Battery charge and discharge control method

- Jan 11, 2019-

The battery charge controller is usually done by control voltage or control current. In general, there are three ways to charge the battery: constant current charging, constant voltage charging, and constant power charging. Each method has different voltage and current charging characteristics.


In the charging controller of the actual photovoltaic power generation system, in order to realize the set charging mode, the charging process must be controlled. The charging and discharging control mainly includes the charging degree judgment, the automatic replacement from the discharging state to the charging state to the charging state, and charging each Stage mode automatic conversion, stop charge control and discharge control brush. The correct control method is beneficial to improve battery charging efficiency and service life.


The charging process is generally divided into three stages: main charging, equal charging, and floating charging. Sometimes, at the end of charging, there is a trickle charging that is continuously charged with a small charging current for a long time.


The main charge is: fast charging and slow charging. The fast charging adopts two-stage charging, three-stage charging, pulse charging, variable-current intermittent charging and variable-pressure intermittent charging. The slow charging uses a constant current of low charging current charging mode. In order to protect the battery from charging, after the battery is quickly charged to 80%~90% capacity, it is generally switched to floating charge (constant voltage charging) mode to adapt to the reduction of charging current in the later stage of charging. When the float voltage value is equal to the battery terminal voltage, it will automatically stop charging. In order to prevent possible battery charging shortage, trickle charging can be added after this, so that there is more activity inside the battery plate which is basically fully charged. The substance participates in the chemical reaction and its charging is relatively thorough.


After the lead-acid battery is deeply discharged or long-term floating, the voltage and capacity of the single-cell battery in the series may be unbalanced. The charging performed to eliminate this imbalance is called equalization charging.


Charge control


The automatic conversion control methods for common charging stages are as follows.


1 Time control the charging time of each stage is preset, and the switching time is controlled by the time relay or CPU.


2 Current voltage control set the threshold value of charging current or battery terminal voltage. When the actual current or voltage reaches the set value, it will automatically switch.


3 Capacity Control The integration circuit is used to monitor the capacity of the battery online when the capacity reaches a certain value. Then the signal is changed to change the magnitude of the charging current.


In the above method, the time control is relatively simple, but this method lacks information from the battery current, the control is relatively rough, and the capacity control method control circuit is relatively complicated, but the control precision is high.


When the battery is fully charged, the charging current must be cut off in a timely manner. Otherwise, the battery will have a large amount of gas over-emission, water loss and temperature rise, which will affect the service life of the battery. Therefore, it is necessary to monitor the charging status of the battery at any time to ensure that the battery is fully charged and not charged. There are four main methods for stopping charge control.


1 Timing control When using the constant current charging method, the charging time of the battery can be easily determined according to the battery capacity and the 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 It becomes trickle charge. This method is simple, but the charging time cannot be automatically adjusted according to the state of the battery charging surface, so the actual charging time. There may be cases of undercharging and sometimes overcharging.


2 When the battery temperature is controlled for normal charging, 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 cause internal heat, and the temperature will rise rapidly (every minute can be raised) A few degrees Celsius). Therefore, by observing the change in the battery temperature, it can be judged whether the battery is full. The two thermistors are usually used to monitor the battery temperature and the ambient temperature respectively. When the temperature difference between the two reaches a certain value, the stop signal is issued. Since the dynamic response speed of the thermistor is slow, the full charge state of the battery cannot be accurately and timely detected, which is not conducive to the maintenance of the battery life.


3 Battery terminal voltage negative increment control When the battery is fully charged, its terminal voltage will show a downward trend. According to this, the time when the battery voltage has a negative growth can be used as the stop time. This method is faster than the temperature control method. In addition, the negative increase in voltage is independent of the absolute value of the voltage. Therefore, this stop-and-charge control method can accommodate battery pack charging with a different number of cells. 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 reduction after the sufficient voltage of the battery is not obvious, and thus it is difficult to control.


4 Using polarization voltage control under normal circumstances, the polarization voltage of the battery is generally protected at 50-100mV after the battery is fully charged. The polarization voltage of each single cell is measured, so that each battery can be charged to its own requirements degree. Since there are at least some slight differences in the geometry, chemistry and electrical properties of each battery, then the characteristics of each cell should be determined to determine the level of charge it requires, rather than taking the battery as a whole. The method of control is more appropriate.