Active charging balancing method for lithium-ion battery packs

The E-Cart is a drivable vehicle that demonstrates the electrical performance of a hybrid vehicle. The vehicle will be powered by a large lithium-ion battery pack, and the developers realized that battery management with charge balancing was absolutely necessary. In this case, active energy transfer between the cells must be used instead of the traditional simple charge balancing solution. The active charge balancing system they developed can provide better performance at a material cost comparable to that of a passive solution (see Figure 1).

Figure 1: E-Cart prototype.

Battery system architecture

Nickel-cadmium batteries, followed by nickel-metal hydride batteries, dominated the battery market for many years. Lithium-ion batteries are growing rapidly in market share due to their greatly improved performance. Lithium-ion batteries have an incredible energy storage capacity, but even so, the capacity of a single battery cell is still too low in terms of both voltage and current to meet the needs of a hybrid engine. Connecting multiple cells in parallel increases the current provided by the battery, while connecting multiple cells in series increases the voltage provided by the battery. Battery assemblers often use abbreviations to describe their battery products, such as "3P50S" for a battery pack with 3 cells in parallel and 50 cells in series. Modular structure is ideal for managing batteries with multiple cells in series. For example, in a 3P12S battery array, each 12 cells are connected in series to form a module (block). The battery cells are then managed and balanced by an electronic circuit with a microcontroller as the 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 about 30V to 45V.

Hybrid vehicles require a DC power voltage of around 450V to drive. In order to compensate for changes in the 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 can also limit the current output by 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, the lithium-ion battery cell is easily damaged (see Figure 2). If the voltage exceeds the upper and lower limits (for example, the lower limit voltage is 2V and the upper limit voltage is 3.6V for nanophosphate lithium-ion batteries), the battery may be irreversibly damaged. The result is at least an accelerated self-discharge rate of the battery. The battery output voltage is stable within a wide range of state of charge (SOC), and the risk of the voltage deviating from the safe range is small. However, at both ends of the safe range, the charging curve is relatively steep. Therefore, as a precaution, the voltage must be closely monitored.

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

If the voltage reaches a critical value, the discharge 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 goal, 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 balance 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 selected cells individually. However, this method is only suitable for suppressing the voltage rise of the strongest battery cell in charging mode. To limit power consumption, such circuits generally only allow discharge at a small current of about 100mA, resulting in charging balancing taking up to several hours.

2. Active balancing method: There are many active balancing methods in the relevant literature, all of which require a storage element for transferring energy. If a capacitor is used as a storage element, connecting it to all battery cells requires a large switch array. A more effective method is to store energy in a magnetic field. The key component in this circuit is a transformer. The circuit prototype was developed by the development team of Infineon Technologies and VOGT Electronics GmbH. Its functions are:

a. Transferring energy between battery cells

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

The circuit is constructed on the flyback transformer principle. This type of transformer is able to store energy in a magnetic field. The air gap in its ferrite core increases the magnetic resistance and thus avoids magnetic saturation of the core material. The circuits on both sides of the transformer are different:

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

b. The secondary coil is connected to each battery cell

A 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 prototype transformer described above has 28 pins.

The switches are MOSFETs from the OptiMOS 3 series , which have extremely low on-resistance, so their conduction losses are negligible (see Figure 3).

Figure 3: Schematic diagram of the battery management module.

Each module in the figure is controlled by an 8-bit advanced microcontroller XC886CLM from Infineon Technologies. This microcontroller comes with its own flash program and a 32KB data memory. In addition, it has two hardware-based CAN interfaces that support communication with the processor load below 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 adopt two different balancing methods according to the situation. After the voltage of all battery cells is scanned (the details of the voltage scan will be introduced later), the average value is calculated, and then the battery cell with the largest voltage deviation from the average value is checked. If its voltage is lower than the average value, the bottom balancing method is adopted, and if its voltage is higher than the average value, the top balancing method is adopted.

1. Bottom balancing method: The example shown in Figure 4 is the bottom balancing method. The scan found that battery cell 2 is the weakest cell and must be strengthened.

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

At this point, the primary switch ("prim") is closed and the battery pack begins charging the transformer. When the primary switch is opened, the energy stored in the transformer can be transferred to the selected battery cell. When the corresponding secondary ("sec") switch - in this case, switch sec2 - is closed, energy transfer begins.

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

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

2. Top balancing: If the voltage of a battery cell is higher than that of other cells, the energy in it needs to be extracted, which is especially necessary in charging mode. If balancing is not performed, the charging process has to be stopped immediately after the first battery cell is full. After balancing, the voltage of all battery cells can be kept equal to avoid the situation of stopping charging too early.

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

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

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

Balanced power and voltage sweeps

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 balancing current of 5A, the power consumption of the entire module is only 2W, so no special cooling measures are required, and the energy balance of the system is further improved.

To manage the state of charge of each battery cell, their individual voltages must be measured. Since only cell 1 is within the ADC range of the microcontroller, 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 can measure the voltage of all battery cells with only a small amount of additional hardware. In this method, the transformer whose main task is charge balancing is also used as a multiplexer.

The flyback mode of the transformer is not used in the voltage scanning 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 a 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, the energy stored in the transformer through this pulse is very small. And in any case, after the switch is opened, the energy stored in the magnetic field will flow back to the entire battery module through the primary transistor. Therefore, the amount of energy in the battery module is not affected. After a cycle of scanning all battery cells, the system returns to the initial state.

Conclusion

Only a good battery management system can fully exploit the advantages of new lithium-ion batteries. The performance of active charge balancing systems is far superior to traditional passive methods, and the relatively simple transformer helps keep material costs low.