Why is it that most of the batteries show they can be recharged only a 1,000 times?

K. JOSEPH

Secunderabad, Andhra Pradesh

It is not true that any rechargeable battery can be recharged only a 1,000 times. Generally the number of recharge cycles is not an exact value.

It depends on many factors such as battery chemistry, charging as well as discharging processes, load borne by the battery etc. In rechargeable batteries, due to the chemical reactions, the chemical energy is converted into electrical energy.

Unlike ordinary chemical reactions, in rechargeable batteries, the chemical reactions take place in a reversible manner.

This means the chemical reaction proceeds in either direction — forward reaction and backward reaction.

In the forward direction chemical energy is converted into electrical energy and so the battery can deliver the voltage. This is the discharging process. Once all the reactant chemicals are converted into product chemicals, then the forward reaction comes to an end and so the battery voltage drops.

So, if we apply an external voltage to the battery (slightly greater than the battery voltage), then the reaction direction is reversed. This is the charging process (backward reaction). So the battery acquires voltage for further discharge. Ideally a single battery cycle is referred to as a complete forward (discharging) as well as reverse (charging) reaction.

And in almost all the rechargeable batteries these reversible reactions, cannot takes place with 100 percent efficiency. This means all the formed product chemicals cannot be completely converted into reactant chemicals in each cycle.

And it depends on many aspects like, memory effect (charging before complete discharging), higher charging and/or discharging rate, other irreversible chemical reactions, load factors, over charging etc.

Thus the recharging process cannot produce the active reactant chemicals completely and so their amount lowers gradually in every cycle. Or in other words, a battery cannot be recharged with 100 per cent efficiency and its life time gradually decreases in each cycle.

So, the number of rechargeable cycles varies for different batteries and it is not an exact value.

For example in lead acid batteries, the maximum charge/discharge efficiency is about 80-90 per cent and so it can have a life cycle of approximately 800-1000 times. In nickel-cadmium alkaline batteries, the life cycle is approximately 1,000 to 1,200 times.

M. KANAGASABAPATHY

Assistant Professor

Department of Chemistry,

Raju’s College

Rajapalayam

Tamil Nadu