Power battery system equalization management
To balance the capacity and energy differences among individual cells in a battery pack and improve the energy utilization rate of the battery pack, an equalization circuit is needed during the charging and discharging process. Based on how the circuit consumes energy during the equalization process, it can be divided into two main categories: energy dissipation type and energy non-dissipation type. Energy dissipation type dissipates excess energy as heat, while energy non-dissipation type transfers or converts excess energy to other batteries.
Energy dissipation-type equilibrium management
Energy dissipation-type equalization circuits achieve equalization by shunting the charging current through parallel resistors in individual battery cells, as shown in Figure 8-12. This circuit structure is simple, and the equalization process is generally completed during charging. However, it cannot replenish the power of low-capacity individual cells, resulting in energy waste and increased load on the thermal management system. Energy dissipation-type electrical appliances generally fall into two categories:
Energy dissipation-type electrical appliances generally fall into two categories: First, a constant shunt resistor equalization charging circuit, where a shunt resistor is always connected in parallel to each battery cell. This method is characterized by high reliability and a large shunt resistor value, reducing differences in individual cell voltage due to self-discharge through a fixed shunt. Its disadvantage is that the shunt resistor constantly consumes power during both charging and discharging, resulting in significant energy loss; it is generally suitable for applications where energy can be replenished promptly.
Second, a switch-controlled shunt resistor equalization charging circuit, where the shunt resistor is controlled by a switch. During charging, when the individual battery voltage reaches the cutoff voltage, the equalization device prevents overcharging and converts excess energy into heat. This equalization circuit operates during charging and can shunt the current to individual cells with higher voltages during charging. Its disadvantage is that due to the limited equalization time, the large amount of heat generated during shunt needs to be dissipated in a timely manner through the thermal management system, which is especially noticeable in battery packs with larger capacities.
For example, in a 10Ah battery pack, a voltage difference of 100mV can result in a capacity difference of over 500mAh. If the equalization time is 2 hours, the shunt current is 250mA, the shunt resistance is approximately 14Ω, and the generated heat is about 2Wh.
Non-energy dissipation type equalization management
Non-energy dissipation circuits consume much less energy than energy dissipation circuits, but their circuit structure is relatively complex. They can be divided into two types: energy conversion equalization and energy transfer equalization.
Energy Conversion Balancing
Energy conversion balancing uses switching signals to either replenish the energy of individual cells from the overall battery pack or convert the energy of individual cells back to the overall battery pack. The conversion from individual cell energy to overall energy typically occurs during the battery pack charging process, as shown in Figure 8-13. This circuit detects the voltage of each individual cell; when the voltage of an individual cell reaches a certain value, the balancing module starts working. It diverts the charging current in the individual cell to reduce the charging voltage, and the diverted current is converted by the module and fed back to the charging bus, achieving balancing. Some energy conversion balancing methods can also use freewheeling inductors to complete the energy conversion from individual cells to the battery pack.
The circuit for converting the energy of the entire battery pack to individual cells is shown in Figure 8-14. This method is also called supplementary balancing. During the charging process, the main charging module first charges the battery pack, while the voltage detection circuit monitors each individual cell. When the voltage of any individual cell is too high, the main charging circuit shuts off, and then the supplementary balancing charging module starts charging the battery pack. Through optimized design, the charging voltage in the balancing module is applied to each individual cell via an independent DC/DC converter and a coaxial coil transformer, adding an identical secondary winding. This ensures that cells with higher voltage receive less energy from the auxiliary charging circuit, while cells with lower voltage receive more energy, thus achieving balancing. The problem with this method is that controlling the consistency of the secondary winding is difficult. Even with identical turns, considering transformer leakage inductance and mutual inductance between secondary windings, individual cells may not receive the same charging voltage. Furthermore, the coaxial coil also experiences some energy dissipation, and this balancing method only addresses charging imbalances, failing to address imbalances in the discharge state.
Energy Transfer Balancing
Energy transfer balancing utilizes energy storage elements such as inductors or capacitors to transfer charge from high-capacity individual cells to lower-capacity cells within a battery pack, as shown in Figure 8-15. This circuit transfers energy between adjacent cells by switching capacitors, moving charge from high-voltage to low-voltage cells to achieve balancing. Alternatively, bidirectional energy transfer between adjacent cells can be achieved using inductive energy storage. This circuit has very low energy loss, but requires multiple transfers during balancing, resulting in a long balancing time and making it unsuitable for multi-cell battery packs. An improved capacitor-switching balancing method can increase the balancing speed by selecting the highest-voltage and lowest-voltage individual cells for energy transfer. However, energy determination and the implementation of the switching circuit in energy transfer balancing are relatively difficult.
Besides the above balancing methods, trickle charging can also be used to achieve battery balancing during charging applications. This is the simplest method and requires no external auxiliary circuitry. It involves continuously charging the series-connected battery pack with a small current. Because the charging current is very small, overcharging has little impact on a fully charged battery. Since a fully charged battery cannot convert more electrical energy into chemical energy, the excess energy will be converted into heat. Batteries that are not fully charged, however, can continue to receive electrical energy until they reach full charge. In this way, after a relatively long period, all batteries will reach full charge, thus achieving capacity equalization. However, this method requires a very long equalization charging time and consumes a considerable amount of energy to achieve equalization. Furthermore, this method is ineffective in managing discharge equalization.
Problems in Application
Existing battery balancing solutions primarily determine battery capacity based on the battery pack's voltage-a voltage-based balancing method. To achieve battery pack balancing, high accuracy and precision in voltage detection are crucial. The leakage current in the voltage detection circuit directly impacts the consistency of the battery pack. Therefore, designing a simple and efficient voltage detection circuit is a key challenge for balancing circuits. Furthermore, voltage is not the sole measure of battery capacity. Internal resistance and contact resistance in the connection method also cause voltage variations. Therefore, solely relying on voltage for balancing can lead to over-balancing and wasted energy. In extreme cases, it may even cause imbalances in the battery pack, despite initial capacity balancing.
Energy dissipation circuits are simple in structure, but the balancing resistors consume energy during current shunting and generate heat, causing thermal management issues. Since they essentially limit excessively high or low terminal voltages in individual cells through energy dissipation, they are only suitable for static balancing. Their high-temperature rise reduces system reliability, making them unsuitable for dynamic balancing. This method is only suitable for small or low-capacity battery packs.
Energy transfer circuits are a method of battery capacity compensation, where a higher-capacity battery contributes some energy to compensate a lower-capacity battery. While feasible, this method is complex, bulky, and costly due to the need for voltage monitoring of individual cells in the actual circuit. Furthermore, the energy transfer is achieved through an energy storage medium, which introduces energy consumption and control issues. This balancing method is generally used in medium to large battery packs.
Energy conversion circuits, on the other hand, use a switching power supply to achieve energy conversion. Compared to energy transfer circuits, they are significantly less complex and less expensive. However, for coaxial coils, the varying lengths and shapes of the wires connecting the windings to each cell result in different transformation ratios, leading to inconsistent balancing of each cell and resulting in balancing errors. Additionally, the coaxial coil itself consumes energy due to electromagnetic leakage and other issues.
