Principle and Characteristics of Lead-Acid Batteries
(1) Charge-Discharge Reactions
The charge-discharge reactions of a lead-acid battery are expressed by the following reaction equations.
- At the positive electrode:
PbO₂ + 4H⁺ + SO₄²⁻ + 2e⁻ ⇄ PbSO₄ + 2H₂O
——— ① - At the negative electrode:
Pb + SO₄²⁻ ⇄ PbSO₄ + 2e⁻
——— ② - For the entire battery system:
PbO2＋2H2SO4＋Pb←→2PbSO4＋2H2Ｏ
―――③
is the overall reaction.
These reactions indicate that, during discharge, equations ①, ②, and ③ proceed from left to right, whereas during charging, they proceed from right to left.
However, as charging approaches its final stage, the reactions represented by equations ④ and ⑤ also occur simultaneously.
In other words, the electrolyte is reduced due to the electrolysis of water.
- At the positive electrode:
H2O→1／2O2＋2H＋＋2e－
―――④ - At the negative electrode:
2H＋＋2e－→H2
―――⑤
Therefore, conventional flooded batteries required periodic water replenishment.
In contrast, valve-regulated stationary lead-acid batteries are equipped with a negative electrode absorption reaction function, so under normal operating conditions the electrolyte does not decrease and water replenishment is not required.
(2) Negative Electrode Absorption Reaction Function
The negative electrode absorption reaction function of valve-regulated stationary lead-acid batteries absorbs oxygen gas generated at the positive electrode during charging at the negative electrode.
As shown in equation ⑦, oxygen gas (O₂) generated at the positive electrode in the final stage of charging reacts with the spongy lead (Pb) which is the negative active material to produce lead sulfate (PbSO₄) and water (H₂O).
In other words, the water lost due to oxygen gas generation at the positive electrode is regenerated.
In addition, because the formation of lead sulfate places the negative electrode in a discharged state, the electrical energy supplied during charging is used to reduce this lead sulfate.
As a result, the amount of hydrogen gas generated at the negative electrode is smaller than that in flooded batteries.
These relationships are shown in equations ⑥ and ⑦ below.
- At the positive electrode:
H2O→1/2O2＋2H＋＋2e－
―――⑥ - At the negative electrode:
Pb＋1/2O2＋SO42-＋2H＋→PbSO4＋H2O
―――⑦ - PbSO4＋2e－→Pb＋SO42－