It is very economical to use lead-acid batteries in independent power supply systems. They are all composed of battery cells, each with a rated voltage of 2V. There are many different battery types in this category that can be adapted to various applications. include:
(1) Traction battery. Used in forklifts, golf carts, etc., these batteries are designed for regular deep cycle and fast charging. They are not suitable for independent power supply systems because their charging/recharging efficiency is usually poor.
(2) Start the battery. Used in cars, it is usually called SLI (starting, lighting and ignition) battery. This battery is designed to provide a large current in a short cycle (start-up time), so it is not suitable for independent power supply systems that require deep-discharged batteries.
(3) Fix the battery. Used in emergency backup occasions, such as the telecommunications field, it does not require frequent deep charging, and its battery is always maintained at the full charge floating voltage. These batteries have been used in independent power supply systems, but they do not have good cycling capabilities.
(4) Deep cycle photovoltaic cells. The design of this battery enables it to perform effective charging and discharging cycles and has a long life.
A battery (usually connected in series) formed by a combination of battery cells can work at any required voltage. For example, a 12V lead-acid battery is composed of 6 battery cells connected in series.
For smaller independent power supply systems, a 12V battery can be used. For systems that require larger battery capacity, batteries are usually sold in 2V units because the weight of a single 12V battery is too heavy (some large-capacity 2V units may already exceed 100kg. Note: some manufacturers will also sell 2 or 3 The unit is sold in a container). The installer must use the cables normally provided by the manufacturer to connect them in series (or in parallel).
Figure 1 shows the structure of a typical lead-acid battery.
In a fully charged lead-acid battery, lead (Pb) forms the negative plate, and lead oxide (PbO2) forms the positive plate. A solution of sulfuric acid (H2SO4) and water constitutes the electrolyte, and the two plates are immersed in it. Pb and PbO2 are called active materials. During the discharge process, H2SO4 reacts with Pb and PbO2 respectively.
The chemical reaction formula is
Pb+PbO2+H2SO4 (discharge)⇌(charge) 2PbSO4+2H2O
After the reaction is over, the acid concentration in the electrolyte decreases, and insoluble lead sulfate (PbSO4) is deposited on the surface of the positive electrode and the negative electrode. The potential difference between the positive and negative electrodes is about 2V.
During the charging process, an external power supply applies a potential difference higher than the battery voltage. In this case, the above reaction is reversed, the battery returns to the initial state of charging, and the two plates switch back to Pb and PbO2.
During the charging process, part of the water in the electrolyte is converted into oxygen and hydrogen through the electrolysis process. The chemical reaction formula is
2H2O→2H2+O2
After that, the gas escapes through the ventilation holes on the top of the battery, and after a period of time, the electrolyte level will drop. Therefore, the battery must be monitored and filled with water regularly.
Hydrogen is highly flammable, so care must be taken to ensure adequate ventilation space above and around the battery.
Valve-regulated lead-acid batteries operate on the same principle as lead-acid batteries, but this type of battery is sealed in a leak-proof device. Valve-regulated lead-acid batteries are designed for applications such as standard immersion batteries and solar deep-cycle batteries. In a sealed battery, the movement of the electrolyte is restricted.
During normal operation, the hydrogen and oxygen generated by the plates during the charging process reconstitute water. The sealed battery needs to be closely monitored to prevent it from overcharging or water loss through the safety hole. Under normal circumstances, these batteries can be transported without worrying about acid spillage.
Valve-regulated lead-acid batteries are less harmful than immersed batteries, because unless the battery is overcharged, it generally produces little or no gas (hydrogen). If overcharged, the regulating valve will open to release oxygen and hydrogen. There is currently no mechanism available to replace the released gas.
VRLA batteries are designed for a variety of applications. It can often provide higher charging and discharging currents than immersed batteries, and can be used in unattended or inaccessible situations, such as solar street lighting.
However, valve-regulated lead-acid batteries are more expensive and use more demanding conditions. If they are overcharged, they cannot be “full”. However, in some cases, its advantages outweigh its disadvantages.
Valve-regulated lead-acid batteries are often referred to as “maintenance-free” batteries, but although they do not require supplemental water, their terminals still need to be cleaned. Therefore, the term “maintenance-free” is not accurate.
There are two different technologies for the manufacture of VRLA batteries:
(1) Absorbing glass mat (AGM). In this type of battery, the lead-calcium plates are separated by an absorbent glass pad that is immersed in the electrolyte between the two plates. The advantage is to avoid the problem of delamination in the battery. The main problem is that the active material easily flows onto the glass fiber mat after the charging cycle, which may cause a short circuit. Therefore, such batteries tend to have poor cycle life.
(2) Gel-like electrolyte. In this type of battery, the electrolyte is combined with a gelling agent (such as quartz powder) to form a thick gel that can fix the electrolyte. This type of battery has better deep cycle performance than AGM batteries.