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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.

Charging Lead Acid Batteries

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.

Discharging Lead-Acid Batteries

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.

Types of Lead-Acid Battery

  • Starting Batteries – Used to start and run engines they can deliver a very large current so a very short time, discharging by about 2-5%. If deep cycled these batteries quickly degenerate and will fail after 30-150 cycles but should last for a very long time when used correctly.
  • Deep Cycle Batteries – Used to store electricity in autonomous power systems (e.g. solar, mini-hydro), for emergency back-up and electric vehicles. These batteries are designed to discharge by as much as 80% of their capacity over thousands of charging and discharging cycles. True deep cycle batteries have solid lead plates however many batteries that do not have solid plates are called semi-deep cycle.
  • Marine Batteries – Usually a hybrid battery that falls between deep cycle and starting batteries although some are true deep cycle batteries. hybrid batteries should not be discharged by over 50%.

Types of Deep Cycle Battery

  • Flooded – These batteries have a conventional liquid electrolyte. Standard types have removable caps so that the electrolyte can be diluted and the specific gravity measured, such batteries are supplied dry and you add distilled water. Standard flooded batteries are cheap and if they are kept topped up they are not overly sensitive to high charging voltages. Sealed batteries are supplied pre-flooded and have fixed valves to allow gases to vent during use however, they will still leak if inverted and the electrolyte can not be replenished so that over charging will cause damage.
  • Gelled Electrolyte – The electrolyte is a jelly and so will not leak. The electrolyte can not be diluted so that over charging must be avoided and these batteries may only last for 2 or 3 years in hot climates although with good care they can last for 5 years.
  • Absorbed Glass Mat (AGM) Batteries – The electrolyte is held between the plates absorbed in a fine boron-silicate mat. Like gelled electrolyte batteries they will not leak acid but they can withstand more careless treatment and are less sensitive to over charging since they are designed to retain vented gases. AGM batteries can also stand for 30 days in a totally discharged state and still be recharged successfully. The major drawback to these batteries is that they cost between 2 or 3 times as much as flooded batteries.