To be clear it's not a simple math issue where 1200 mah bat on a 100 mah charger is 12 hours. You loose some in the transfer to the point where I beleive it may be 85% or so of the 100mah charge rate gets to the battery. Normally a 15 hour charge of 600 mah battery at 50 mah will fully charge the system. There is also something called a forming charge which should initially be considered as well which ensures new batteries will be able to be "fully charged" going forward. The typical wall wart type chargef can do this since its not a timed or peak detection charge and will just go and go until you unplug it.