1c= 1x a given battery's mah capacity. A 4000mah pack means 1c=4amps, 2c= 8amps, etc.. A 9000mah pack means 1c=9amps, 2c=18amps, etc..

While some batteries are "rated" to charge at 5c does not mean they should be. 1-2c is plenty fast, safer and attainable for most chargers.