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Storage battery operation

Most batteries that are used to make up a storage battery bank are a form of lead-acid battery. A lead-acid battery comprises a number of cells, each of which has two sets of sheets of lead, called plates, which are immersed in diluted sulphuric acid, which is called electrolyte. The electricity is taken from connections to the two sets of plates, called terminals.

Discharging the cell

When the cell is fully charged the set of plates that is connected to its positive terminal is coated with lead dioxide (a compound that comprises lead and oxygen.) As the cell is discharged, oxygen from the lead dioxide on the plate is exchanged for sulphur from the sulphuric acid in the electrolyte. The sulphur forms a coating of lead sulphate on the positive plates that progressively replaces the coating of lead dioxide. 

The oxygen released into the electrolyte by this process combines with hydrogen that is left over from the break-down of the sulphuric acid, and forms water. This water further dilutes the electrolyte.

Sulphur from the sulphuric acid also attaches to the negative plates, forming a coat of lead sulphate.

When all of the lead oxide on the positive plate has been replaced with lead sulphate the cell is completely discharged.

Charging the cell

When the cell is recharged the process is reversed. The lead sulphate coating on the positive plate is progressively replaced with lead dioxide. The lead sulphate on the negative plates reverts to lead, and the strength of the sulphuric acid in the electrolyte increases to its original level.

Heavily discharging the cell

The lead sulphate on the plates of a discharged battery grows into crystals. The larger the crystals are, the more stable they become, and the more difficult it becomes to engage them in the necessary chemical reactions of the charging process. For this reason it is best to keep lead-acid batteries as charged as possible, and if they are heavily discharged to recharge them as soon as possible so that crystals don’t start to form. 

If the cell is left discharged for too long the crystals will become too big and stable, and it will not be possible to break them down again. This reduces the capacity of the cell, and permanently damages it. This effect is called sulphation.

Overcharging the cell

If the cell is charged beyond fully-charged (overcharged) the plates will no longer be able to take up the sulphur from the electrolyte and the charging energy will go into breaking down the water of the electrolyte into hydrogen and oxygen gasses. If the cell is not a sealed type the gasses can escape into the atmosphere, creating an explosion hazard. The water lost from the electrolyte in this way must be replaced before the level drops sufficiently to expose the plates.

If the overcharged cell is a sealed type the gasses will pressurise the casing. When the overcharging stops catalysts built into to the cell slowly recombine the oxygen and hydrogen gasses into water, replenishing the electrolyte, and releasing heat in the process. If the overcharging doesn’t stop, the pressure may distort and damage the cell casing, or the gasses may be released through a pressure relief valve into the atmosphere, and be lost to the electrolyte.

The charging process

The entire charging process is managed by the controller in the charging equipment, which may be a generator set, solar panels or a battery charger. Good quality charging equipment will ensure that the storage battery bank is not damaged during the charging process, and that the entire charging process can occur properly without any attention from you.

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