Valve regulated sealed lead-acid batteries (VRLA batteries) are a common type of battery with characteristics such as sealed structure and maintenance free. Their working principle is based on the electrochemical reaction of lead-acid batteries, while achieving sealing and gas recombination functions through special design. The following is its detailed working principle:
1、 Basic composition and materials
Electrode material:
Positive electrode: Lead dioxide (PbO ₂).
Negative electrode: Sponge like metallic lead (Pb).
Electrolyte: Dilute sulfuric acid (H ₂ SO ₄), usually adsorbed in glass fiber separators or colloids (known as "lean solution" or "colloid").
Diaphragm: Made of ultra-fine glass fiber (AGM) or colloidal electrolyte, used to isolate positive and negative electrodes and maintain electrolyte.
Shell: Sealed plastic shell to prevent electrolyte leakage.
2、 Principle of charge discharge chemical reaction
1. Discharge process (chemical energy → electrical energy)
Positive electrode reaction: (PbO2+3H ^++HSO4 ^ -+2e ^ - \ rightarrow PbSO4+2H2O) Lead dioxide reacts with sulfuric acid to obtain electrons, generating lead sulfate and water.
Negative electrode reaction: (Pb+HSO4 ^ - \ rightarrow PbSO4+H ^+2e ^ - \) Metal lead reacts with sulfuric acid, losing electrons and generating lead sulfate and hydrogen ions.
Total reaction: Lead compounds with positive and negative electrodes react with sulfuric acid to form lead sulfate and water, while releasing electrical energy.
2. Charging process (electrical energy → chemical energy)
Positive electrode reaction (electrolysis): \ (PbSO4+2H2O \ rightarrow PbO2+3H ^++HSO4 ^ -+2e ^ - \) Lead sulfate decomposes under the action of electrical energy, producing lead dioxide, hydrogen ions, and sulfate ions, releasing electrons.
Negative electrode reaction (electrolysis): \ (PbSO4+H ^+2e ^ - \ rightarrow Pb+HSO4 ^ - \) Lead sulfate obtains electrons and is reduced to metallic lead and sulfate ions.
Total reaction: \ (2PbSO4+2H2O \ rightarrow PbO2+Pb+2H2SO4 \) Lead sulfate and water are re converted into positive and negative active substances and sulfuric acid under charging conditions, completing the storage of electrical energy into chemical energy.
3、 Sealing and gas recombination mechanism
1. Side effects during overcharging
When the battery is overcharged, a side reaction of water splitting occurs:
Positive electrode (oxygen evolution): \ (2H2O \ rightarrow O2 \ uparrow+4H ^++4e ^ - \)
Negative electrode (hydrogen evolution): \ (4H ^+4e ^ - \ rightarrow 2H_2 \ uparrow \) In traditional lead-acid batteries, oxygen and hydrogen gas will escape, resulting in a decrease in electrolyte and requiring regular water replenishment; VRLA batteries solve this problem through sealing design and gas recombination mechanism.
2. Principle of Gas Recombination
Oxygen Recombination (Core Design):
The oxygen released from the positive electrode diffuses to the negative electrode surface through the separator.
The metal lead on the negative electrode surface reacts with oxygen to form lead oxide (PbO), which then reacts with sulfuric acid to form lead sulfate and water.
Inhibition of hydrogen evolution:
VRLA batteries increase the initial potential of hydrogen evolution reaction by controlling the negative electrode potential and electrolyte distribution, significantly reducing hydrogen evolution (the amount of hydrogen evolution during normal charging is only less than 1% of traditional batteries).
4、 Key design features
Poor liquid or colloidal electrolyte:
The electrolyte is adsorbed in the AGM membrane (lean solution type) or mixed with colloids (colloidal type) to prevent flow and provide channels for gas diffusion.
Unidirectional exhaust valve:
When the internal pressure of the battery exceeds the threshold (such as excessive gas caused by overcharging), the exhaust valve opens to release pressure, preventing the shell from expanding. After the pressure drops, it automatically closes to maintain sealing.
Tight assembly structure:
The positive and negative electrode plates are in close contact with the separator to improve reaction efficiency and promote oxygen diffusion to the negative electrode.
5、 Summary of job characteristics
Advantages:
Sealed maintenance free, no need for regular water replenishment;
Small size, light weight, suitable for portable devices;
Low self discharge rate and long lifespan (usually 3-10 years);
High safety, minimal gas release, and no acid mist pollution.
matters needing attention:
Overcharging or overdischarging can cause lead sulfate crystallization (sulfurization), affecting its lifespan;
It is recommended to control the working temperature at 20-25 ℃, as high temperatures can accelerate dehydration and aging.
Application scenarios
Valve regulated sealed lead-acid batteries are widely used in:
Communication base station, UPS power supply, emergency lighting system;
Electric vehicles, solar/wind energy storage systems;
Medical devices, security equipment, and other scenarios that require maintenance free power supply.
Through the above principles, VRLA batteries have achieved efficient energy conversion, sealed maintenance free operation, and long lifespan, becoming one of the important technologies in the field of modern energy storage.
