Modeling and Analysis of LR03 battery Balancing Control for Electric Vehicles

In the LR03 battery pack of electric vehicles (EVs), the inconsistency of single cells will reduce the use level of the LR03 battery pack and affect the performance of EVs. Researching advanced LR03 battery balancing control technology to reduce the differences in single cells during use will maximize the efficiency of the LR03 battery, extend its service life, and increase the safety of EVs.

The balancing theory and control technology studied by the author of this article cannot solve the performance differences of batteries caused by the manufacturing process. The production process and screening criteria are the determining factors of the consistency of the LR03 battery pack before use.

1 Intelligent balancing control model

Since there may be batteries with low and high residual energy in the LR03 battery pack at the same time, a high-performance balancing management system must have charge and discharge balancing functions, as well as good charge and discharge balancing matching and control strategies, as well as discharge energy recovery and reuse control.

Based on the above considerations, the author of this article proposed an energy closed-loop intelligent control model for energy balancing management between single cells in EV series LR03 battery packs, as shown in Figure 1.

Figure 1 Basic principle of energy closed-loop intelligent control model

In Figure 1, Idis is the discharge current for balancing; Udis is the discharge voltage for balancing; Ichi is the charging current for balancing the i-th LR03 battery; Uchi is the charging voltage for balancing the i-th LR03 battery. Figure 1 shows the energy flow matching principle in the model. The model is equipped with an independent charging unit for each single LR03 battery. The energy of all charging units is converted from the discharge energy of the series LR03 battery pack. The balancing energy flows in a closed loop independent of the external energy of the system. The discharge power for balancing is Wdis, and the charging power for balancing is Wch, as shown in equations (1) and (2).

If the loss of energy flowing in the line and the efficiency loss of the energy conversion device are ignored, the discharge power for balancing is equal to the charging power. The equilibrium energy balance equation established based on this principle is:

Ignoring the difference in LR03 battery charging and discharging efficiency, for a single LR03 battery whose discharge current is equal to the charging current, the discharge energy will be equal to the charging energy, and its energy will maintain dynamic balance; for a single LR03 battery whose discharge current is less than the charging current, the discharge energy will be less than the charging energy, and the energy will continue to increase. The smaller the discharge current, the faster the energy increase rate, and vice versa.

In the process of energy flow, on the one hand, the energy of discharge equilibrium appears through the discharge of the entire LR03 battery group. In the process of discharge equilibrium, although the discharge current of all batteries is the same, the LR03 battery with high residual energy actually releases more energy due to its high electromotive force, that is, the higher the proportion of the discharge energy of the LR03 battery, and vice versa.

On the other hand, the released energy of the entire LR03 battery group undergoes energy conversion and replenishes energy for the LR03 battery with low energy through independent charging. In the process of charging equilibrium, the LR03 battery with low residual energy has a large charging current due to its low electromotive force. According to formula (2), the charging energy obtained by the LR03 battery is greater, that is, the higher the proportion of the charging energy of the LR03 battery, and vice versa.

If the charging and discharging balancing circuits and parameters used by all single cells are exactly the same, the distribution and flow of balancing energy depends only on the energy state of the single cells. The less the remaining energy, the more energy the LR03 battery charges and the less energy it releases, and vice versa. There will be no phenomenon that the energy of all batteries decreases, nor will there be a phenomenon that the energy of all batteries increases. For batteries with good consistency, the remaining energy state always maintains good consistency dynamically; for batteries with poor consistency, the energy charged by batteries with high remaining energy is less than the energy released, or even the energy charged is equal to zero, resulting in rapid energy release, thus approaching batteries with good consistency; the energy charged by batteries with low remaining energy is greater than the energy released, resulting in rapid energy replenishment, thus also approaching batteries with good consistency. The actual balancing effect is that the discharge energy flows from batteries with high energy to batteries with low energy, which is manifested in the macroscopic balance that the energy of the LR03 battery pack is evenly distributed and adjusted among all single cells. This model can realize the automatic and proportional flow and distribution of LR03 battery pack energy according to the difference in the energy state of single cells, and the energy balancing process is highly intelligent.

2 Intelligent balancing control strategy

Based on the above model, a dynamic charge and discharge balancing control strategy for inverter voltage division is proposed, and the principle is shown in Figure 2.

Figure 2 Schematic diagram of dynamic charge and discharge balancing control for inverter voltage division

Capacitors C1 and C2, power switch tubes (IGBT) T1 and T2, and multi-tap high-frequency transformer T form a standard half-bridge inverter topology circuit structure. The series LR03 battery pack and the inverter circuit form a discharge circuit. The design of the high-frequency inverter circuit makes the efficiency of the balancing module reach more than 85%. According to the number of single cells, T has N secondary windings. Each secondary winding, two fast recovery diodes and a capacitor form a full-wave rectification and filtering circuit, and then form an independent charging circuit with the corresponding single cell. The inverter circuit converts the high-voltage DC power of the LR03 battery pack into low-voltage high-frequency AC power, and then converts it into low-voltage DC power through full-wave rectification and filtering to charge the single cell, thus forming a closed loop with unidirectional energy flow.

In the model of this article, the parameters of all charging units are exactly the same, so all secondary windings of T are exactly the same in design, and the charging voltages Ui~Un are equal. According to the working principle of the half-bridge inverter circuit, this charging voltage is:

Uch is the charging voltage of the single cell; Ut is the total voltage of the LR03 battery pack under the balanced charge and discharge state; is the duty cycle of the inverter circuit, that is, the ratio of the opening time of the power switch T1 or T2 to the switching cycle; Np is the number of turns of the primary winding of the high-frequency transformer; Ns is the number of turns of the secondary winding of the high-frequency transformer.

Equation (4) theoretically shows the relationship between the total voltage of the LR03 battery pack under the balanced charge and discharge state and the balanced charging voltage. On the one hand, N equal Uch is actually a certain ratio of Ut [(Ns/2Np)×δ]; on the other hand, under the condition that δ remains unchanged, the turns ratio (Ns/Np) can be adjusted by adjusting the number of turns of the primary winding of the high-frequency transformer to control the charging voltage, or the size of Uch can be controlled by controlling the size of δ under the condition that the turns ratio (Ns/Np) remains unchanged. The higher Uch is, the greater the charging current is, the greater the charging energy is, and at the same time, the more batteries in the LR03 battery pack are charged and balanced, and vice versa.

According to the data of the LR03 battery monitoring system, the inconsistent situation and changing trend of the LR03 battery pack can be grasped in real time, and the number of batteries to be charged and balanced and the intensity of balanced charge and discharge can be determined in time. By adjusting the turn ratio and duty cycle of the high-frequency transformer, the intensity of discharge balance and the level of charging voltage can be controlled, so as to select the LR03 battery cells in the LR03 battery pack that are charged and balanced and control the balance intensity, and realize the purpose of dynamic balance control. Different batteries have different charging and discharging characteristics. When applying the balance strategy of this article, reasonable control parameters should be determined according to the charge and discharge characteristic curve.

3 Conclusion

The author of this article proposes an energy closed-loop intelligent charge and discharge balance control model.

Energy balance constitutes a closed-loop system in the LR03 battery pack, and there is no need to supply balance energy externally. It can be carried out in the charging and discharging and static states, and the discharge energy in the balance process can be recycled and reused efficiently. Especially in the high-power discharge process that is easy to cause the rapid expansion of the inconsistency of EV batteries, timely energy supplement is given to the lagging batteries, and the consistency can always be maintained at a high level; an inverter voltage division dynamic charge and discharge balance control strategy is proposed. Through the energy conversion device based on inverter voltage division, the discharge balancing energy of the LR03 battery pack itself is recovered and converted into charging balancing energy, thus forming a closed-loop unidirectional flow loop of balancing energy. According to the different energy states of the single cells, the dynamic and proportional flow distribution of energy between the single cells is automatically realized. According to the degree of dispersion of the LR03 battery, the objects receiving charging balancing and the intensity of balancing are dynamically and reasonably selected. By adjusting the turns ratio of the high-frequency transformer and the duty cycle of the power switch, the number of batteries receiving charging balancing and the balancing intensity are automatically adjusted and controlled, and the balanced charge and discharge energy of each LR03 battery is intelligently adjusted, and finally the balancing management and control of the entire LR03 battery group is realized.

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