I started to notice the similar problem last week, as I mostly use it plugged into AC power. But the battery seemed to start degrading since April without my realization.
My SP3 was purchased late Jan 2015. It hasn't been used much, mostly on travel.
PERIOD FULL CHARGE
CAPACITY DESIGN CAPACITY
2015-07-26 - 2015-08-04 42,343 mWh 42,157 mWh
2015-08-04 - 2015-08-09 42,358 mWh 42,157 mWh
2015-08-09 - 2015-08-16 42,362 mWh 42,157 mWh
2015-08-16 - 2015-08-23 42,374 mWh 42,157 mWh
2015-08-23 - 2015-08-30 42,268 mWh 42,157 mWh
2015-08-30 - 2015-09-06 42,248 mWh 42,157 mWh
2015-09-06 - 2015-09-13 42,423 mWh 42,157 mWh
2015-09-13 - 2015-09-20 42,431 mWh 42,157 mWh
2015-09-20 - 2015-09-30 42,404 mWh 42,157 mWh
2015-09-30 - 2015-10-04 42,553 mWh 42,157 mWh
2015-10-04 - 2015-10-11 42,583 mWh 42,157 mWh
2015-10-11 - 2015-10-18 42,583 mWh 42,157 mWh
2015-10-18 - 2015-10-25 42,583 mWh 42,157 mWh
2015-10-25 - 2015-11-01 41,841 mWh 42,157 mWh
2015-11-01 - 2015-11-08 41,535 mWh 42,157 mWh
2015-11-08 - 2015-11-15 41,100 mWh 42,157 mWh
2015-11-15 - 2015-11-22 40,948 mWh 42,157 mWh
2015-11-22 - 2015-12-04 41,708 mWh 42,157 mWh
2015-12-04 - 2015-12-12 40,668 mWh 42,157 mWh
2015-12-12 - 2015-12-13 40,592 mWh 42,157 mWh
2015-12-13 - 2015-12-21 41,061 mWh 42,157 mWh
2015-12-21 - 2015-12-29 41,314 mWh 42,157 mWh
2015-12-29 - 2016-01-14 40,934 mWh 42,157 mWh
2016-01-14 - 2016-01-28 40,949 mWh 42,157 mWh
2016-01-28 - 2016-02-04 40,911 mWh 42,157 mWh
2016-02-04 - 2016-02-11 40,326 mWh 42,157 mWh
2016-02-11 - 2016-02-25 42,340 mWh 42,157 mWh
2016-02-25 - 2016-03-17 42,583 mWh 42,157 mWh
2016-03-17 - 2016-04-01 42,583 mWh 42,157 mWh
2016-04-01 - 2016-04-04 42,583 mWh 42,157 mWh
2016-04-04 - 2016-04-16 42,151 mWh 42,157 mWh
2016-04-16 - 2016-04-20 41,635 mWh 42,157 mWh
2016-04-20 - 2016-04-29 42,583 mWh 42,157 mWh
2016-04-29 - 2016-05-13 40,751 mWh 42,157 mWh
2016-05-13 - 2016-05-20 30,477 mWh 42,157 mWh
2016-05-20 - 2016-05-27 28,014 mWh 42,157 mWh
2016-05-27 - 2016-06-03 21,374 mWh 42,157 mWh
2016-06-03 - 2016-06-10 18,559 mWh 42,157 mWh
2016-06-10 - 2016-06-17 18,194 mWh 42,157 mWh
2016-06-17 - 2016-06-24 13,025 mWh 42,157 mWh
2016-06-25 12,312 mWh 42,157 mWh
2016-06-26 10,996 mWh 42,157 mWh
2016-06-27 7,955 mWh 42,157 mWh
2016-06-28 7,752 mWh 42,157 mWh
2016-06-29 7,752 mWh 42,157 mWh
2016-06-30 7,301 mWh 42,157 mWh
2016-07-01 6,992 mWh 42,157 mWh