After understanding the charging and discharging characteristics of battery, how to control the charging and discharging process is the key to the application of battery in photovoltaic power generation system. In 1967, American scientist J.A. MAS made a lot of experimental research on the gas evolution problem in the charging process of lead-acid batteries, and put forward an acceptable charging current curve of lead-acid batteries on the premise of the lowest gas evolution rate. In the charging process, as long as the charging current does not exceed the acceptable current of the battery, a large number of bubbles will not be generated in the battery. A large number of test results show that for the lead-acid battery with nominal voltage of 2V, when the charging voltage is below 2.32 ~ 2.35v, no matter how large the current or current density is, the gas precipitation is very small. It is significant only when the voltage reaches 2.35 ~ 2.40v, and the gas precipitation is intense above 2.40v. During intense gas evolution, the initial oxygen content is 50% ~ 70%, and the later hydrogen accounts for about 80%. Only in the middle period of intense gas evolution, H: 0 is close to 2:1. This is consistent with the theoretical conclusion that oxygen evolution begins when the positive charging power reaches 70% and hydrogen evolution begins when the negative charging power reaches 90%. Therefore, for the battery that reaches about 80% of the power during charging, the charging voltage has a considerable impact on the gas evolution. The critical charging voltage that causes a large amount of gas evolution on the positive and negative plates is called the gas evolution voltage of the battery. For a single lead-acid battery, this value is about 2.4V, and the gas evolution voltage of the old battery is lower than this value. When the charging voltage exceeds the gas evolution voltage of the battery, the charging current is mainly consumed to precipitate gas from the battery rather than charging electric energy into the battery. At this time, it not only wastes electric energy, but also damages the service life of the battery.
In order to make the lead-acid battery have short charging time, high charging efficiency and not shorten the service life of the battery, it is best to control the charging voltage of the battery below the gas evolution voltage of the battery. When the terminal voltage of lead-acid battery rises to the gas evolution voltage, the amount of charge depends on the charging current value: the greater the charging current, the greater the polarization voltage, the faster the voltage rise, and the less charge. Therefore, in order to improve the charging efficiency of the battery and achieve the purpose of rapid charging, measures must be taken to reduce the polarization voltage as much as possible and charge as much as possible before the voltage reaches the gas evolution voltage.
- Charging control method of battery
In the charger of the actual photovoltaic power generation system, the charging process must be controlled in order to realize the set charging mode. The charging control mainly includes the judgment of charging degree, the automatic conversion from discharge state to charging state, the automatic conversion of charging mode in each stage and the stop charging control. Mastering the correct control method is helpful to improve the charging efficiency and service life of the battery.
(1) Stage division of charging process
The charging process is generally divided into three stages: main charging, equalizing charging and floating charging. Sometimes at the end of charging, there is slip current charging with small charging current for a long time.
The main charging mode is generally fast charging, such as two-stage charging, variable current intermittent charging and pulse charging, which are common main charging modes at this stage. Slow charging as the main charging mode generally adopts the constant current charging mode with low charging current. After deep discharge or long-term floating charge of lead-acid battery, the voltage and power of single battery in series may be unbalanced. The charging to eliminate this imbalance is called equalizing charging, which is called equalizing charging for short.
In order to protect the battery from overcharge, after the battery is quickly charged to 80% ~ 90% power, it is generally switched to floating charge (constant voltage charging) mode to adapt to the reduction of acceptable charging current of the battery in the later stage of charging. When the floating charge voltage is equal to the battery terminal voltage, it will automatically stop charging. In order to prevent the possible insufficient charging of the battery, trickle charging can be added after that, so that more active substances in the battery plate that has been basically fully charged can participate in the chemical reaction, and its charging is relatively complete.
(2) Charging degree judgment
When charging the battery, the charging degree of the battery must be judged at any time in order to control the charging current. The main methods to judge the charging degree are as follows.
① Detect the terminal voltage change of the battery after depolarization. Generally speaking, in the initial stage of charging, the change rate of battery terminal voltage is very small; In the middle stage of charging, the change rate of battery terminal voltage is very large; At the end of charging, the change rate of terminal voltage is very small. Therefore, by observing the change of terminal voltage in unit time, we can judge the charging stage of the battery.
② Detect the actual power value of the battery and compare it with its rated power value to judge its charging degree.
③ Check the battery terminal voltage. When there is a large difference between the battery terminal voltage and its rated value, it indicates that it is in the initial stage of charging; When the difference between the two is very small, it indicates that the charging process is close to the end.
(3) Automatic conversion of charging stages
At present, the common automatic conversion methods of each charging stage are:
① Time control, that is, the charging time of each stage is preset, and the conversion time is controlled by time relay or CPU;
② Set the charging current or battery terminal voltage value of the conversion point. When the actual current or voltage value reaches the set value, it will be automatically converted;
③ The integrating circuit is used to monitor the power of the battery on line. When the power reaches a certain value, a signal is sent to change the charging current.
Among the above methods, the time control is relatively simple, but this method lacks real-time information from the battery, and the control is relatively rough; The control circuit of power monitoring method is complex, but the control accuracy is high.
(4) Stop charging control
When the battery is fully charged, the charging current must be cut off in time, otherwise the battery will have a large number of overcharge reactions such as gas evolution, water loss and temperature rise, endangering the service life of the battery. Therefore, the charging condition of the battery must be monitored at any time to ensure that the battery is fully charged but not charged. The main methods of stopping charging are as follows.
① When the constant current charging method is adopted for timing control, the charging time required by the battery can be easily determined according to the battery power and charging current. Therefore, as long as the charging time is preset, once the time is up, the timer can send a signal to stop charging or reduce to trickle charging. The timer can be acted by the time relay, or its function can be undertaken by the single chip microcomputer. This method is simple, but the charging time cannot be automatically adjusted according to the state of the battery before charging. Therefore, sometimes under charging and sometimes overcharging may occur during actual charging.
② When the battery temperature is controlled and charged normally, the temperature change of the battery is not obvious, but when the battery is overcharged, the internal gas pressure will increase rapidly, the oxidation reaction on the negative plate will heat the interior, and the temperature will rise rapidly (several degrees per minute). Therefore, observing the change of battery temperature can judge whether the battery is full. Usually, two thermistors are used to detect the battery temperature and ambient temperature respectively. When the temperature difference between them reaches a certain value, the charging stop signal is sent. Due to the slow dynamic response speed of thermistor, the full charge state of battery can not be detected timely and accurately, which is not conducive to the maintenance of power storage and battery life.
③ Negative increment control of battery terminal voltage generally speaking, when the battery is fully charged, its terminal voltage will show a downward trend. Therefore, the time when the battery voltage increases negatively can be regarded as the time of stopping charging. Compared with the temperature control method, this method has fast response speed. In addition, the negative increment of voltage is independent of the absolute value of voltage. Therefore, this stop charging control method can adapt to the charging of battery packs with different number of single cell batteries. The disadvantage of this method is that the sensitivity and reliability of the general detector are not high. At the same time, when the ambient temperature is high, the voltage reduction after the battery is fully charged is not obvious, so it is difficult to control.
④ Polarization voltage control is used. Generally, after the battery is just full, the polarization voltage of the battery is generally maintained at the order of 50 ~ 100mV. Measuring the polarization voltage of each single battery can make each battery charge to the degree required by itself. The research shows that because each battery has at least some slight differences in geometric structure, chemical properties and electrical characteristics, it is more appropriate to determine the required charging level according to the characteristics of each single battery than to control the battery pack as a whole.
- Several common problems in the process of battery charging and discharging
(1) Inconsistency and balanced charging of lead-acid batteries
The inconsistency of storage battery refers to the difference of voltage, internal resistance, power and other parameters of storage battery of the same model and specification. The inconsistency of the battery parameters of the battery pack will make it easier for the low-power battery in the battery pack to overcharge and over discharge, so that the battery pack will fall into a vicious cycle of aggravating the vulcanization of the battery plate and further expanding the power gap of the battery, which will not only shorten the service life of the battery, but also increase its internal resistance and reduce the effective active substances due to the vulcanization of the battery plate, Thus, the energy conversion efficiency and output power of the battery pack are reduced, and even the performance state of the battery is deteriorated rapidly and scrapped prematurely.
The vulcanization of battery plate is the main reason for the expansion of power gap in the process of battery use. The traditional equalizing charging methods of “voltage balance” and “state of charge balance” can not reduce and eliminate the effect of battery plate vulcanization, but increase the power gap of each battery in the battery pack. Balanced charging based on eliminating battery plate vulcanization can be adopted to appropriately “overcharge” the low-power storage battery to reduce or eliminate its plate vulcanization, so as to restore its power and enable other batteries to be charged to a “fully charged” state, so as to achieve the purpose that the performance parameters of each storage battery do not expand and tend to be consistent.
(2) Effect of discharge on battery performance
In order to prevent the memory effect of the battery, a discharge link needs to be added in the charger design. The battery with insufficient discharge depth should be discharged to a certain extent before recharging. The discharge condition of the battery has a certain impact on the charging performance of the battery. According to the electrochemical theory, for any given discharge current, the acceptable charging current of the battery is inversely proportional to the square root of the amount of charge it has discharged. Therefore, the greater the discharge depth, the more the battery discharges, the smaller the acceptable charging current of the battery, which will slow down the charging speed of the battery. Therefore, the deep discharge of the battery should be avoided.
The greater the discharge current of the battery, the greater the acceptable initial charging current during recharging, which helps to improve the recharging speed. However, the heat generated when the battery discharge current flows through the internal resistance will cause the temperature to rise, so the discharge current should not be too large.
(3) Fast charging and polarization
Fast charging is an effective method to improve the energy storage speed of battery quickly and safely. Fast charging will bring large current, which will bring the polarization of the battery, causing the active material of the battery plate to fall off and damage. The polarization phenomenon of battery seriously hinders the progress of electrolytic chemical reaction, which will eventually lead to the irreversible reaction of battery. Therefore, fast charging and improving battery life are a pair of contradictions. In order to charge smoothly with continuous high current, the polarization phenomenon caused by high current must be properly eliminated. Eliminating polarization is the key of high current fast charging technology.
High current charging and polarization are a pair of contradictions, which will exist in the whole process of battery charging. Polarization phenomenon destroys the reversibility of battery chemical reaction. The theoretical basis for solving this contradiction is the reversibility of battery chemical reaction. There are three main ways to eliminate polarization.
① Forced elimination. The basic meaning of reversible battery is that when the current in the electrolytic cell is reversed, the chemical reaction is also reversed. In the process of high current charging, the forced current is reversed, that is, the battery is discharged instantaneously at a certain depth. In this process, the excess charge on the positive and negative plates of the battery that has not participated in the chemical reaction moves in the opposite direction to the original charge, and the original accumulated excess charge on the plates decreases rapidly, so the electrochemical polarization will be eliminated or weakened. At the same time, during the discharge process, the positive and negative ions in the electrolyte will also move in the opposite direction to the original charging, which plays the role of stirring the electrolyte solution and can effectively control the concentration polarization. At the same time, during the discharge process, the battery will transfer part of the heat energy formed by ohmic polarization to the load, which can also effectively control the temperature rise of the battery.
② Natural elimination. In the process of high current charging, make the battery stop charging instantaneously, and the ohmic polarization will disappear rapidly. At the same time, it can buffer the electrochemical polarization and concentration polarization caused by the differences caused by charge movement, ion migration and chemical reaction speed.
③ Feedback control. Suppression of outgassing and temperature rise are two main problems to be solved in fast charging. The practice shows that they are closely related to the terminal voltage generated by the battery during charging. On the premise of eliminating polarization, the air outlet volume and temperature rise of a single battery are not significant until the voltage of a single battery reaches 2.3V. Therefore, it is appropriate to control the charging current by detecting the terminal voltage generated by the battery in the charging process and taking it as a feedback command. The purpose of feedback is to reduce the charging current in time after continuous high current charging for a period of time when the battery is in the degassing stage, so as to attenuate it according to the exponential function, which can effectively inhibit the degassing and temperature rise in the later stage of charging.