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

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
A maintenance free battery has a relatively short shelf life when compared with standard batteries
Recharging of a maintenance free battery is neither required nor possible

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Automobile Engineering in ME
The difference between DOT 3 and DOT 4 brake fluids is

DOT 4 fluids have a lower boiling point than DOT 3 fluid
DOT 4 fluid is more resistant to freezing than DOT 3 fluid
DOT 3 and DOT 4 fluids have the same boiling point, but DOT 4 fluid has a longer service life
DOT 4 fluids have a higher boiling point than DOT 3 fluid

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