What Is A Lead-acid Battery?
Oct 31, 2022
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Lead acid battery is a rechargeable battery that uses lead and sulfuric acid to play a role. Lead is immersed in sulfuric acid to allow controlled chemical reactions.
This chemical reaction is the cause of battery power generation. The reaction is then reversed to charge the battery.
Materials for lead-acid batteries
The main active materials required for the construction of lead-acid batteries are:
Lead peroxide (PbO2): dark brown, hard and brittle material, forming a positive plate.
Sponge lead (Pb): Pure lead under the condition of soft sponge forms the negative plate.
Dilute sulfuric acid (H2SO4): a strong acid and good electrolyte. It is highly ionized, and most of the heat released during dilution comes from the hydration of hydrogen ions. For lead acid batteries with water: acid=3:1.
How do lead-acid batteries work?
Lead acid battery is made of lead peroxide electrode plate and sponge lead electrode plate immersed in dilute sulfuric acid. The current is externally connected between these boards. In dilute sulfuric acid, acid molecules split into positive hydrogen ions (H+) and negative sulfate ions (SO4 --). When it reaches the PbO2 plate, hydrogen ions receive electrons from there and become hydrogen atoms, attacking PbO2 again and forming PbO and H2O (water). This PbO reacts with H2SO4 to form PbSO4 and H2O (water).
SO4 - ions (anions) move to the electrode (anode) connected to the positive pole of the DC power supply, where they will give up excess electrons and become free radical SO4. This radical SO4 cannot exist alone; Therefore, it reacts with PbSO4 of anode to generate lead peroxide (PbO2) and sulfuric acid (H2SO4).
When the battery is charged
Charging is a process of reversing electrochemical reaction. It converts electrical energy from the charger into chemical energy in the battery. However, the battery does not store power. It retains the chemical energy needed to generate electricity.
If the voltage of the charger is higher than the battery, the battery charger will reverse the current. The charger generates too many electrons on the negative plate, and positive hydrogen ions are attracted to them. Hydrogen reacts with lead sulfate to generate sulfuric acid and lead. When most of the sulfate disappears, hydrogen rises from the negative plate. The oxygen in the water reacts with the lead sulfate on the positive plate to make it become lead dioxide again. When the reaction is about to be completed, oxygen bubbles rise from the positive plate. This is called bleeding.
Self discharge
A bad feature of lead-acid batteries is that they discharge by themselves, even when not in use. The general rule of thumb is one percent self discharge rate per day. The rate increases at high temperature and decreases at low temperature.
The change of electrolyte in lead-acid battery after charging and discharging
When the lead battery is discharged, the sulfuric acid in the electrolyte decreases, the water content increases, and the specific gravity of the solution decreases.
When the lead-acid battery is charged, the sulfuric acid in the electrolyte increases continuously, the water decreases gradually, and the specific gravity of the solution increases.
In fact, the change of electrolyte specific gravity determines the charging state of lead-acid battery. This is why it is important to consistently water the battery to ensure that you make the most of it.
