A storage battery is an energy storage device that can convert between electrical energy and chemical energy.
At present, the electricity from distributed power sources installed by users is either for personal use or directly uploaded to the grid, and most of them do not have energy storage equipment. In photovoltaic systems that are not connected to the public grid, that is, photovoltaic off-grid systems, energy storage devices are required to store and regulate the electrical energy generated by solar cells. The storage battery function of the solar photovoltaic power generation system is to store the electric energy generated by the solar cell array when exposed to light, and can supply power to the load at any time. As the energy storage device in the solar photovoltaic power generation system, the storage battery can improve the power supply quality of the system from the following three aspects:
(1) Storage and backup of surplus energy. When there is sufficient sunshine, the energy storage device stores the excess electric energy of the system, and then outputs the energy at night or on cloudy and rainy days, which solves the problem of inconsistency between power generation and electricity consumption.
(2) Ensure the stable power output of the system. The working hours and power levels of various electrical equipment have their own changing laws. It is impossible to make solar energy and electrical loads naturally cooperate. The use of energy storage devices, such as the storage space of batteries and good charge and discharge performance, can play a role in regulating the power and energy of the photovoltaic power generation system.
(3) Improve power quality and reliability. Some loads in photovoltaic systems (such as water pumps, lawn mowers, and refrigerators, etc.), although not large in capacity, will generate surge currents and rush currents during startup and operation. When the photovoltaic module cannot provide a large current, the low resistance and good dynamic characteristics of the storage battery energy storage device can be used to meet the requirements of the above-mentioned inductive load on the power supply.
Classification of storage batteries
The types of storage batteries are divided into two categories according to the type of electrolyte: acid batteries and alkaline batteries.
Storage batteries that use acidic aqueous solutions as electrolytes are called acidic batteries. Since the electrodes of acidic batteries are mainly made of lead and lead oxides, they are also called lead-acid batteries; another type of storage battery that uses alkaline aqueous solution as electrolyte is called alkaline storage battery.
Storage batteries can be divided into recycle batteries and floating charge batteries according to their uses. Recycled batteries include solar storage batteries, railway batteries, car batteries, and electric vehicle batteries. Floating charge uses batteries mainly as a backup power source.
According to the use environment of the storage battery, it can be divided into fixed battery and mobile battery. Fixed batteries are mainly used for backup power sources, and are widely used in post and telecommunications, power stations, and hospitals. Because they are fixed in one place, weight is not a critical issue. The biggest requirement is safety and reliability. Currently used for stationary batteries mainly include sealed batteries and traditional flooded batteries. Mobile batteries mainly include batteries for internal combustion engines, batteries for railway passenger cars, batteries for motorcycles, and batteries for electric vehicles.
The voltage of the storage battery
The nominal voltage of each cell of the storage battery is 2V, and the actual voltage varies with charging and discharging. At the end of charging, the voltage is 2.5~2.7 V, and then slowly drops to a steady state of about 2.05 V.
If the storage battery is used as the power source, the voltage will drop to about 2V quickly when the discharge starts, and then slowly drop to keep it between 1.9~2.0V. When the discharge is close to the end, the voltage will quickly drop to 1.7 V; when the voltage is lower than 1.7 V, no more discharge, otherwise the electrode plate will be damaged. After stopping use, the battery voltage can rise to 1.98 V by itself.
The capacity of the storage battery
The amount of electricity released by the battery under certain discharge conditions is called the battery’s capacity, which is represented by the symbol C. When the discharge current is a constant value, the capacity of the battery is expressed by the product of the discharge current and time, and the unit is ampere hour, abbreviated as ampere hour (A·h) or milliampere hour (mA·h). The discharge current of a battery is usually expressed by the length of the discharge time (that is, the discharge rate), which is called the “discharge rate”, such as the 30-hour discharge rate, the 20-hour discharge rate, and the 10-hour discharge rate. Among them, the 20-hour discharge rate is the normal discharge rate. The so-called 20-hour discharge rate means the rated capacity that can be discharged in 20 hours with a certain current. Usually the rated capacity is indicated by the letter “C”. Therefore, C20 represents the 20-hour discharge rate, and C30 represents the 30-hour discharge rate. The capacity of a lead-acid battery refers to the capacity of the battery to store electricity, which is usually expressed as the total amount of electricity discharged by the battery when the fully charged battery is discharged to the end voltage of the specified discharge end voltage.
The capacity of the battery can be divided into theoretical capacity, actual capacity, rated capacity and nominal capacity.
The theoretical capacity is the highest theoretical value calculated by Faraday’s law for the mass of the active material. In order to compare different series of batteries, the concept of specific capacity is commonly used, that is, the theoretical power that a unit volume or unit mass battery can give. The unit is A·h/kg or A·h/L.
The actual capacity refers to the amount of power that the battery can output under certain conditions. It is equal to the product of the discharge current and the discharge time, the unit is A·h, and its value is less than the theoretical capacity. Because when the battery is composed and designed, in addition to the active material, it also includes non-reactive components such as casing, conductive parts, etc., and it is also related to the extent to which the active material is effectively used.
The rated capacity is the minimum capacity that the battery should discharge under certain discharge conditions according to the standards promulgated by the country or relevant departments.
The nominal capacity is the amount of electricity that the battery can provide when the voltage of a single cell battery drops to a specified value under a certain discharge current and a certain electrolyte temperature when the battery leaves the factory. It is used to identify the battery ampere-hour value. Only the capacity range of the battery is indicated without the exact value, because the capacity of the battery cannot be determined without specifying the discharge conditions.
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