Question

During charging, the electrolyte of a lead acid battery becomes

• A. stronger
• B. weaker
• C. diluted with water
• D. None of these

Hint:

During the charging process of a lead-acid battery, the concentration of sulfuric acid in the electrolyte increases, making it stronger.

Answer 1

The Correct Option is A

In practice, technicians check the state of charge of a lead-acid battery using a hydrometer, which measures the electrolyte's specific gravity. A fully charged battery typically shows a specific gravity around 1.28, while a discharged battery shows a much lower value (around 1.10-1.15). This practical measurement directly tracks how "strong" (concentrated) the acid is.

1. Stronger: As the battery charges, the measured specific gravity rises toward its fully-charged value, which is the practical definition of the electrolyte becoming stronger.
2. Weaker: A falling specific gravity reading is what technicians associate with a discharging (not charging) battery.
3. Diluted with water: Dilution would show up as a falling, not rising, specific gravity reading on the hydrometer.
4. None of these: Since the hydrometer reading clearly rises during charging, this option does not apply.

The rising specific-gravity reading during charging confirms the electrolyte becomes stronger.

Therefore, the correct answer is stronger.

Answer 2

We can also reason by symmetry with the discharge process. It's well established that during discharge, the electrolyte becomes weaker because sulfuric acid is consumed and water is released as the plates convert to lead sulfate. Charging is simply the reverse of discharging — restoring the plates to their original lead and lead-dioxide form. Let's check each option against this reversal logic.

1. Stronger: Since charging reverses everything that happens in discharge, and discharge weakens the electrolyte, charging must do the opposite and strengthen it.
2. Weaker: This describes what happens during discharge, the opposite process, so it cannot also describe charging.
3. Diluted with water: Water being added is also a discharge-side effect (water is produced as the reaction proceeds during discharge); charging instead consumes water, so dilution cannot describe charging.
4. None of these: Since the reversal argument clearly points to the electrolyte becoming stronger, this option is incorrect.

The symmetry between charging and discharging confirms that charging makes the electrolyte stronger.

Therefore, the correct answer is stronger.