Lead-acid batteries

Lead-acid battery (VRLA) is a storage battery whose electrodes are mainly made of lead and its oxides, and the electrolyte is sulfuric acid solution. In the discharge state of a lead-acid battery, the main component of the positive electrode is lead dioxide and the main component of the negative electrode is lead; in the charged state, the main component of the positive and negative electrodes are lead sulfate. Divided into exhaust-type batteries and maintenance-free lead-acid batteries.

1. What is a lead-acid battery

The most obvious feature of lead-acid batteries is the unscrewed plastic sealing cover on the top and the vent hole on the top. These filling caps are used for filling pure water, checking electrolyte and exhaust gas. Theoretically speaking, lead-acid batteries need to check the density and liquid level of the electrolyte during each maintenance, and add distilled water if there is a shortage. However, with the upgrading of battery manufacturing technology, lead-acid batteries have developed into lead-acid maintenance-free batteries and colloidal maintenance-free batteries, and there is no need to add electrolyte or distilled water in the use of lead-acid batteries. The main purpose is to use the positive electrode to generate oxygen, which can be absorbed in the negative electrode to reach the oxygen cycle, which can prevent the reduction of moisture. Lead-acid water batteries are mostly used in tractors, tricycles, car starting, etc., while maintenance-free lead-acid batteries have a wider range of applications, including uninterruptible power supplies, electric vehicle power, electric bicycle batteries, etc. Lead-acid batteries are divided into constant current discharge (such as uninterruptible power supply) and instantaneous discharge (such as car starter batteries) according to application needs.

The battery is mainly composed of tubular positive plate, negative plate, electrolyte, separator, battery tank, battery cover, pole, injection cover, etc. The electrodes of the vented battery are composed of lead and lead oxides, and the electrolyte is an aqueous solution of sulfuric acid. The main advantages are stable voltage and low price; the disadvantages are low specific energy (that is, the electric energy stored per kilogram of battery), short service life and frequent daily maintenance. Old-fashioned ordinary batteries generally have a lifespan of about 2 years, and it is necessary to regularly check the height of the electrolyte and add distilled water. However, with the development of technology, the life of lead-acid batteries has become longer and maintenance has become easier.

2. The working principle of lead-acid batteries

The anode (PbO₂) and cathode (Pb) in the lead battery are immersed in the electrolyte (dilute sulfuric acid), and 2V electricity is generated between the two electrodes. This is based on the principle of the lead battery. After charging and discharging, the cathode, anode and electrolyte will be The following changes have occurred:

(Anode) (Electrolyte) (Cathode) PbO₂+2H₂SO₄+Pb=PbSO₄+2H₂O+PbSO₄ (discharge reaction). (Lead dioxide) (Sulfuric acid) (Sponge lead) The valence of Pb in PbO₂ decreases and is reduced, and negative charges flow; the valence of Pb in spongy lead increases, and positive charges flow.

(Anode) (Electrolyte) (Cathode) PbSO₄+2H₂O+PbSO₄=PbO₂+2H₂SO₄+Pb (charge reaction) (must be energized) (lead sulfate) (water) (lead sulfate).

The valence of lead in the first lead sulfate increases and is oxidized, and the positive charge flows into the positive electrode; the valence of lead in the second lead sulfate decreases and is reduced, and the negative charge flows into the negative electrode.

1. Chemical changes in discharge: When the battery is connected to an external Circuit for discharge, the dilute sulfuric acid will react with the active materials on the anode and cathode plates to form a new compound, lead sulfate. The sulfuric acid component is released from the electrolyte through the discharge, and the longer the discharge, the thinner the concentration of sulfuric acid. The consumed component is proportional to the discharge amount. As long as the concentration of sulfuric acid in the electrolyte is measured, that is, the specific gravity is measured, the discharge amount or residual electricity can be known.

2. Chemical changes during charging: Because the lead sulfate produced on the anode and cathode plates during discharge will be decomposed and reduced to sulfuric acid, lead and lead dioxide during charging, the concentration of electrolyte in the battery gradually increases. That is, the specific gravity of the electrolyte rises and gradually returns to the concentration before the discharge.

This change shows that the active material in the battery has been converted to a state where it can be powered again. When the lead sulfate at the two poles is converted into the original active material, it is equal to the end of charging, while the cathode plate produces hydrogen and the anode plate produces oxygen. , When charging to the final stage, the current is almost used in the electrolysis of water, so the electrolyte will be reduced. At this time, it should be supplemented with pure water.

3. Lead-acid battery maintenance method

1. The ambient temperature has a greater impact on the battery. If the ambient temperature is too high, the battery will be overcharged to produce gas. If the ambient temperature is too low, the battery will be undercharged, which will affect the service life of the battery. Therefore, the ambient temperature is generally required to be around 25°C, and the UPS float voltage value is also set according to this temperature. In practical applications, the battery is generally charged within the range of 5°C to 35°C. Lower than 5°C or higher than 35°C will greatly reduce the capacity of the battery and shorten the service life of the battery.

2. The depth of discharge has a great influence on the battery life. The deeper the discharge depth of the battery, the fewer the number of cycles it uses, so deep discharge should be avoided during use. Although UPS has a battery low-potential protection function, generally when a single battery is discharged to about 10.5V, the UPS will automatically shut down. However, if the UPS is under light-load discharge or no-load discharge, it will also cause deep discharge of the battery.

3. During storage, transportation, and installation, the battery will lose part of its capacity due to self-discharge. Therefore, before putting into use after installation, the remaining capacity of the battery should be judged according to the open circuit voltage of the battery, and then different methods should be used to recharge the battery. The storage battery that has been left on standby shall be recharged every 3 months. You can judge the quality of the battery by measuring the open circuit voltage of the battery. Take a 12V battery as an example. If the open circuit voltage is higher than 12.5V, it means that the battery has more than 80% energy storage. If the open circuit voltage is lower than 12.5V, it should be charged immediately. If the open circuit voltage is lower than 12V, it means that the battery stores less than 20% of the electric energy and the battery is unusable.

4. Charging voltage. Because the UPS battery is a backup mode of operation, the mains is in a charging state under normal conditions and will only be discharged when the power fails. In order to prolong the service life of the battery, UPS chargers are generally controlled by constant voltage and current limiting. After the battery is fully charged, it will switch to a floating charge state, and the floating charge voltage of each cell is set to about 13.6V. If the charging voltage is too high, the battery will be overcharged, and vice versa, the battery will be undercharged. The abnormal charging voltage may be caused by a wrong battery configuration or caused by a malfunction of the charger. Therefore, when installing the battery, you must pay attention to the correctness of the battery specifications and quantity, and do not mix batteries of different specifications and different batch numbers. Do not use inferior chargers for external chargers, and consider heat dissipation issues when installing. At present, in order to further improve battery life, advanced UPS adopts an ABM (Advanced Battery Management) three-stage intelligent battery management solution, that is, charging is divided into three stages: initial charging, floating charging and rest: the first stage is constant current balance Charge, charge the battery capacity to 90%; the second stage is floating charge, charge the battery capacity to 100%, and then stop charging; the third stage is natural discharge, in this stage, the battery uses its own leakage current to discharge, Until the specified voltage lower limit, and then repeat the above three stages. This method changes the previous full charge and still keeps the battery in a floating charge state for 24 hours a day, thus prolonging the life of the battery.