Batteries have a nominal (normal/acceptable) charging rate, most are around 1.5 amps. If you go above that you may decrease the batteries lifespan, sort of like how if you Discharge batteries too fast you will shorten their lifespan. There is a chart somewhere that shows nominal charge rates for different batteries...I know ilumn.com usually includes this number on their website.
Higher amp means faster charging, but going too fast could be bad