Balanced charging technology for battery packs
A single battery has a limited voltage and capacity, and in many cases it is necessary to form a series battery pack for use. However, the battery in the battery pack has a problem of balance. How to improve the service life of the battery pack, improve the stability of the system and reduce the cost is an important issue before us. The service life of the battery is determined by many factors, the most important of which is the physical properties of the battery itself. In addition, the low battery management technology and unreasonable charging and discharging system are also important reasons for shortening battery life. For the battery pack, inconsistencies between the individual cells are also an important factor in addition to the above reasons. Aiming at the phenomenon that the single battery is unbalanced during the charging and discharging process of the battery, the author analyzes and compares several current equalization methods, and puts forward the non-destructive equalization method in combination with the actual test. Existing equalization charging method To achieve equalization of each unit battery of the series battery pack, there are mainly the following methods. 1. Add a parallel equalization circuit to each unit of the battery pack to achieve the shunt function. In this mode, when a battery first reaches full charge, the equalization device can prevent it from overcharging and convert excess energy into heat, continuing to charge the unfilled battery. This method is simple, but it will bring energy loss and is not suitable for fast charging systems. 2. Discharge each monomer one by one through the same load one by one before charging, and then perform constant current charging to ensure a more accurate equilibrium between the individual cells. However, for the battery pack, due to the physical differences between individuals, it is difficult to achieve a completely consistent ideal effect after each unit is deeply discharged. Even if the same effect is achieved after discharge, a new imbalance will occur during the charging process. 3. Timing, sequencing, and separate detection and uniform charging of the single battery in the battery pack. When charging the battery pack, it can ensure that each battery in the battery pack does not overcharge or overdischarge, thus ensuring that each battery in the battery pack is in a normal working state. 4. Using the time-sharing principle, through the control and switching of the switch components, additional current flows into the battery with a relatively low voltage to achieve balanced charging. This method is relatively efficient, but the control is more complicated. 5. The voltage parameters of each battery are equalized, and the voltages of the batteries are restored. As shown in FIG. 2, during equalization charging, the capacitor is alternately connected to the adjacent two batteries through the control switch, accepting the charging of the high voltage battery, and discharging to the low voltage battery until the voltages of the two batteries tend to be uniform. This kind of equalization method better solves the problem of battery pack voltage imbalance, but the method is mainly used in the case of a small number of batteries. 6. The whole system is controlled by single-chip microcomputer, and the single battery has a separate set of modules. The module performs charge management on each of the individual batteries according to the setting procedure, and automatically disconnects after the charging is completed. The method is relatively simple, but the cost is greatly increased when the number of single cells is large, and the system volume is also not reduced. Non-destructive charging circuit This paper proposes a lossless uniform charging path. After the equalization module is activated, the overcharged battery will transfer excess power to the battery that is not fully charged to achieve dynamic balance. Its high efficiency and low loss, all battery voltage is monitored by the equalization module. 1 circuit design A battery pack consisting of N cells in series, the main circuit current is Ich. Each series battery is connected to a balanced bypass, as shown in Figure 3. In the figure, BTi is a single cell, Si is a MOSFET, and inductor Li is an energy storage element. Si, Li, and Di form a shunt module Mi. In a charging cycle, the circuit operation process is divided into two phases: the voltage detection phase (time ä Tv) and equalization phase (time is Tc). In the voltage detection phase, the equalization bypass circuit does not work, the main power source charges the battery pack, and simultaneously detects the cell voltage in the battery pack, and calculates the duty ratio of the MOSFET according to the control algorithm. In the equalization phase, the MOSFET that is triggered in the bypass controls the switching state from the calculated duty cycle, and equalizes the corresponding battery. At this stage, the current flowing through each cell is constantly changing and different. Excluding M1 connected at both ends of B1, all bypass shunt modules are composed the same. In the shunt bypass, due to the unidirectional conduction of the diode Di, all shunt modules transfer excess power from the corresponding battery to the upstream battery, while M1 transfers excess power to the downstream battery. 2 Switch tube duty cycle calculation The state of charge (SOC) of the state of charge of the battery during charging can be derived from the empirical formula below, where V is the terminal voltage of the battery. SOC=-0.24V 2+7.218V- 53.088 (1) The SOC is the ratio of the current capacity of the battery to the rated capacity, SOC = Q / Q TOTAL × 100%. By converting the battery voltage detected at the end of the voltage detection phase into a state of charge, and the storage capacity Qest,n of the single cell has a corresponding relationship with the SOC, Qest,n can be estimated. In the charge balancing phase, the charge from the main charger to the single cell is IchTcep. Where Tcep is the time of the equalization phase in a charging cycle. In order to achieve a balance of single-cell battery storage capacity during the equalization phase, the target Q tar of the charge should be: (2) However, the charge conversion between the activated bypass and other batteries is mutually influential, and the current output by the bypass battery to the other battery and the received charging current are difficult to calculate with a simple formula. However, the Gauss-Seidel iteration method can solve this problem. The expected storage capacity Q n can be calculated by the following formula: (3) Where I dis,n is the average current in one switching cycle, and I o^,n is the current obtained from the other triggered bypass. Q tar is the amount of charge when the battery reaches the equalization state during the charging period Ts in an ideal state, and Q n is the desired storage capacity, and Q tar=Q n , that is, (2) and (3) are equal. Through the corresponding conversion, the formula for calculating the duty ratio is obtained: (4) The function f N here is just a schematic function, indicating that D n has a certain relationship with D 2...D 3 . 3 experimental design In order to verify the equalization charging method of this paper, the battery pack composed of two single cells is taken as an example for experiment and analysis, mainly to verify the regulation effect of the switching tube on the voltage in the bypass. Since there are no ready-made batteries, an alternative battery is required for the experiment. During the charging process, the internal resistance and terminal voltage of the battery are constantly changing, and the battery accumulates energy during the charging process. According to the analysis of the physical properties of the battery and related materials, the "resistor series capacitor" is used instead of the single battery to carry out the experiment. In this experiment, two low-power NPN tubes C1815 (Q1, Q2) were used instead of the switch tube, and the switches Q1 and Q2 were controlled by the P1.0 and P1.1 pins of the 89C51 chip. At the same time, the terminal voltages V1 and V2 of the battery are collected by the differential amplifier circuit and sent to the CPU via A/D conversion. Throughout the process, the voltage is sampled every 20ms, uploaded to the host computer every 1s and saved and automatically plotted. Experimental results and analysis It can be seen from the experimental results that the voltage difference is 1.98V at the beginning of charging, and the voltage difference is about 0.2V after charging for 140s. During the equalization process, the battery voltage tends to be consistent. The equalization method can shorten the inconsistency between the battery packs according to the difference of the single cells, so that the overall performance of the battery pack is improved and the life is prolonged. At the same time, from the experimental results, the method also has an unsatisfactory effect, that is, the difference between the voltages of the two battery terminals is large. The reason is that the first is to replace the battery with "resistance series capacitor" in this experiment, which is different from the real battery, and can not reach the ideal simulation state. Second, the experiment is mainly to verify the balanced effect of the switch on the voltage of the switch. Simplified processing in many aspects, ignoring some secondary factors, and these factors also have a certain impact on the experimental results. In general, however, the experiment achieved its intended purpose and proved the feasibility of the non-destructive filling method. Metal Jacket Gasket,Spiral Wound Gasket,Metal Gasket Weisike Sealing Co., Ltd. , http://www.wzgasket.com