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Lead Acid Battery

Lead acid battery is a secondary battery. The chemical reactions that take place in secondary cells are reversible. The reaction uses an ingredient called the reactant when the cell delivers current, which can be reproduced by passing current through the cell in the opposite direction.

The chemical process of obtaining current from a secondary battery, that is, the forward reaction, is called discharge. The process of reproducing active materials is called charging. The most common secondary cells are lead acid accumulator and alkali accumulator.

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Sealed Lead Acid Battery

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The sealed lead acid battery has six cells mounted side by side in a single case. The cells are coupled together so that each 2.0V cell adds up to the 12.0 V overall battery capacity. Lead-acid batteries are very heavy, but still much better than the light weight options, since lead acid batteries can deliver the large surges of electricity needed to start a cold automobile engine.

A fully charged lead-acid cell is made up of a stack of alternating lead, Pb, and lead (IV) oxide, PbO2, plated, isolated from each other by layers of porous separators. All these parts sit in a concentrated solution of sulfuric acid. Inter cell connectors link the positive end of one cell to the negative end of the next cell, so the six cells are in series.

When the cell discharges, it acts as a galvanic cell. The following reactions occur:
Negative: Pb(s) + HSO4 - + H2O(l) $\to$ 2e- + PbSO4(s) + H3O+(aq)
Positive: PbO2(s) + HSO4 - (aq)+ 3H3O+(aq) + 2e- $\to$ PbSO4(s) +5 H2O(l)

Lead sulfate is produced at both the electrodes. Also, two electrons are transferred in the overall reaction. The lead acid battery is housed in a thick plastic or rubber case. This prevents leakage of the corrosive sulfuric acid.

The reactions both go to the right when current is being drawn from the battery. That is, lead (II) sulfate, PbSO4 is made at both the negative and positive plates. However, this happens by two different processes and for each mole of lead sulfate produced, two moles of electrons travel through the external circuit. These are the electrons, that, say for example- make the lights, indicators, and radio in the car work.

When a car or lorry moves, an electric current is generated by the alternator. This current is passed into the battery in the direction that forces both the reactions to go to the left. That is, the lead sulfate is decomposed. In theory, at least, the charging and discharging can go on indefinitely. However, in practice, this is not so.

A typical car battery provides a voltage of 12V. This is not a large voltage, but the battery can provide a large current, say 10A, without being destroyed.

Lead Acid Battery Charging

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The sulfuric acid present in the lead discharge battery decomposes and has to be replaced. Also, sometimes, the plates themselves change their structure. Eventually, the battery becomes much less efficient and has to be changed or charged.

One of the worst things that can happen to a car battery is for it to spend most of its time in a discharged state. This causes so much lead sulfate to build up that it is almost impossible to remove.Thus, the lead acid batteries should be charged as soon as possible to avoid the building up of lead sulfate. Charging of the lead acid batteries are usually done, by providing an external current source.

Usually, a plug is inserted which in turn is connected to the lead acid battery. A completely opposite reaction begins to take place.Sometimes, charging cannot be done efficiently, if the sulfuric acid or one or the other substances have decomposed already. So, we need to check them from time to time, to guarantee the efficiency of the battery.

Lead Acid Battery Recharging

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Lead acid batteries are made from a combination of secondary cells, the cells that can be recharged. The lead acid battery recharging is a reversible process, to that of its normal process. The whole reaction, by which the electric current is produced by a lead acid battery is reversed in a recharge process.

During the recharging cycle, the automobile battery functions like an electrolytic cell. The energy to drive the recharging of the cell comes from an external source, such as an alternator of a car engine. It is also necessary to make sure that overcharging of the battery does not take place. If it happens, the byproducts formed, like oxygen and hydrogen gas will escape, and thereby, it will be lost to the battery.

The reactions involved in recharging the battery are:
Negative: 2e- + PbSO4(s) + H3O+(aq) Pb(s) + HSO4 - + H2O(l)
PbSO4(s) +5 H2O(l) PbO2(s) + HSO4 - (aq)+ 3H3O+(aq) + 2e-
Both the negative and positive reactions are complete opposite of what happens normally in a lead acid battery.

Lead Acid Accumulator and Electrolyte

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The lead acid accumulator consists of a container made of hard rubber or glass or celluloid. The container contains dilute sulfuric acid which acts as the electrolyte.

Spongy lead (Pb) variety starts acting as the negative electrode and the lead oxide (PbO2) will act as the positive electrode. These electrodes are separated by insulating them and are assembled in such a way so that it provides a low internal resistance. After the cell is connected to a given circuit, the oxidation reaction takes place at the negative electrode, and the spongy lead variety reacting with dilute sulfuric acid will start producing the lead sulphate product along with two electrons.

These electrons start flowing through the external circuit from the negative electrode to the positive electrode.
The reduction process takes place at the positive electrode which results in the production of lead sulfate due to the lead oxide reacting with sulfuric acid (the electrolyte) along with the two electrons which are also neutralized in this process. 

The conventional current now flows from positive electrode to the negative electrode through the external circuit. The cell having low internal resistance starts delivering high current. The cell is then discharged by gradual drawing of current from it, and the overall emf falls.

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