English .   Español  .

The application of Appropriate Technology

# Part 1: How Lead-Acid Batteries Work

### Structure and Operation

Most lead-acid batteries are constructed with the positive electrode (the anode) made from a lead-antimony alloy with lead (IV) oxide pressed into it, although batteries designed for maximum life use a lead-calcium alloy. The negative electrode (the cathode) is made from pure lead and both electrodes are immersed in sulphuric acid.

When the battery is discharged water is produced, diluting the acid and reducing its specific gravity. On charging sulphuric acid is produced and the specific gravity of the electrolyte increases. The specific gravity can be measured using a hydrometer and will have a value of about 1.250 for a charged cell and 1.17 for a discharged cell, although these values will vary depending on the make of battery. The specific gravity also depends on the battery temperature and the above values or for a battery at 15°C.

Specific gravity is defined as:

$\text{Specific Gravity}=\dfrac{\text{mass of a specific volume of electrolyte}}{\text{mass of the same volume of pure water}}$

The chemical reactions that occur during charging and discharging are summarised in figures 1 and 2.

Figure 1: Charging. Lead (IV) oxide is formed at the anode, pure lead is formed at the cathode and sulphuric acid is liberated into the electrolyte causing the specific gravity to increase.

Figure 2: Discharging. Lead sulphate is formed at both electrodes and sulphuric acid is removed from the electrolyte causing the specific gravity to reduce.

If lead-acid batteries are over discharged or left standing in the discharged state for prolonged periods hardened lead sulphate coats the electrodes and will not be removed during recharging. Such build-ups reduce the efficiency and life of batteries. Over charging can cause electrolyte to escape as gases.