apple bluetooth keyboard model no: a1255

I got this second hand keyboard from a good friend at very cheap price.  It requires 3 AA-size battery.  On the first week. I was really shocked to see that its battery level went down so fast like around 5% to 10% everyday.  I thought this keyboard might be damaged or something.  However, after the battery level reached like 60%, it then started dropping very slow.  I think after 2 months, it is now still left 36% which is amazing!