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Lead-Acid Batteries Tutorial

Key Concepts

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A Lead-Acid Galvanic Cell Producing Electricity (discharging)

Discharge of a lead-acid cell:

Below is a diagram of a lead-acid cell during discharge:

anode (-)
cathode (+)
e-
electrolyte

At the anode during discharge:

At the cathode during discharge:

Wen can use the oxidation reaction at the anode and the reduction reaction that occurs at the cathode to write an overall redox equation for the lead-acid cell:

anode
reaction:
Pb(s) + SO42-(aq) PbSO4(s)+ 2e-
cathode
reaction:
PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- PbSO4(s) + 2H2O(l)  
 
 
overall
reaction:
Pb(s) + PbO2(s) + 4H+(aq) + 2SO42-(aq) 2PbSO4(s) + 2H2O(l) E ≈ +2 V

As the lead-acid cell discharges:

Connecting lead-acid galvanic cells in a series to make lead-acid batteries allows us to make batteries with voltages greater than 2 V:

number of cells in series 2 3 4 5 6
approximate voltage produced 2 × 2 = 4 V 3 × 2 = 6 V 4 × 2 = 8 V 5 × 2 = 10 V 6 × 2 = 12 V

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Recharging a Lead-Acid Galvanic Cell

Recharging a lead-acid cell:

The diagram below shows a lead-acid cell during recharge:

cathode (-)
anode (+)
← e-
electrolyte

At the cathode during recharge:

At the anode during recharge:

Using the half-equations for the reactions occuring at the anode and cathode we can write an overall redox reaction for the lead-acid cell during recharge:

cathode
reaction:
PbSO4(s)+ 2e- Pb(s) + SO42-(aq)  
anode
reaction:
PbSO4(s) + 2H2O(l) PbO2(s) + 4H+(aq) + SO42-(aq) + 2e-  
 
 
overall
reaction:
2PbSO4(s) + 2H2O(l) Pb(s) + PbO2(s) + 4H+(aq) + 2SO42-(aq) E ≈ -2 V

As the lead-acid cell recharges:

Recharging a lead-acid battery requires a minimum of 2 V per cell:

number of cells in series 2 3 4 5 6
recharge voltage required > 4 V > 6 V > 8 V > 10 V > 12 V

Overcharging a battery electrolyzes water, producing hydrogen gas and oxygen gas.

The bubbles of gas degrade the surfaces of the electrodes causing the PbSO4(s) to fall off the electrodes.

This reduces the capacity of the cell.

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