Automobile Engineering in ME
The main characteristics of a maintenance free (MF) battery is that

A maintenance free battery has a relatively short shelf life when compared with standard batteries
A maintenance free battery requires little maintenance during normal use and it is sufficient to add water instead of an electrolyte containing sulphuric acid
Since it is sealed, the water in maintenance free battery is not lost through evaporation thus accordingly it is not necessary to top up the cells with water
Recharging of a maintenance free battery is neither required nor possible

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Automobile Engineering in ME
The function of a second ring is that it is a

Back up oil control ring which will work in place of the bottom ring when it is broken
Compression ring and aids the top ring in sealing and cooling
Cushion ring which prevents piston slap and knock
Back up compression ring which will work in place of the top ring when it is broken

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