The battery numeral system

The sexagesimal numeral system is a place-value notation using the powers of 60 rather than the powers of 10 and this numeral system was extensively used by the ancient Babylonians, albeit without a zero (they used extra spacing instead). Although nowadays, the decimal numeral system is used extensively; like duodecimal, sexagesimal shows many vestiges in our current usage, like the subdivisions of time (an hour into 60 minutes, and a minute into 60 seconds) or angle (an arc degree into 60 arc minutes, and a arc minute into 60 arc seconds).

Back to batteries

"C" specifically refers to the discharge capacity which is generally considered to be the shortest time a battery can be safely and completely discharged. "C" is either quantified in fractions of one hour or fractions of minutes, i.e. fractions of 60.
A 20C battery can be discharged in 3 minutes, where a 30C battery can be discharged in 2 minutes and 60C in a minute flat. 

Very important*** Battery manufacturer data sheets should be considered for optimal discharge rates.

Battery discharge table showing the C rating as well as duration the battery should last depending on the rate of discharge

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