode from the energy storage mode. Energy is fed into the entire battery pack via a huge primary coil.

　　The current and timing conditions in the top balancing method are very similar to those in the bottom balancing method, except that the sequence and direction of the current are opposite to those in the bottom balancing method.

　　Balanced power and voltage sweep

　　According to the prototype configuration in Infineon E-Cart, the average balancing current can reach 5A, which is 50 times higher than the current of the passive balancing method. At a balanced current of 5A, the entire module consumes only 2W, eliminating the need for special cooling measures and further improving the energy balance of the system.

　　In order to manage the state of charge of each battery cell, their individual voltages must be measured. Since only cell 1 is within the microcontroller's ADC range, the voltages of the other cells in the module cannot be measured directly. One possible solution is to use an array of differential amplifiers, and they must support the voltage of the entire battery module.

　　The method described below requires only a small amount of additional hardware to measure the voltage of all battery cells. In this method, the transformer whose main task is charge balancing is also used as a multiplexer.

　　Flyback mode of the transformer is not used in voltage sweep mode. When one of the switches S1 to Sn is closed, the voltage of the battery cell connected to it is transferred to all windings of the transformer.

　　After simple preprocessing by a discrete filter, the measured signal is sent to the ADC input port of the microcontroller. The duration of the measurement pulse generated when one of the switches S1 to Sn is closed may be very short, with an actual conduction time of 4us. Therefore, very little energy is stored into the transformer by this pulse. And anyway after the switch is opened, the energy stored in the magnetic field flows back through the primary transistor to the entire battery module. Therefore, the energy of the battery module is not affected. After a cycle of scanning of all battery cells, the system returns to the initial state.

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