Energy storage battery for photovoltaic power generation system

Energy storage batteries and accessories are an indispensable part of an independent photovoltaic power generation system. Its main function is to store the excess power of the load and absorb the output power of the photovoltaic power generation system, and to supply power to the load when the amount of sunlight is insufficient and at night.

In photovoltaic power generation systems, commonly used energy storage batteries and accessories include lead-acid batteries, alkaline batteries, lithium-ion batteries, lithium iron phosphate batteries, nickel-hydrogen batteries, and super capacitors. At present, considering the comprehensive consideration of cost-effectiveness and other factors, lead-acid batteries are more commonly used in independent photovoltaic power generation systems. According to the structure of the product, lead-acid batteries are divided into open type, fixed valve control seal maintenance-free type and valve control seal colloid type. The most widely used in photovoltaic power generation systems are fixed valve-regulated sealed maintenance-free lead-acid batteries and valve-regulated sealed gel batteries.

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1. The working principle of lead-acid batteries
The working process of a lead-acid battery is a process of converting electrical energy into chemical energy through an electrochemical reaction, and then converting chemical energy into electrical energy.

Energy storage battery for photovoltaic power generation system
Figure 1 The working principle of lead-acid batteries

2. Common technical terms for batteries
(1) Capacity Capacity refers to the amount of electricity that a battery can release under certain discharge conditions. It is represented by the symbol C and the unit is A.h or mA.h. Capacity=discharge current×discharge time, namely

(2) Discharge rate Discharge rate is a discharge parameter that measures the speed of battery discharge. The discharge rate is usually expressed in “hour rate”, that is, the rate of discharge is expressed in terms of discharge time. Commonly used discharge rates are 20HR, 10HR, SHR and 2HR. Denoted as C20, C10, C5 and C2, they represent the discharge efficiency of 20h, 10h, 5h, and 2h, respectively. The greater the discharge rate, the longer the discharge time, the smaller the required discharge current, the higher the efficiency of battery discharge, and the greater the measured capacity. For example, a set of 60A.h batteries with a rated capacity are discharged in 10 hours, which is called the 10-hour discharge rate.

(3) Nominal battery name. For example, a battery is called “12V8Ah/10HR”, where “12V” is the rated voltage, “8Ah” is the rated capacity, and “10HR” is the 10-hour discharge rate, which means “12V8Ah/10HR” This battery can be used for 10 hours under 0.8A constant current discharge, and “12V8Ab/SHR” can be used for 5 hours under 1.6A constant current discharge. The rated capacity of the battery for starting is expressed at a rate of 20 hours, and the symbol is C20; the rated capacity of a stationary battery is expressed at the rate of 10 hours, and the symbol is C10.

(4) Discharge current The current formed when the storage battery discharges the stored electric energy to the load. Due to the chemical characteristics of the battery itself, the battery voltage will gradually decrease during discharge. When the discharge current is smaller, the discharge time is longer; when the discharge current is larger, the discharge time is shorter.
“10AhC2” means that the rated capacity of the battery is 10A.h, and the discharge hour is 10A-h/2h=5A in constant discharge.
“54AhC20” means that the battery has a rated capacity of 54A-h and a small discharge capacity of 54A.h/20h=2.7A.

(5) End-of-discharge voltage End-of-discharge voltage (abbreviated as “end-of-discharge”) refers to the lowest voltage at which the battery discharges when the voltage drops to the point when it is no longer suitable for discharging. For lead-acid batteries (25℃), the rated voltage of a single cell is 2.0V, and the final single cell is 1.75V. If the rated voltage is 12V, the final voltage is 1.75V.

(6) End-of-charge voltage The end-of-charge voltage (referred to as “end of charge”) refers to the voltage value. The voltage at the end of charging is not allowed to exceed this value, but the charging end voltage is higher than the rated voltage. For lead-acid batteries (25℃), the rated voltage of a single cell is 2.0V, and the final charging of a single cell is 2.3~2.35V. If the rated voltage is 12V, the final charging is 2.3Vx6=13.8V or 2.35Vx6=14.1V

(7) According to the relevant regulations of JB/T 2599-2012 “Lead-acid battery name, model establishment and naming method”, the model of lead-acid battery is usually expressed in three parts: the first part is Arabic numerals , Indicates the number of single batteries in series, the nominal voltage of each single battery is 2V, when the number of single batteries in series is 1, the first part can be omitted; the second part is 2-4 Chinese pinyin letters, Indicates the type, function and purpose of the battery; the third part indicates the rated capacity of the battery, expressed in Arabic numerals. The composition is shown in Figure 2.

Energy storage battery for photovoltaic power generation system
Figure 2 Model compilation and composition of lead-acid batteries

For example: 6-QAW-54a means that it is composed of 6 single cells, each cell has a voltage of 2V, that is, the rated voltage is 12V, Q indicates the battery for starting, A and W indicate the type of battery, and A indicates dry charge Electric battery, w means maintenance-free battery, if not marked, it means ordinary battery. 54 indicates the rated capacity battery of the battery, which is discharged at a rate of 20h at room temperature, and the external output power of the 20h battery is 54A)

(8) Depth of Discharge The depth of discharge (Depth of Discharge, DOD) refers to the percentage of the battery’s rated capacity that the battery discharges during its use. The discharge in which the battery is basically discharged but has not been damaged to the battery is called deep discharge. It is used to call deep discharge after 70% of the battery is discharged, that is, 70% DOD, and shallow discharge within 30% of the discharge, that is 30%DOD

(9) The series-parallel voltage of the battery is connected in series with the capacity of two batteries with the same parameter. The total voltage is twice the voltage of a single battery, and the capacity of the battery is equal to the capacity of a single battery; if two batteries with the same parameter are connected in parallel, the total voltage is For the voltage of a single battery, the capacity of the battery is equal to twice the capacity of a single battery.

3. Gel battery
(1) The working principle of the colloidal battery. The colloidal battery is the abbreviation of the colloidal lead-acid rose battery. It replaces the liquid sulfuric acid electrolyte in the lead-acid battery with the colloidal sulfuric acid electrolyte. Its working principle is similar to that of the lead-acid battery. A variety of surfactants are added to the colloid to help prevent the plate from being vulcanized, reduce the corrosion of the separator, and improve the reaction utilization rate of the plate’s active materials.

(2) Characteristics of colloidal batteries
1) Sealed structure, no leakage, no acid mist during charging and discharging, no pollution, safety and environmental friendliness.
2) The self-discharge electrode is small.
3) Long service life, its normal service life can reach 10-15 years.
4) The performance of deep discharge cycle is good, and it can still be used normally after discharge to OV.

4. Factors affecting battery capacity
The larger the capacity of the battery, the more electric energy stored. The size of the battery capacity is related to the discharge rate, ambient temperature, electrolyte density and plate structure.

(1) The impact of discharge rate on capacity The design of stand-alone photovoltaic systems should consider choosing a suitable discharge rate for the battery. The discharge rate of ordinary batteries is generally 20h, while the discharge rate of photovoltaic batteries is generally 100~200h. The discharge rate of the battery follows the relationship curve between the discharge rate and the capacity shown in Figure 3. Taking 10 hours as an example, the battery capacity will reach about 110%, which is nearly 10% more than the nominal capacity; and for high current discharge, if you choose When the battery is discharged at a rate of 2 hours, the battery capacity will shrink from the rated capacity, which is only about 75% of the nominal capacity. Therefore, in actual projects, if the design capacity of the battery is determined and the load discharge current is large, the actual capacity of the battery will be smaller than the designed capacity, which will cause insufficient power supply for the system; conversely, if the system load current is small, the actual capacity of the battery will be Compared with the design, the cost is increased.

Energy storage battery for photovoltaic power generation system
Figure 3 The relationship between battery discharge time rate and capacity

(2) The influence of ambient temperature on battery capacity. The rated capacity of the battery is generally calibrated at an ambient temperature of 25°C. The capacity of the battery will change with changes in temperature. As shown in Figure 4, when the temperature drops to 0°C, the battery capacity The capacity drops to 90%-95% of the rated capacity, and drops to 80%~90% of the nominal capacity at ﹣10℃, and drops to only about 25% of the nominal capacity at ﹣40℃. When the temperature exceeds 25℃ , The battery capacity will increase slightly. Therefore, the influence of temperature must be considered when calculating the battery capacity. When the temperature is too low, it can be buried in the ground, moved into the room or switched to a colloidal lead-acid battery.

Energy storage battery for photovoltaic power generation system
Figure 4 The relationship between battery capacity and temperature

5. Battery selection
The selection and design of batteries should generally consider the following aspects:
1) The nominal voltage of the battery: the nominal voltage of the battery should match the rated output voltage of the controller.
2) The capacity of the battery: the capacity of the battery should be greater than the capacity required by the load. For example: the calculated current of the load is 25A, and the continuous power supply time is not less than 40h, then the minimum capacity of the battery is 25A x 40h = 1000A.h
3) The type of battery: First, check whether there is an impact load in the load. For example, if there is a direct-starting motor with a larger capacity in the load, a starting battery should be selected, otherwise a fixed battery should be selected. Of course, the battery of the corresponding model should be selected for special places. As for whether to choose lead-acid batteries, gel batteries, or lithium batteries, it must be considered from the perspective of cost performance, environmental adaptability, and compliance with local environmental protection requirements.