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When the lead-acid cell is fully charged, the electrolyte assumes ______________ appearance
dull
reddish
bright
milky
milky
Quick Summary: When a lead-acid cell is fully charged, the chemical reaction results in the formation of lead peroxide ($PbO_2$) at the positive plate and spongy lead ($Pb$) at the negative plate. The evolution of hydrogen and oxygen gases at the electrodes causes the electrolyte to bubble, giving it a characteristic milky or cloudy appearance.
When a lead-acid cell is fully charged, the chemical reaction results in the formation of lead peroxide (PbO2тАЛ) at the positive plate and spongy lead (Pb) at the negative plate. The evolution of hydrogen and oxygen gases at the electrodes causes the electrolyte to bubble, giving it a characteristic milky or cloudy appearance.
PbSO4тАЛ+2H2тАЛO+PbSO4тАЛ+EnergyтЖТPbO2тАЛ+2H2тАЛSO4тАЛ+Pb
The charging process involves the electrolysis of water and the conversion of lead sulfate (PbSO4тАЛ) back into its active materials. As the state of charge approaches 100%, the internal resistance increases, and the excess charging energy is consumed by the electrolysis of the electrolyte, releasing bubbles of H2тАЛ and O2тАЛ. These rising gas bubbles displace the electrolyte particles, creating a milky, opaque suspension.
The milky appearance is a physical indicator of gassing, signaling the completion of the charging cycle.
The specific gravity of the electrolyte increases as the cell reaches a full charge.
Overcharging after the 'milky' stage can lead to excessive water loss and electrode degradation.
High surge current capability
Low internal resistance
Heavy weight compared to lithium-ion
Requires regular maintenance (topping up distilled water)
Automotive starter batteries
UPS systems and emergency lighting
Off-grid renewable energy storage
Dull or reddish colors often indicate sulfation or plate damage.
Milky electrolyte indicates that the charging current is effectively performing electrolysis at the plates.
D is correct тАФ The milky appearance is caused by the liberation of gas bubbles during the final stages of charging.
Always remember that gassing (milky electrolyte) occurs at the end of the charging cycle, which is a key maintenance cue in battery room operations.