I've got 2 Galxy S2 with the same problem.
This is what happens.
Booth phones with 60% of battery or more, if i shut down, remove the battery and wait about 5 seconds, then put it back.
My battery returns to 20% or less.
Tried with new batterys, different rom's (cm11, omni, stock), reseted the chip fuel gauge.
Has anyone ever seen this happen?
Is driving me crazy.