In the photovoltaic power generation system, the input of electric energy generated by solar energy is not stable, so the control of the photovoltaic system brings uncertainty. In addition, the various components of the system also change with the environment and use time, so reasonable charging control to reduce the damage to the battery has become a prominent problem. Since the current photovoltaic power generation system is dominated by lead-acid batteries, here we only take lead-acid batteries as an example to introduce the basic principle of the controller’s charging control.
- Basic characteristics of charging
At present, the main charging control has several types such as parallel control, series control and PWM (pulse width modulation) control. Among them, the parallel control connects the output of the solar cell array to a resistor or other power devices in parallel through electronic components, and the energy after the battery is fully charged is consumed in the form of heat on the parallel line. This control method is simple and reliable, and there is no voltage drop in series, but it is easy to cause heat island effect. The series control works directly after the diode is connected in series between the solar cell module and the battery. This kind of charging control is relatively simple but has low efficiency, and there are many restrictions on the matching of the battery and the solar cell module, so it is rarely used. PWM control is to connect a switch tube in series between the solar cell module and the battery, and control the charging voltage or current by controlling the conduction pulse width of the series switch tube. According to the actual situation, you can choose to control the charging voltage or charging current, and realize the charging control of the battery at different stages according to the preset battery charging strategy. This control method realizes optimal charging according to battery characteristics and can protect the battery from overcharging, so more small independent systems will choose this control method.
The basic principle of using PWM control is: when the battery starts to charge, the pulse width of the control charging current is wide, so that the charging current can be large, so that the battery can be charged quickly; The turn-on pulse width will be narrowed, making the charging current smaller; when the battery reaches a fully charged state, the charging current will drop below the trickle charging current. In PWM control, the switch tube mainly uses MOSFET (metal oxide semiconductor field effect transistor) and IGBT (insulated gate bipolar transistor), so the PWM switching frequency can work in a higher range, and the MOS-FET switching frequency can work in dozens of KHz to hundreds of KHz range, IGBT can work in the range of several KHz to 20 KHz. This type of PWM control constitutes a charger with complex structure and high cost, and has gradually become the main method of solar photovoltaic charging control. If you would like to learn more about battery charging, please visit tycorun.com.
The lead-acid battery charging characteristic curve is shown in Figure 1. It can be seen from the charging characteristic curve that the battery voltage changes with the capacity, the charging process can be divided into three stages: in the early stage of charging (OA), the battery terminal voltage rises rapidly; in the middle stage (AC), the voltage rises slowly and lasts for a long time, the battery , the charging capacity is increased rapidly; at the end of charging (CD), the voltage rises faster, but the capacity growth rate decreases. From point C, the electrochemical reaction of the battery is nearing the end, and the voltage begins to rise rapidly. When approaching point D, hydrogen gas is released from the negative electrode, oxygen gas is released from the positive electrode, and water is decomposed. Therefore, the voltage at point D indicates that the battery is fully charged. When the battery voltage reaches point D, stop charging, otherwise it will damage the lead-acid battery. According to this principle, a voltage measurement and voltage comparison circuit is set up in the controller, and by monitoring the voltage value at point D, it can be judged whether the battery should end charging. For VRLA batteries, the end-of-charge voltage under standard conditions (25°C, 0.1C charge rate) is about 2.35V. The same comparison voltage as the voltage at point D is set in the control system, which is called “threshold voltage” or “voltage threshold”. Since the charging rate of the photovoltaic power generation system is generally very small, the charging point of the VR-LA battery is generally set at 2.3~2.35V.
Figure 2 shows the curve relationship of battery charging current and terminal voltage with charging time. The capacity and life of the battery are important parameters of the battery, and they are greatly affected by the charging method. As can be seen from Figure 2, at the beginning of charging, the voltage at OA first rises sharply, and then slowly rises along the ABC line for a long time. After reaching point C, the voltage rises quickly. At this time, the water in the battery is blocked. Electrolysis, the battery is fully charged when it reaches point D. It can also be seen from Figure 2 that the curves corresponding to different current charging will be very different. The higher the current, the shorter the charging time, and the higher the corresponding end-of-charge voltage. In addition, the charging process is also affected by the electrolyte concentration, temperature, and charging history. The charging strategy is very important for the battery. An optimized charging strategy can improve the performance and life of the battery. Conversely, an unreasonable charging strategy will significantly reduce the usable capacity and life of the battery.
The purpose of the control system is to make the battery as full as possible and avoid electrolysis of water as much as possible. Both the redox reaction and the electrolysis reaction of water during battery charging are related to temperature. When the temperature rises, the redox reaction and the decomposition of water become easier, and the electrochemical potential decreases. At this time, the full threshold voltage of the battery should be reduced to prevent the decomposition of water; when the temperature decreases, the redox reaction and the decomposition of water both change. If it is difficult, its electrochemical reaction potential will rise. At this time, the charging threshold voltage of the battery should be increased to ensure that the battery is fully charged without a large amount of water decomposition. In photovoltaic power generation systems and wind-solar hybrid power generation systems, the temperature of the battery electrolyte not only changes with the seasonal cycle, but also fluctuates due to environmental influences. Therefore, the control system is required to perform automatic temperature compensation for the battery full threshold voltage. The temperature coefficient of a single lead-acid battery is generally -(3~5) mV/℃ (standard condition is 25℃), that is, when the electrolyte temperature (or ambient temperature) deviates from the standard condition, the battery is fully charged for every 1℃ increase. The threshold voltage is adjusted downward by 3~5mV for each battery; for every 1°C drop, the battery full threshold voltage is adjusted upward by 3~5mV for each battery.
- Main charging method
(1) Series charging
The solar cell module is directly charged to the battery after being isolated by series diodes. This method must match the operating point voltage range of the solar cell module with the range where the battery can be charged stably, also known as voltage matching charging. With this method, as the charging time increases, the terminal voltage of the battery increases, the current input to the battery by the solar cell assembly decreases, and the operating point moves toward the open-circuit voltage direction along the volt-ampere characteristic curve of the solar cell assembly. The function of the series diode is to prevent the reverse discharge of the battery to the solar cell module during the period of low solar irradiance or at night. The diode can use silicon rectifier diodes and Schottky diodes. Since Schottky diodes have a lower tube voltage drop, their The conduction loss is small. Since the volt-ampere characteristic curve of the solar cell module is affected by the change of light, its working curve is uncertain, so the voltage generated by the solar cell module may be lower than the voltage of the battery when the light is not good, and the battery cannot be charged. This charging method Only fully charge the battery when there is sufficient solar radiation. Therefore, the utilization efficiency of solar energy is very low, especially when the battery is deeply discharged and the sunlight is not good, the charging capacity of the battery is very low, so multiple discharges will damage the battery. This kind of charger has low charging efficiency, and it is difficult to match the charging voltage of the battery with the solar cell module. When the battery discharge depth is low, the efficiency is particularly low, and it is greatly affected by the sunlight conditions, so it is not used much at present.
The principle of the improved method of series charging is to connect a switch tube in series between the solar cell module and the battery, and the series switch tube replaces the series diode. When the charging voltage exceeds the high limit of the charging voltage set by the battery, the charging switch is turned off, the battery stops charging, and the voltage of the battery terminal is lowered; when the charging voltage is lower than the low limit of the charging voltage set by the battery, the charging is turned on. Switch tube, the battery starts to charge, so that the battery terminal voltage rises. The controller can keep the charging voltage of the battery within the allowable range, and realize the overcharge and overdischarge protection of the battery, as shown in Figure 3. It uses a typical hysteresis type two-point switching charging control method, the output has a relatively high voltage ripple, and the charging switch operates at a very low variable frequency, so relays can be used as switches, and most of them use solid-state switches such as MOSFETs and IGBTs, etc. This charger is simple and reliable, and is mostly used in small photovoltaic systems, but its charging control scheme lacks intelligence. The output of photovoltaic cells is equivalent to an open circuit when the battery does not lose power, so the utilization rate of photovoltaic cells is relatively low.
(2) Parallel charging
This is the most commonly used method for small photovoltaic power generation systems, mainly because the price is relatively cheap. The principle of parallel charging is to connect a parallel discharge circuit before the series isolation diode. When the charging voltage exceeds the high limit of the charging voltage set by the battery, the bypass discharge circuit is opened to pull down the charging voltage; when the charging voltage of the battery is lower than When the charging voltage set by the battery reaches the low limit, disconnect the bypass discharge circuit, and the charging voltage will rise. This control keeps the charging voltage of the battery within the allowable range and realizes the overcharge protection of the battery, as shown in Figure 4.
This charging method is simple and reliable, but the charging control scheme lacks intelligence, and the utilization efficiency of photovoltaic cells is relatively low. This charging method is mainly used for occasions where the depth of discharge of the battery is not high and the power of the system is small. Otherwise, the utilization efficiency of solar energy will be very low, especially when the battery is deeply discharged and the sunlight is not good, the charging capacity of the battery is very low, so that the battery will be damaged after multiple discharges. In order to improve the utilization rate of photovoltaic cells, the auxiliary load can be connected to the parallel shunt circuit to supply power for it, and the purpose of adjusting the charging voltage can be achieved by controlling the load current. However, the control of the load current in a wide range requires the adjustment characteristics of the load There are certain difficulties in matching it.
(3) PWM charging
In a high-power independent solar system, due to the nonlinearity of the photovoltaic cell itself and the influence of its output by light intensity and temperature, the traditional battery constant voltage, constant current and exponential pulse charging methods are difficult to apply to photovoltaics. charge controller. In the photovoltaic system, the speed of charging the battery is no longer a concern; instead, it is how to maximize the utilization of photovoltaic energy during the charging process and reasonably charge the battery to reduce losses and prolong life. . There are many factors that affect battery charging, such as temperature, battery electrolyte concentration, and aging degree of battery plates. It is not easy to get an optimal battery charging strategy, and it is even more difficult to implement it in the system. In the actual system, better results can be obtained only by grasping the main factors. The general charging strategy is a comprehensive strategy in which the photovoltaic cell voltage, current and battery voltage, current and capacity are both variables and control objects. It is divided into three parts.
① The maximum power point tracking strategy is adopted when the battery is short of power, that is, the maximum possible output energy of the solar cell module should be fully utilized in the fast charging stage. At this time, the battery has a relatively strong power receiving capacity and is not easy to be damaged. However, the charging current should not be too large. Generally, the charging rate should be controlled to be less than Cio. Otherwise, the temperature will be too high due to the excessive charging current, which will easily bend the electrode plate, cause the active material to fall off, and accelerate the corrosion of the positive electrode plate. At the same time, the charging efficiency is also low. In addition, if the charging current is too large, the water in the electrolyte of the battery will be electrolyzed, which will affect the life of the battery. This stage requires the battery to be charged to 80% to 90% of its rated capacity.
②When the terminal voltage of the battery exceeds the threshold set in the overcharge stage (this voltage can generally be set to 2.5~2.7V for each cell of the battery), the constant voltage control is adopted. In this process, due to the constant charging voltage, the charging current attenuates with the rise of the battery electromotive force, the current components that generate heat and electrolyzed water are greatly reduced, the temperature is lowered, and the overflow of acid mist is reduced. This improves the operating and maintenance conditions of the battery during charging and increases electrical efficiency. During this charging phase, the battery can be charged to more than 97%.
③ When the battery is full or very close to full, it will enter the floating charging stage, which adopts the second-level constant voltage control with PI regulation. For each cell of the battery, the voltage can be set to 2.27V. This process is to give the battery a very small current to supplement the loss of battery self-discharge.
The judgment and switching of the three states of the battery charging process should follow the precise control of the battery input characteristics. The judgment signs here are the voltage, current and capacity of the battery. At the same time, different types of batteries also have different parameters, so in the actual system, some necessary thresholds must be checked and measured in advance to realize the above control strategy.
During the charging of the battery, the temperature of its electrolyte will increase, and the voltage threshold of its charging will also change as the temperature changes. The charge and discharge controller needs to detect the temperature of the battery and compensate the charge and discharge voltage of the battery.
After the battery is fully charged, the best way to keep the power is to add a constant voltage to the battery to float the battery. This puts forward requirements for the charging circuit to be suitable for floating charging voltage. The float voltage value should not only compensate the self-discharge current of the battery, but also not be too large, so as not to cause the decomposition of the chemical composition inside the battery due to overcharging. The fully enclosed maintenance-free lead-acid battery can work stably for 6 to 10 years under the proper floating charge state, but as long as there is a 5% deviation in the floating charge voltage, the life of the battery will be halved.
The voltage characteristics of lead-acid batteries have an obvious negative temperature coefficient, and a 2V battery is about -4mV/C. Reasonably considering the temperature variation range, the charger should give some form of compensation according to the temperature coefficient of the battery, which can effectively reduce the impact of the float voltage on the battery life. In order to improve the service life of the battery, according to the different charging conditions, the cut-off voltage of the charging process to keep the battery in good condition has the following principles.
①In the case of continuous use and infrequent charge-discharge cycles, the average cut-off voltage of each battery cell is 2.40V.
② Periodic charge and discharge cycle operation, the average cut-off voltage of each battery unit is 2.35V, and the photovoltaic charging system generally works in this state.
③ One charge-discharge cycle per day, the average cut-off voltage of each battery unit is 2.45V, which is maintained for 2h.
④For a full charge once a month, the average cut-off voltage of each battery unit is 2.45V, and it is maintained for 5h.
⑤Equalize charging once every two months, the average cut-off voltage of each battery unit is 2.45V, and keep it for 15h.
In addition, the following principles need to be followed in the use (discharging) of lead-acid batteries.
①Avoid deep discharge.
②Charge to full state of charge at least once a month (at a single battery cell voltage of 2.4V, charge for 4h).
③ Carry out “equalization charging” at least once every 3 months.
④ Try to keep the battery in the state of “floating charge voltage” (the voltage of the single battery is 2.27V).
⑤ Avoid high temperature.
⑥ Avoid freezing due to low temperature.
⑦Replenish the steaming water (maintainable lead-acid battery) in time.
For batteries used in photovoltaic power generation systems, the working environment conditions are harsh, that is, it is difficult to strictly control the charging and discharging laws under the above ideal conditions due to the limitation of meteorological conditions. For example, taking a photovoltaic lighting system as an example, as long as the battery is not overcharged, the battery should be charged as much as the solar cell module generates during the day. The maximum charging current is pre-planned by the system design.
The charging current cannot be selected and controlled during specific operation. It is determined by the maximum power point current of the solar cell module, and the discharge current is also determined by the load current. It is difficult to achieve optimal charging and optimal discharging. be managed to avoid overcharging and overdischarging.
Read more: What is battery charge and discharge control and what are the common problems with it?