Equalization control of power battery charging

At present, both the charging speed and the effect of the voltage-limiting and current-limiting charging method are not scientific enough. In the early stage of charging, the polarization effect is not intense. The battery has the strongest current acceptance ability, and the charging current should be increased. The battery temperature rises in the later stage of constant current. , The internal pressure increases, the current has exceeded the battery’s ability to accept, and the current should be reduced. At the same time, polarization and skin effect reduce the activity of the material reaction. Reverse current pulses can be used to weaken these adverse effects. To realize the equalization control of the single battery voltage, the equalizer is the core component of the battery management system. Without the equalizer, even if the management system obtains the measurement data of the battery pack, it will do nothing, and there will be no management.

Equalization control of power battery charging
Battery Charge Equalizer System

1. Cut-off and shunt
The equalizer is divided into cut-off and shunt according to the way of energy loop processing. Current cut-off means that on the basis of monitoring cell voltage changes, when certain conditions are met, the charging or load circuit of the cell is disconnected, and the switch matrix is ​​formed through mechanical contacts or power electronic components to dynamically change the cells in the battery pack. The connection structure between the batteries. The power of the battery pack for electric vehicles is very large, the instantaneous current can reach hundreds of amperes and the bipolar changes. Considering many factors such as feasibility, cost performance, practicability, reliability, etc., it is extremely difficult to implement the current cutoff, which is not suitable for electric vehicles. Used on car power battery packs.

The shunt does not disconnect the battery working circuit, but adds a bypass device to each single battery, just like a battery partner, the combined characteristics of the two tend to be the average characteristics of each single battery in the battery pack.

2. Energy consumption type and feedback type
Energy-consumption type refers to providing parallel current branches to each single battery, and the over-voltage single battery transfers electric energy through shunting to achieve the purpose of balancing. The device that realizes the parallel current branch can be a controllable resistor or Energy-power converters drive power-consuming equipment such as air conditioners and fans. Its essence is to limit the excessively high or low terminal voltage of the single battery through the method of energy consumption. It is only suitable for use in static equilibrium. Due to the characteristics of high temperature rise, it reduces the reliability of the system and consumes energy, so it is not suitable for dynamic Used in equilibrium.

The feedback type is different from the energy consumption type. The feedback type is to feed the deviation energy between the single batteries back to the stalk battery pack or some single battery cells in the group through the energy power converter. In theory, when the conversion efficiency is neglected, the feedback type does not consume energy and can achieve dynamic equilibrium. The feedback type has higher research value and use value, and is most likely to achieve practical design.

3. energy power converter
Battery voltage equalization can be realized by energy conversion device. According to the principle and technical design of high-frequency switching power supply (SMPS), the basic power circuit includes non-isolated Buck, Boost, Buck Boost, Cuk, Sepic, Zeta, and isolated Forward, Fly back, Push Pull, Half Bridge, Full Bridge, Iso-Cuk, etc. The small-capacity storage battery is charged with less energy when charging, the shunt circuit absorbs electric energy, and the shunt circuit supplements energy when discharging, and the energy-to-power converter should be able to achieve bidirectional conversion. In principle, all kinds of power circuits can be bidirectional after improved design. The simplest solution is to use two power supplies, the input and output are crossed in parallel, and the two circuits are controlled separately. Due to factors such as cost, volume and weight, and long-term reliability, bidirectional single power converters have more advantages than unidirectional dual power converters and are the direction of development.

4. Charge, discharge and dynamic balance
According to the characteristics of the equalization function, it is divided into charging, discharging and dynamic equalization. In the middle and later stages of the charging process, when the voltage of the single battery reaches or exceeds the cut-off voltage, the equalization circuit starts to work to reduce the current of the single battery in order to limit the voltage of the single battery. Higher than the charge cut-off voltage. Similar to charge equalization, discharge equalization limits the voltage of a single battery not to be lower than a preset discharge termination voltage by supplementing electric energy when the battery pack outputs power. The setting of charge cut-off voltage and discharge cut-off voltage is related to temperature. Different from charging and discharging equalization, dynamic equalization can achieve the voltage balance of the single battery in the battery pack through the energy conversion method regardless of the charging state, the discharging state, or the floating state, and maintain a similar degree of charge in real time.

The only function of charge equalization is to prevent overcharging, and it brings negative effects during discharging use. When the charge equalization is not applied, the battery with small capacity is overcharged to a certain extent. Before any single battery in the battery pack is overdischarged, The output Ah of the battery pack is slightly higher than the minimum capacity of the single battery. When using charge equalization, the small-capacity battery is not overcharged, and the amount of electricity that can be released is less than the energy that can be released when the equalizer is not used for light overcharge, so that the discharge time of the single battery is shorter, and the possibility of overdischarge is greater . In addition, when the motor controller reduces or stops the output power based on the battery pack voltage being reduced to a certain extent, the large-capacity battery is charged with more electric energy due to the charge balance and exhibits a higher platform voltage, which masks the small capacity If the voltage of the battery drops, the battery pack voltage will be high enough, and the small-capacity single battery has been over-discharged.

Discharge equalization is similar to charge equalization, that is, large-capacity shallow and full discharge, small-capacity over-full discharge, the result of accelerating the difference in the performance of the single battery is the same, and neither can form a truly practical product. Only dynamic balance concentrates two types. The advantage of equalization, despite the difference in initial capacity between single batteries, it can ensure the consistency of the relative charge and discharge intensity and depth during work, and gradually reach the common end of life.

5. Unidirectional and bidirectional
According to the possible flow of energy processed by the equalizer, it is divided into one-way and two-way equalization. The two-way equalizer uses a two-way power converter, and the input and output directions are dynamically adjusted. In comparison, the two-way equalizer has more advantages. Based on the consideration of equalization efficiency, for the one-way equalizer, the power converter from the high voltage of the battery pack to the low voltage of the single battery is suitable for discharge equalization, as shown in Figure 1(a) It shows that the inverter from the low voltage of the single battery to the high voltage of the battery pack is suitable for charge equalization, as shown in Figure 1(b).

Equalization control of power battery charging
Figure 1 Buck or Boost unidirectional and bidirectional converters

The most advanced equalization scheme is from a single battery to a single battery, and directly converts energy from a high-voltage single battery to a low-voltage single battery, which has the best equalization efficiency and is difficult to achieve. Sort by the size of single battery C1>C2>…Cn, n is the number of single battery in series, the average capacity is Cn=(C1+C2 +…Cn)/n, set the capacity of the kth single battery to be the closest Average value, ie Ck=Ca, the goal of the equilibrium system is to extract energy from C1, C2, .., Ck-1, Cout=( C1+C2 +… +Ck-1)-(k-1)Ca , Transfer to Ck+1, Ck+2, .., Cn. Considering the energy conversion efficiency, the value of k needs to be shifted back appropriately.

6. concentration and decentralization
After combining all the windings of the unidirectional and bidirectional power converters connected to the battery voltage into one winding, the centralized power converter as shown in Figure 2 is obtained. The advantage is that the cost and technical complexity of the power converter are greatly reduced. The main disadvantages are that the length and shape of the wires between the low-voltage winding and the individual batteries are different, the transformation ratio is different, and the equalization error is large. On the other hand, the wiring process of n+1 power wires between the power converter and the battery pack is not easy to design, and the stretching and shearing of the wires during the driving of the vehicle brings hidden dangers to safety.

Equalization control of power battery charging
Figure 2 Centralized power converter

Based on the consideration of cost and balanced efficiency, the centralized type can be applied to small and medium power such as mopeds and occasions where the battery pack has no vibration or is less mobile. A method of using a single capacitor to balance each battery cell is called the flying capacitor method, which is also a centralized method, as shown in Figure 2 (d). Its characteristic is that the equalization function is carried out directly through the charging and discharging of the capacitor, but the instantaneous opening current on the switch is large, which is prone to arc or electromagnetic interference, and the voltage drop of the switch contact directly affects the equalization effect.

7. independent and cascading
A design idea of ​​an equalizer allows every two adjacent single batteries to be balanced, and then to achieve the balance between each single battery. Figure 3 lists three cascaded converters. The bidirectional Buck Boost power converter uses inductance to transfer energy, and the bidirectional Cuk and switched capacitor network uses capacitors to transfer energy. The interval between the high-voltage and low-voltage single-cell batteries in the battery pack Several single batteries, exporting energy from a high-voltage single battery to a low-voltage single battery requires multiple cascaded power converters to work at the same time, and the energy conversion efficiency of the single battery to reach the destination is extremely low. Extreme conditions and energy-consuming power converters near.

Equalization control of power battery charging
Figure 3 Cascade converter

8. efficiency and safety
In the process of applying dynamic balance to the discharge process, the heat consumption of the power converter is taken from the energy of the battery pack. Due to the low voltage of the single battery, the efficiency of the power converter is a design difficulty, and the latest design technology of contemporary power circuits must be adopted and borrowed , Such as synchronous rectification, soft switching, etc.

In the design of the equalizer, conventional detection functions such as parameter over-limit alarm and thermal protection are indispensable. The internal environment of electric vehicles is subject to turbulence and vibration for a long time. The wiring process and fastening structure must be carefully designed. , May cause fire hazards unrelated to battery performance. As far as power converters are concerned, surge suppression, overvoltage and overcurrent protection also need to be considered.

9. Control and management
The balance control scheme is different, and the complexity of the management system is different. The passive balance is adjusted by the charger to adjust the output voltage and current, the control is the simplest, and the balance ability is also the worst. Existing products adopt a distributed management structure of main and auxiliary modules. The auxiliary module is equivalent to an independent equalizer. The main module completes the function of the management system, and the two are connected through a field bus. Some adopt hierarchical management, the upper-level module manages the lower-level module, and the lower-level module manages 12 batteries.

In terms of control strategies, it is required to combine the electrochemical characteristics of the battery, power supply technology, and control technology. During the operation of electric vehicles, acceleration, landslides, stalls, braking, etc. may occur at any time, and the current and power output by the battery pack are bipolar. Changes, various impedance characteristics and the modulation characteristics of the motor controller all bring complexity to the voltage changes of the battery pack. Management decision-making cannot be calculated based on simple formulas, and reciprocating balance should be avoided, resulting in waste of battery energy.