The basic characteristics of the battery directly affect the photovoltaic power generation system, and the operating characteristics of the photovoltaic power generation system equipped with the battery are largely determined by the characteristics of the battery.
(1) Self-discharge of the battery
The phenomenon that the capacity of the battery will gradually decrease during storage without any load after being charged is called the self-discharge phenomenon of the battery. Usually, the capacity of the battery will decrease due to self-discharge when it is not connected to any external circuit, and it will drop to zero for a long enough time.
Since the negative electrode active materials are mostly active metals, their potential in the electrolyte is often lower than that of hydrogen, so the chemical reaction of replacing hydrogen gas is easy to occur, so self-discharge usually occurs at the negative electrode of the battery. There are metal impurities with lower potential than hydrogen in the negative electrode. These impurities and the negative electrode active material can form a corrosive micro-battery. As a result, the metal on the negative electrode self-dissolves, and hydrogen replacement occurs while reducing the capacity. Impurities present in the electrolyte also undergo a similar reaction to reduce capacity. The self-discharge of the positive electrode itself is not large, but if there are impurities that are easily oxidized in the electrolyte or on the separator, the positive electrode acts as a strong oxidant to reduce the positive electrode active material, thereby reducing the capacity of the battery. Batteries can be recharged to restore capacity, and batteries should be pre-charged when installed in photovoltaic systems, and recharged if not used for a long time. In general, the charge retention capacity of alkaline batteries is better than that of acid batteries.
(2) Service life of the battery
The effective life period of the battery is called the service life. When the battery cannot be used due to internal short circuit or external damage, the battery will be invalid, and the service life of the battery will be terminated.
The service life of the battery includes the use period and the use cycle. The service period refers to the time that the battery can be used including the storage time of the battery; the service cycle refers to the number of times the battery can be reused. For example, the life of a typical alkaline battery is stipulated: at an ambient temperature of 20 ° C ~ ° C, the number of cycles of full charge and full discharge can exceed 900; when the floating charge method is used, the service life can reach 15 to 20 years. The battery life in the photovoltaic power generation system is the bottleneck of the life of the photovoltaic system.
Each time the battery undergoes a full charge and full discharge process is called a cycle or a cycle. The useful life of a battery includes the cycle life experienced. If the life is assessed in a cycle way, among the commonly used batteries, zinc-silver batteries have the shortest lifespan, generally only 30-500 cycles; lead-acid batteries have 300-500 cycles; alkaline nickel-cadmium batteries have the longest lifespan, which is 500-1000 cycles .
There are two main reasons for the end of battery life: one is that its power gradually decreases, and the main factors causing the decline of power are the change of the crystal form of the active material inside the battery, the expansion and fall of the active material, and the corrosion of the battery skeleton or substrate; Internal short circuits, such as perforation due to degradation and aging of the diaphragm material, short-circuiting of the two poles due to the shedding and expansion of active materials, and dendrites penetrating the diaphragm during charging may all cause internal short circuits in the battery.
(3) Operation mode of the battery
According to the application requirements of the photovoltaic power generation system, multiple batteries of the same type can be connected in series, in parallel or in series and parallel to form a battery pack. There are three main modes of operation of the battery pack: cyclic charge and discharge system, continuous float charge system and regular float charge system.
a. Cyclic charge and discharge system The cycle charge and discharge system belongs to the full charge and full discharge method. The cycle operation of the battery is relatively simple. The photovoltaic cell directly charges the battery, and then the battery directly discharges the load. However, its service life is short, because the battery is often fully charged and fully discharged, and the electrolyte is consumed more. Small household photovoltaic power generation systems often use this type of charging and discharging method.
b. Continuous floating charging system Continuous floating charging system is also called full floating charging system. The operation mode is to connect the battery pack to the load circuit in parallel for a long time. The battery maintains a small amount of charging current and compensates for the fluctuating load current. Under normal circumstances, there is always a photovoltaic DC voltage applied to both ends of the battery. As long as the battery voltage is lower than the photovoltaic array DC power supply, the power supply will charge the battery. When the photovoltaic array power supply is insufficient or completely out of power, the battery is enabled to supply power to the load, so as to ensure that the load power supply is not interrupted.
c. Regular floating charging system Regular floating charging system is also called semi-floating charging system, which is a working method in the parallel power supply system of photovoltaic array DC power supply and battery. Part of the time is powered by the battery, and part of the time is directly powered by the photovoltaic array, while supplementing the power released by the battery pack and the power lost by self-discharge. Using the regular floating charging system, the life of the battery is longer than that of the cyclic charging and discharging system, and the use efficiency of the photovoltaic array is also higher. However, the output current of this method has a pulsating AC component, and a voltage regulator or inverter is required. Voltage the battery to offset the voltage higher than the load allows. The continuous floating charging system is superior to the regular floating charging system, the service life of the battery can be extended by 1~2 times compared with the cyclic charging and discharging system, and the power of the battery used is greatly reduced, the electric energy efficiency is improved, the maintenance is simple, and the utilization efficiency of the entire photovoltaic power generation system is high. This type of photovoltaic power generation system generally adopts continuous float charging.