When a lead-acid battery is in fully charged condition, the colour of its positive plate is
A
dark grey
B
brown
C
dark brown
D
none of the above

Question

When a lead-acid battery is in fully charged condition, the colour of its positive plate is

Examoogle · Accepted Answer

dark brown

Explanation: In a fully charged lead-acid battery, the positive plates consist of lead peroxide ($PbO_2$), which is characterized by a distinct dark brown color. The negative plates, conversely, consist of spongy lead ($Pb$), which appears slate grey.

Working Principle: During the charging process, the electrical energy converts the lead sulfate ($PbSO_4$) on the positive plates into lead peroxide ($PbO_2$). As the concentration of $PbO_2$ increases and the conversion nears completion, the plate transitions to a rich dark brown color. This visual change serves as a chemical indicator of the state of charge in traditional lead-acid batteries.

Key Formulas: $PbSO_4 + 2H_2O + SO_4^{2-} \rightarrow PbO_2 + 2H_2SO_4$ — Reaction at the positive plate during charge

Key Points: Positive plate active material: Lead peroxide ($PbO_2$); Negative plate active material: Spongy lead ($Pb$); Discharged positive plate color: Dull brown or white due to lead sulfate ($PbSO_4$) formation; Fully charged positive plate color: Dark brown

Advantages: Visual inspection of battery health is possible; High reliability in chemical composition

Disadvantages: Requires access to plates (not possible in sealed VRLA batteries); Subjective observation depending on lighting

Applications: Automotive starter batteries; Uninterruptible Power Supplies (UPS); Off-grid solar energy storage

Additional Info: Dark grey (Option A) is often confused with the negative plate, which appears slate grey.. The brown color is a hallmark of the high oxidation state of lead in $PbO_2$.

Answer: C is correct — The positive plate of a lead-acid battery is composed of lead peroxide ($PbO_2$), which exhibits a dark brown color when fully charged.

Exam Tip: Always remember the positive plate is $PbO_2$ (dark brown) and negative is $Pb$ (grey); in a discharged state, both tend to turn into white lead sulfate ($PbSO_4$).

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