Rechargeable battery
QUESTION: How does a rechargeable battery work? What is the life of such batteries and how are they different from ordinary batteries?
S. Abdul Althaf, Tirupati
ANSWER: 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.
S. Vijayanand
Head- Research and Engineering
Amara Raja Batteries Ltd
Tirupati
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