Design scheme of charging management circuit for power lithium battery pack

As the international crude oil price soars, the research of various new energy sources has become the focus of public attention. Electric energy has been widely used in various vehicles as a power energy source. Lithium batteries have become the first choice for power energy due to their high energy-to-mass ratio and energy-to-volume ratio, no memory effect, many rechargeable times, and long service life.

As a new type of power technology, lithium batteries must be connected in series to meet the voltage requirements. The uneven performance of single cells is not entirely due to battery production technology issues. Even if the voltage and internal resistance of each battery are exactly the same when it leaves the factory, there will be differences after a period of use. This makes solving the power battery charging technology problem an urgent technical problem to be solved. Based on full consideration of industrial cost control and stability requirements, this design uses an energy-consuming partial shunt method to balance the charging of power lithium batteries, improve the imbalance of battery pack charging, and improve working performance.

1 Lithium battery charging solution selection

1.1 Single-cell lithium battery charging requirements

The charging requirements for single-cell lithium-ion batteries (GB/T18287-2000) are first constant current charging, that is, the current is constant, and the battery voltage gradually increases during the charging process. When the battery terminal voltage reaches 4.2 V (4.1 V), the constant current charging is changed to constant voltage charging, that is, the voltage is constant, and the current gradually decreases according to the saturation level of the battery cell as the charging process continues. When it decreases to 10 mA, the charging is considered to be terminated. The charging curve is shown in Figure 1.

Figure 1 Lithium battery charging curve

1.2 Lithium battery pack charging characteristics

In a power battery pack, there are inconsistencies between individual cells. The differences caused by continuous charge and discharge cycles will accelerate the decay of the capacity of some individual cells. The capacity of a series battery pack is determined by the minimum capacity of the individual cells, so these differences will shorten the service life of the battery pack. The main reasons for this imbalance are:

●During the battery manufacturing process, due to process and other reasons, there are differences in the capacity and internal resistance of the same batch of batteries;

●The difference in battery self-discharge rate, after a long period of accumulation, causes the difference in battery capacity;

●During battery use, differences in usage environment such as temperature and circuit boards lead to imbalance in battery capacity.

1.3 Charging solution selection

In order to reduce the impact of imbalance on the lithium battery pack, a balancing circuit should be used during the charging process.

There are two main solutions for balancing lithium battery packs: energy consumption type and feedback type. The energy consumption type is to provide parallel branches for each single cell, and transfer the electric energy of the single cell with excessive voltage through shunt to achieve the purpose of balancing. The feedback type is to feed the deviation energy between cells back to the battery pack or some cells in the battery pack through the energy converter.

Theoretically, when the conversion efficiency is ignored, feedback does not consume energy and dynamic balance can be achieved. However, due to the complex control method of feedback design and high manufacturing cost, this charger adopts energy consumption design.

Energy consumption type can be divided into current interruption and current shunting according to the energy circuit processing method. Current interruption means that on the basis of monitoring the voltage change of the single cell, when certain conditions are met, the charging circuit of the single cell is disconnected, and the charging current completely passes through the bypass resistor. Through the switch matrix composed of mechanical contacts or power electronic components, the connection structure between the single cells in the battery pack is dynamically changed. Shunting does not disconnect the working circuit, but adds a bypass resistor to each battery. When a single cell is higher than other batteries in the group, all or part of the charging current is directed to the bypass resistor. In this way, balanced charging of each single cell is achieved. Due to the high power of the power lithium battery pack, after comprehensively considering factors such as charging efficiency and thermal management, we use partial shunting as the design solution for the charger. [page]

2 System Design and Analysis

2.1 Overall system structure

As shown in the system block diagram of Figure 2, the industrial frequency AC power is converted into 18 V/5 A DC power through the switching power supply and output to the boost circuit. The boost circuit provides a certain charging current for charging the battery pack according to the control signal of the CPU. The voltage monitoring circuit feeds back the real-time voltage of the battery to the CPU. The CPU controls the overall charging voltage and current of the battery pack through the boost circuit. The charging rate of each single battery is adjusted through the balancing circuit to ensure the consistency of charging of the entire battery pack.

Figure 2 System overall block diagram

2.2 Boost circuit

The input conversion link of electric energy consists of two parts: the switching power supply circuit and the voltage regulation circuit. The switching power supply converts the input AC power into 18V/5A DC output. Since the current switching power supply technology is quite mature, it will not be described here.

The function of the boost circuit is to convert the DC power output by the switching power supply into the voltage and current required for charging the battery pack, and to adjust the output voltage and current in real time according to the charging status.

The boost circuit is shown in Figure 3.

Figure 3 Boost circuit

Among them, R1, R2, and Q1 constitute the power reverse protection circuit, Q5 is the switch of the entire boost circuit, Q2, Q4, and U1 constitute the field effect tube Q3 driving stage circuit, Q3, L1, D1, C4, and C5 constitute the BOOST boost regulation circuit, and R9, R10, and C6 are the voltage sampling circuit.

When the charger is working normally, the positive and negative outputs of the switching power supply are connected to DC+ and DC- respectively, and the switch tube Q5 is turned off. The CPU calculates the PWM duty cycle based on the voltage feedback from the battery monitoring circuit and outputs the corresponding modulation signal. The PWM modulation signal is amplified and adjusted by the driver stage to control the switching state of Q3 to generate the required output voltage.

Under steady-state conditions, the average value of the voltage across the inductor in a switching cycle is zero. It can be obtained that:

Among them, UL is the average value of the voltage across the inductor in one switching cycle; U0 is the output voltage; Ui is the input voltage; T is the switching cycle; ton is the time when Q3 is in the on state; toff is the time when Q3 is in the off state. Let UL = 0, in the continuous working process of the inductor current:

in

Therefore, the battery charging voltage can be effectively controlled by only adjusting the duty cycle of the PWM output.

[page]

Since the voltage of a single lithium battery is too small, in order to obtain a higher working voltage, lithium batteries generally need to be connected in series. During the charging process of the battery pack, the voltage of each battery needs to be monitored in real time to ensure that each battery works in a normal working state and avoid overcharging and damage to the lithium battery.

In a series lithium battery pack, the reference voltage of each lithium battery is different. Assuming that the battery voltages in the battery pack are a1, a2, ?, the voltage of the first battery to ground is a1, the voltage of the second battery to ground is a1 + a2, and so on.

In voltage monitoring, we need to compare the real-time voltage of each battery, so we must design a certain circuit to convert the voltage of each battery to the same reference. The method of optocoupler isolation sampling can achieve level conversion. Considering that the price of linear optocouplers is more than 10 times that of ordinary optocouplers, for the purpose of cost control in engineering, ordinary optocouplers are linearly connected to achieve voltage collection and real-time monitoring.

In the single cell voltage monitoring circuit shown in Figure 4, two common optocouplers and two operational amplifiers of the same model and batch are used. One of the two optocouplers is used for output and the other is used for feedback. Feedback is used to compensate for the nonlinearity of the time and temperature characteristics of the light-emitting diode.

Figure 4 Voltage monitoring circuit

In Figure 4:

Where: K1, K2 are the current transfer ratios of the optocouplers U1, U2 in the circuit.

From the circuit we can see that:

Where V bat is the voltage across the battery. Since the optocouplers of the same model and batch are used, the current transfer ratio is approximately equal, that is, K1 = K2.

So, we have:

From formula (5), it can be seen that the voltage gain of the measurement circuit is only related to the resistance value of resistors R1 and R2, and has nothing to do with the current transmission parameters of the optocoupler, thereby achieving linear isolation of the voltage signal. After the conversion of the circuit shown in the figure, the battery voltage is converted into an output voltage Vout with a unified reference ground.

2.4 Partial current shunt control circuit

As shown in the shunt control circuit in Figure 5, during the charging process, when the voltage of a single cell is significantly higher than that of other batteries in the group, the CPU pulls up the control port, Q1 is turned on, the base potential of Q2 is pulled down, Q2 is turned on, and part of the electric energy is shunted from the bypass resistor R4, reducing the charging rate of the battery, thereby achieving synchronization of the charging rates of each single cell in the battery pack.

[page]

Where Iequ is the current flowing through the bypass resistor R4, that is, the balancing current; P is the power consumed by the bypass resistor R4; Ubat is the voltage across the battery.

Figure 5 Shunt control circuit

The choice of equalization current will directly affect the performance of the charger.

When the current is large, the overall heat generated by the charger is large and the working stability is poor. When the current is small, the voltage adjustment range is small and the rate adjustment range is small. After repeated experiments, when Iequ≈0. 1 Icharge, the adjustment ability and heat generation reach the best balance.

Since the range of Ubat during charging is 3~4 V, the nominal capacity of the rechargeable battery is 2000 mAh, and the maximum charging current is 2 A. Considering the above factors, R4 is selected to connect two 47 Ω resistors in parallel.

3 Conclusion

Due to the differences in manufacturing process and working environment of single lithium batteries, the charging of lithium battery packs in series will be unbalanced. The energy-consuming lithium battery pack equalization charger designed by partial shunt method solves the problem of unbalanced charging of battery packs. It effectively prevents overcharging, improves the safety of lithium battery use, increases the charging capacity of battery packs, and prolongs the service life of lithium battery packs. After repeated tests, the most suitable parameters are selected, the heat generation is controlled, and the long-term stable operation of the charger is guaranteed. In the design process, the actual production needs are fully considered. Under the premise of ensuring practicality and reliability, the design is simplified, common devices are selected, the cost performance is improved, and it has good application prospects.