Not sure what charger you have then.. 5A is fast for most airsoft batteries. Personally I wouldn't charge at a rate over 1C, it's good practice if you ever make the switch to LiPo too.
By 1C, I mean if you have a 2000mah battery you charge at 2A, a 1600mah, 1.6A etc. etc. if you can't be that precise with your charger, I'd round down. So long as you aren't in a rush then you'll still have them done in 2 hours max.