How does a rechargeable battery work? What is the life of such batteries and how are they different from ordinary batteries?

Electrochemical cells and batteries are identified generally as primary and secondary batteries. The primary batteries cannot be easily or effectively re-charged electrically and hence are discharged (used) and discarded. The electrochemical reactions in primary cells are not easily reversible. When the battery delivers current (during use) the active materials undergo changes and the active materials slowly will become inactive because the discharged active materials can't deliver current. In secondary batteries (example., lead-acid) the reactions are said to be reversible because once the battery is used, the inactive materials can be converted back to active materials by re-charging and the battery will be again ready for use.

These systems are also called as `storage batteries'. (example., lead-acid, nickel-cadmium) In the primary category, for example., zn-carbon cells, the anode is zinc and cathode is manganese dioxide. During discharge (when battery in use), the simplified reaction can be written as (the actual electrochemical process is more complicated)

Zn + 2 MnO{-2} ZnO + Mn{-2}O{-3}

Discharge (delivers current)

The discharged products (right hand side) cannot be formed back into original active materials (left hand side) by passing current in an opposite direction (charging). It is said to be `irreversible'

Where as in secondary batteries, for example., lead-acid, the active materials can be formed back after discharge (use) and it will be ready for use again after charge.

Pb + PbO{-2} + 2H{-2}SO{-4}

Technically some primary batteries can be recharged for several cycles but may not deliver full capacity and may have poor charge retention after recharge. Generally the cells are not designed for that type of use. The life of a secondary battery (lead-acid or nickel-cadmium) may vary from 200-1200 cycles (one cycle represents one discharge and charge) depending on its design parameters.



How does a compact fluorescent lamp consume less electricity than conventional fluorescent lamps and bulbs?

Fluorescent lamps are based on the phenomenon of gas discharge between two electrodes at the ends of a glass tube. Generally these tubes contain a little mercury in the low-pressure vapour phase. When sufficiently large voltage is applied between the electrodes, some atoms of the vapour get ionised.

The process of ionisation usually starts with stray electrons and ions that are generally present in the vapour. The electron-ion pairs so formed get accelerated towards electrodes of opposite electrical polarity, gaining kinetic (speed-dependent) energy.

When they collide neutral mercury atoms, some of them are ionised and some are electronically excited. Excited (higher-energy) atoms release their energy in the form of electromagnetic radiation, part of which is in the visible and in the infrared regions of the spectrum. But it is rich in the invisible ultraviolet region.

A fluorescent light source has the inner surface of its glass tube painted with a material called phosphor. Zinc sulphide is the commonest example of a phosphor. But phosphors used in practice are complex mixtures of the sulphides and phosphates of barium, strontium and rare earth elements.

These phosphors have the property of absorbing ultraviolet component of the radiation and re-emitting a major fraction of the corresponding energy in the form of visible light. This enhances the lamp's efficiency of converting electrical energy into visible light.

The ordinary fluorescent lamp works with a supply voltage of about 220 volt. Since the start of discharge process demands a little higher voltage, it also employs a starter and ballast (a choke coil) that together produce the desired voltage. On the other hand, the compact tube works at about 400 volt (constant), which is produced by a transformer arrangement embedded in its base.

Working at a higher voltage improves its efficiency of producing electromagnetic radiation. Another factor adding to its efficiency is the phosphor composition, which produces light richer on the violet side of the spectrum. This makes the light of a compact lamp somewhat more bluish than that of the ordinary fluorescent lamp. These newer phosphors are not yet being used in ordinary fluorescent lamps perhaps for cost reasons.

Higher efficiency means low consumption of electrical energy. A filament lamp has the lowest efficiency, because it is based on the fact that a material body heated to a high temperature emits radiation of all wavelengths. This radiation is richer in the infrared part of the spectrum and since there is no mechanism of converting this into visible light, it has poor efficiency. The three types of lamps may have a typical efficiency ratio of 8:6:3.

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"Life is either a daring adventure or nothing."
Helen Keller