That C refers to the discharge rating and its also used in charging a lipo. A 30C 3000mah lipo theoretically has a discharge capability of 3000/1000 x 30 = 90amps continuous discharge. If the lipo says it can be charged at 5C, that means 5 x 3000/1000 = 15amps.

After reading a survey some time back taken by the members of a big plane club that indicated a majority of their puffed lipos were 20C variety, I buy higher C rated lipos than 20.


A beginning read for lipo can be found here -

http://www.rchelicopterfun.com/rc-lipo-batteries.html