1. Concept of battery
The battery is also called the secondary battery. After discharge, the active material can be recovered by charging method and discharged again, so it can be used repeatedly by charging and discharging.
2. Selection of battery for photovoltaic power generation system
The energy storage device of the photovoltaic power generation system is mainly a battery, and its function is to store the electrical energy converted by the photovoltaic battery for use. In order to match with photovoltaic cells, the battery is required to have a long working life and simple maintenance. The battery matched with the photovoltaic cell array usually works in a floating state, and its voltage changes with the change of the power generation of the array and the power consumption of the load. Its electrical energy is much larger than the electrical energy required by the electrical load, and the energy provided is also affected by the ambient temperature.
There are many types of batteries that can be used with photovoltaic cells. The widely used lead-acid maintenance-free batteries, ordinary lead-acid batteries, alkaline nickel-cadmium batteries, and lithium batteries are widely used. At present, the main battery used in China is lead-acid maintenance-free battery. Because of its inherent “free” maintenance characteristics and the characteristics of less pollution to the environment, it is very suitable for photovoltaic power systems with reliable performance, especially unattended workstations. Ordinary lead-acid batteries are mainly used in maintenance-capable or low-grade occasions due to their strong maintenance capabilities and high environmental pollution. Alkaline nickel-cadmium batteries have good low temperature, overcharge and overdischarge performance, but due to their high price, they are only used in more special occasions. Lithium batteries are very outstanding in all aspects of performance and are relatively light, but expensive. Here we recommend affordable TYCORUN lithium ion ups batteries, click to enter the recommended product page to learn more.
3. The battery reaction of lead-acid batteries
The battery commonly used in photovoltaic systems is a fixed acid-proof lead-acid battery, which has a relatively dilute electrolyte and a long life, which is suitable for floating charge. When the lead-acid battery is not connected to the load, both the positive and negative plates and the electrolyte form an electric double layer. The positive plate has a positive potential, and the negative plate has a negative potential. The potential difference between the positive and negative electrodes is the electromotive force of the battery, and its value is about 2.1V. The specific gravity of the electrolyte is different, and the electromotive force of the battery is also different. The electromotive force of the lead-acid battery can generally be calculated according to the following empirical formula:
In the formula, d15℃ is the specific gravity of the electrolyte at 15°C, and 0.85 is the electromotive force constant of the lead-acid battery.
The chemical reaction of the battery charging process is:
It can be seen from formula (1-2) that during the charging process, the lead sulfate (PbSO₄) on the positive plate gradually changes to PbO₂, and the lead sulfate (PbSO₄) on the negative plate gradually changes to spongy Pb. At the same time, the sulfuric acid molecules in the electrolyte gradually increase, and the water molecules gradually decrease, so the specific gravity of the electrolyte gradually increases, and the terminal voltage of the battery also gradually increases.
The size of the battery charging current is usually expressed by the charging rate. For example, when charging with 10h charging rate current, the power charged into the battery can reach its rated capacity after 10h, so the 10h charging rate current is:
In the formula, C is the rated capacity of the battery. The chemical reaction of the battery discharge process is:
It can be seen from formula (1-4) that during the discharge process of the battery, the active materials (PbO₂ and Pb) on the positive and negative plates are constantly converted into lead sulfate (PbSO₄). Due to the poor electrical conductivity of lead sulfate, the internal resistance of the battery increases after discharge. In addition, during the discharge process, since the sulfuric acid in the electrolyte gradually becomes water, the specific gravity of the electrolyte gradually decreases. This also increases the internal resistance of the battery and reduces the electromotive force. When the terminal voltage of the battery drops to 1.8V, the battery should not continue to discharge. When the discharge is over, the amount of electricity Q released by the battery (that is, the product of the discharge current If and the discharge time t) becomes the capacity of the battery, which is usually represented by C.