Photovoltaic energy storage batteries, due to long-term use, the positive electrode plate will gradually corrode and grow under the action of the electrolyte, and the growth of the grid will reduce the binding between the active material and the grid, resulting in gradual loss of battery power. The corrosion and growth of the positive electrode grid are mainly affected by the alloy composition of the grid, electrolyte density, and the shape of the grid ribs.

(1) The positive electrode active material softens and falls off

The main cause of battery failure in VRLA batteries during cyclic use is the softening and detachment of the positive electrode active material (PAM).

During the cycling process of lead-acid batteries, the active materials of the positive and negative electrodes undergo reversible dissolution and re deposition processes, thereby changing the structure of the porous lead dioxide electrode. Especially for lead dioxide electrodes, it may lead to an increase in apparent volume, alter particle and pore size distribution, and reduce the mechanical bonding and conductivity between particles in porous lead dioxide structures. As the cycle continues, this situation will become worse, leading to softening and detachment of active substances in the area. 

(2) The impact of discharge current on battery life

In photovoltaic systems, the discharge current of the battery is very small. The conversion of PbSO4 formed under low current conditions is much more difficult than that formed under high current conditions. This is because the PbSO4 crystal particles formed under low current conditions are larger than those formed under high current conditions. Coarse PbSO4 crystal particles will reduce the effective area of PbSO4, thereby accelerating the polarization of the plate during the charging process. As a result, the conversion of PbSO4 is difficult. As the cycle continues, this situation will become even more severe. As a result, the electrode cannot be charged and the battery life is over.

(3) Recovery of battery capacity after deep discharge

In photovoltaic systems, the discharge rate of batteries is lower than that of batteries in other applications, usually between C20 and C240, or even lower. Deep discharge at low current means that the active materials on the board will be more fully utilized. In many photovoltaic systems, deep discharge usually does not occur unless the charging system malfunctions or severe weather persists for a long time. In this case, if the battery cannot be charged in a timely manner, the sulfurization problem will become more severe and further lead to capacity loss. 

(4) The impact of acid stratification on battery life

The phenomenon of electrolyte stratification is caused by the gravity of the battery during charging and discharging, which means that H2SO4 is produced on the surfaces of the positive and negative plates during the charging process. It has a high density and sinks due to gravity. During the discharge process, H2SO4 is consumed on the surface of the positive and negative plates, resulting in a lower density of the surface liquid layer. The low-density electrolyte rises between the plates, while the high-density electrolyte is located in the upper part of the electrode group and descends from the side of the electrode group. As a result of liquid flow, the upper part has low density, while the lower part has high density. The generation of delamination has adverse effects on the service life and capacity of the battery, accelerating the corrosion of the gate and the detachment of the positive electrode active material, and leading to the sulfation of the negative electrode plate.

(5) The influence of electro-hydraulic density on the lifespan of lead-acid batteries

The concentration of electrolyte is not only related to the capacity of the battery, but also to the corrosion of the positive electrode grid and the sulfation of the negative electrode active material. Excessive sulfuric acid concentration accelerates the corrosion of the positive electrode grid and the sulfation of the negative electrode active material, leading to increased water loss.

(6) The influence of grid alloy

VRLA batteries, due to long-term use, the positive electrode plate will gradually corrode and grow under the action of the electrolyte, and the growth of the plate mesh will reduce the binding force between the active material and the plate, resulting in the gradual loss of the positive electrode material. Battery capacity. The corrosion and growth of the positive electrode grid are mainly affected by the alloy composition of the grid, electrolyte density, and the shape of the grid ribs.

During the charging process of the battery, a non-conductive layer is formed at the interface between the gate and the active material. These non-conductive or low conductivity layers cause high impedance at the interface between the gate and PAM, resulting in heat generation during charging and discharging and PAM generation near the gate. Expansion, thereby (9) the influence of temperature

High temperature can accelerate the loss of moisture and drying of the battery, thermal runaway, corrosion and deformation of the positive electrode grid, low temperature can cause negative electrode failure, and temperature fluctuations can accelerate dendrite short circuits, thereby affecting the battery life. When discharging within a certain temperature range, the usage capacity increases with the increase of temperature and decreases with the decrease of temperature. The capacity of lead-acid batteries increases with temperature within the range of 10-45 ℃. For example, the discharge capacity of a valve regulated sealed lead-acid battery at 40 ℃ is approximately 10% higher than that at 25 ℃, but exceeds a certain level. The temperature range is opposite. If the battery is discharged at an ambient temperature of 45-50 ° C, the battery capacity will be greatly reduced. At low temperatures (<5 ° C), the battery capacity will decrease as the temperature decreases. When the temperature of the electrolyte decreases, its viscosity increases, the movement of ions is greatly hindered, and its diffusion ability decreases; At low temperatures, the resistance of the electrolyte also increases. The increase in electrochemical reaction resistance leads to a decrease in battery capacity. Secondly, low temperature will also lead to a decrease in the utilization rate of the negative electrode active material, which will affect the battery capacity. For example, when the battery is discharged at an ambient temperature of -10 ° C, the negative electrode capacity will only reach 35% of the rated capacity.

Normally, when used at 25 ℃, the lifespan of the battery is 3 years, but when used at 30 ℃, it drops to 2.5 years; At 40 ℃, it drops to 1.5 years. Based on 25 ℃, for every 10 ℃ increase, its service life is reduced by half

4 Design Practice of Energy Storage VRLA Battery for Photovoltaic System

Based on the working conditions of the photovoltaic system battery and the special requirements for the performance of the photovoltaic system battery, combined with the factors that affect the battery life mentioned above, a series of research and technical improvements were carried out on the original VRLA battery, and a photovoltaic system specific VRLA battery was designed and developed. Specific improvement measures include the following aspects:

(1) Grid alloy: Suitable for cyclic use of lead antimony or lead cadmium grid alloys are used, which can prevent corrosion growth of the plates during use and also serve as a barrier layer at the interface between the grid and the active material, eliminating early capacity decay. Its charging efficiency and recovery performance after deep discharge are both ideal. Due to cadmium being a toxic element, its use is now restricted. However, due to the strict water loss of lead antimony alloy batteries (7), the ratio of positive and negative active substances has been adjusted to improve the cycle life of energy storage VRLA batteries used in photovoltaic systems based on their charging and discharging characteristics.

(8) Safety valve: The impact of high-altitude climate above 2500m on the safety valve has also been considered, and the pressure of the opening and closing valve has been specially adjusted, using a dedicated safety valve.

(9) Battery structure: reduces the overall height of the battery. Using a short structure for production can greatly reduce the adverse effects on the service life and capacity of the battery caused by the phenomenon of electro-hydraulic stratification. However, due to the fact that colloidal batteries are less prone to electrolyte stratification on-site, there is no such limitation.

(10) The consistency of each individual cell in the battery: The consistency mentioned here not only refers to the open circuit voltage and initial capacity of the battery, but also includes the internal resistance, self discharge, and charging efficiency of the battery. This requires sufficient manufacturing accuracy, that is, from lead powder, casting, paste, smear, curing, formation, drying assembly, acid addition, charging to the final four functional tests must be controlled within a small tolerance range. Therefore, the use of machine casting, machine coating, assembly and precise acid injection is a reliable guarantee to ensure the consistency of the battery, minimizing human factors.

Summary

Due to the low conversion efficiency and high cost of photovoltaic power generation systems, as well as the lack of corresponding laws and regulations to encourage their development, the development of photovoltaic systems has been slow. But the development of new energy is the trend and will inevitably develop rapidly. At present, energy storage batteries mainly include cadmium nickel batteries and lead-acid batteries, among which cadmium nickel batteries are gradually being phased out. Lead acid batteries, including both rich and poor liquid types, are expected to be widely used in photovoltaic power generation systems in recent years.