calculate Amp Hours when voltage is given
12v - 11.5v = 0.5v

i=v/r  i=0.5/1 = 0.5Amps charge current.

battery = 20Ah (note: not A/hrs!) 

20/0.5= 40 hours to charge.

amp hours= 20 amps for 1 hour or 10 amps for 2 hours or .5 amps for 40 hours.

it is a measurement for how much capacity the battery has and is useful for calculating how long it will last.