Probably just worn.Sorry to revive a post so old, but I'm going through the same problem.
I have a Defy MB525 and bought a battery original Defy + MB526 and these crazy drops happen throughout the charge cycle.
Can anyone really help because I tried several tips and nothing worked. Today I am using CM10.2 which in theory has the correct driver for the Defy + battery and yet the drops continue.
One thing I noticed is that the original Defy the load drop occurs in 61% to 49% and after 30% to 19%, but when I use a charger Samsung Galaxy S4 (which has 2000 mha output) the happens only drop from 43% to 19%. Do not be a hardware failure in the Defy does not allow the battery to be fully charged?
Where did you buy this Defy+ battery? Anything under $20 on ebay are used worn batteries. Charging your phone with a charger not meant for the Defy isn't good ether as is charging till 100, keep in charger long time and dropping below 20% all the time.
I bought a genuine new Defy+ battery and use it with no problems in my old Defy. Just installed CM10.2, that's it.