Electricity, as a power source, has been widely used in various vehicles. Lithium-ion batteries have become the preferred choice for power sources due to their advantages such as high energy-to-weight ratio and energy-to-volume ratio, no memory effect, high rechargeability, and long service life.
As a novel power technology, lithium-ion batteries must be connected in series to achieve the required operating voltage. The inconsistent performance of individual cells is not entirely due to battery manufacturing issues; even if each battery has identical voltage and internal resistance at the factory, differences will emerge after a period of use. This makes addressing the charging technology of power lithium-ion batteries an urgent technical problem. This design, while fully considering industrial cost control and stability requirements, employs an energy-efficient partial current shunt method for balanced charging of power lithium-ion batteries, improving the imbalance in battery pack charging and enhancing performance.
Lithium-ion battery pack charging method selection
1. Charging requirements for a single lithium-ion battery
The charging requirements for a single lithium-ion battery (Gb/T18287-2000) are as follows: First, constant current charging, i.e., the current is constant, while the battery voltage gradually increases during the charging process. When the battery terminal voltage reaches 4.2V (4.1V), the constant current charging is changed to constant voltage charging, i.e., the voltage is constant, and the current gradually decreases as the charging process continues, depending on the saturation level of the cell. When it decreases to 10mA, the charging is considered to be terminated. The charging curve is shown in Figure 1.
2. Lithium-ion battery pack charging characteristics
In power lithium-ion battery packs, inconsistencies exist between individual cells. These differences, resulting from continued charge-discharge cycles, accelerate the capacity decay of some cells. Since the capacity of the series-connected battery pack is determined by the minimum capacity of each individual cell, these differences shorten the pack's lifespan. The main reasons for this imbalance include:
●During battery manufacturing, due to factors such as processes, there may be differences in capacity, internal resistance, etc., even within the same batch of batteries;
● Differences in battery self-discharge rates, accumulated over a long period, result in differences in battery capacity;
●During battery use, environmental factors such as temperature and differences in circuit boards can lead to an imbalance in battery capacity.
3. Charging method selection
To reduce the impact of imbalance on lithium-ion battery packs, an equalization circuit is used during charging.
Currently, there are two main methods for balancing lithium-ion battery packs: energy dissipation and feedback. Energy dissipation involves supplying parallel branches to each individual cell, diverting energy from cells with excessively high voltage to achieve balancing. Feedback involves using an energy converter to return the energy imbalance between cells to the battery pack or specific cells within the pack.
In theory, when conversion efficiency is ignored, feedback does not consume energy and can achieve dynamic equilibrium. However, due to the complexity of the control method and the high manufacturing cost of feedback design, this charger adopts an energy-efficient design.
Energy-consuming types can be further divided into current interruption and current shunting based on the energy circuit solution. Current interruption refers to disconnecting the charging circuit of a single battery cell when certain conditions are met, based on monitoring changes in the individual cell voltage, allowing the charging current to flow entirely through a bypass resistor. This is achieved by dynamically changing the connection structure between cells within the battery pack through a switching matrix composed of mechanical contacts or power electronic components. Current shunting, on the other hand, does not disconnect the working circuit but adds a bypass resistor to each battery. When a single battery cell's voltage is higher than that of other cells in the pack, all or part of the charging current is diverted to the bypass resistor, thus achieving balanced charging of all individual cells. Due to the high power of power lithium-ion battery packs, after comprehensively considering factors such as charging efficiency and thermal management, we use a partial current shunting method for the charger design.
System Design and Decomposition
1. System Overall Structure
As shown in the system block diagram in Figure 2, the mains frequency AC power is converted into 18V/5A DC power by a switching power supply and output to the boost circuit. The boost circuit supplies a certain charging current to the battery pack according to the control signal of the CPU. The voltage monitoring circuit feeds back the real-time voltage status of the battery to the CPU. The CPU controls the overall charging voltage and current of the battery pack through the boost circuit. The equalization circuit adjusts the charging rate of each individual battery to ensure the consistency of charging of the entire battery pack.
2. Boost circuit
The electrical energy input conversion process consists of two parts: a switching power supply circuit and a voltage regulation circuit. The switching power supply converts the input AC power at the mains frequency into an 18V/5A DC output. Since current switching power supply technology is quite mature, it will not be elaborated upon further here.
The purpose of a boost circuit is to convert the DC output from a switching power supply into the voltage and current required for battery pack charging, and to adjust the output voltage and current in real time according to the charging status.
R1, R2, and Q1 form a reverse power supply protection circuit; Q5 is the switch for the entire boost circuit; Q2, Q4, and U1 form the driver stage circuit for the field-effect transistor Q3; Q3, L1, D1, C4, and C5 form the bOOST boost regulation circuit; and R9, R10, and C6 form 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 switching transistor Q5 is turned off. The CPU calculates the PWM duty cycle based on the voltage feedback from the battery monitoring circuit and outputs a corresponding modulation signal. The PWM modulation signal is amplified and adjusted by the driver stage to control the switching state of Q3 to produce the desired output voltage.
Because under steady-state conditions, the uniform value of the voltage across the inductor is zero over one switching cycle, we can obtain:
Where UL is the uniform value of the voltage across the inductor during one switching cycle; U0 is the output voltage; Ui is the input voltage; T is the switching cycle; ton is the time Q3 is in the on-state; and tof is the time Q3 is in the off-state. Let UL = 0. During the continued operation of the inductor current:
in
Therefore, by adjusting the duty cycle of the PWM output, the charging voltage of the battery can be effectively controlled.
Because the voltage of a single lithium-ion battery is too low, they are usually connected in series to obtain a higher operating voltage. During the charging process of the battery pack, the voltage of each battery must be monitored in real time to ensure that each battery is operating under normal conditions and to prevent overcharging, which could damage the lithium-ion batteries.
In a series-connected lithium-ion battery pack, each lithium-ion battery has a different reference voltage. Assuming the battery voltages in the pack are a1, a2, ..., then the voltage of the first battery relative to ground is a1, the voltage of the second battery is a1 + a2, and so on.
In voltage monitoring, we need to compare the real-time voltages of each battery, which requires designing a circuit to convert the voltages of each battery to the same reference. Optical coupler isolation sampling can achieve this level conversion. However, considering that linear optocouplers are more than 10 times more expensive than general optocouplers, for cost control in engineering, we linearize the connection of general optocouplers to achieve voltage acquisition and real-time monitoring.
The single-cell voltage monitoring circuit uses two general optocouplers and two operational amplifiers of the same model and batch. One optocoupler is used for output, and the other for feedback. The feedback is used to compensate for the nonlinearity of the time and temperature characteristics of the LED.
Where: K1 and K2 are the current transfer ratios of optocouplers U1 and U2 in the circuit.
From the circuit, we can see that:
Where Vbat is the voltage across the battery terminals. Since the same model and batch of optocouplers are used, the current transfer ratios are approximately equal, i.e., K1 = K2.
Therefore, we have:
As shown in equation (5), the voltage gain of this measurement circuit is only related to the resistance values of resistors R1 and R2, and is independent of the current transmission parameters of the optocoupler, thus achieving linear isolation of the voltage signal. After conversion by the circuit shown in the figure, the battery voltage is converted into an output voltage Vout with a unified reference ground.
4. Partial shunt control circuit
As shown in Figure 5, during the charging process, when the voltage of a certain cell is significantly higher than that of other cells in the pack, the CPU pulls the control port high, Q1 turns on, the base potential of Q2 is pulled low, Q2 turns on, and some electrical energy is diverted from the bypass resistor R4, reducing the charging rate of that cell, thereby synchronizing the charging rates of all cells in the battery pack.
in
Iequ is the current flowing through the bypass resistor R4, i.e., the balancing current; p is the power consumed by the bypass resistor R4; Ubat is the voltage across the battery terminals.
The choice of equalization current directly affects the performance of the charger.
High current results in high overall heat generation and poor operational stability of the charger. Low current results in limited voltage and speed adjustment. Through repeated testing, the optimal balance between adjustment capability and heat generation was achieved when Iequ≈0.1Icharge.
Since the Ubat voltage range is 3~4V during charging, the nominal capacity of the rechargeable battery is 2000mAh, and the maximum charging current is 2A, R4 chooses to connect two 47Ω resistors in parallel.
Conclusion
Due to differences in manufacturing processes and operating environments among individual lithium-ion batteries, imbalances can occur during series charging of lithium-ion battery packs. A power-saving lithium-ion battery pack equalizer designed using a partial shunt method effectively addresses this charging imbalance. It effectively prevents overcharging, improves the safety of lithium-ion battery use, increases the charging capacity of the battery pack, and extends its lifespan. Through repeated experiments, optimal parameters were selected, and heat generation was controlled, ensuring the charger's long-term stable operation. The design process fully considered the needs of actual manufacturing. While ensuring practicality and reliability, the design was simplified, commonly used components were selected, and the cost-effectiveness was improved, demonstrating promising application prospects.
