Capacity (Ah)  Model(s)  Cell Configuration  Capacity per cell (Ah) 
1.5  OP4015  10S (or 10S1P)  1.5Ah 
2.0  OP40201  10S (or 10S1P)  2.0Ah 
2.6  OP4026, OP40261  10S2P  1.3Ah 
3.0  OP4030, OP40301  10S2P  1.5Ah 
4.0  OP4040, OP40401  10S2P  2.0Ah 
5.0  OP4050, OP40501  10S2P  2.5Ah 
6.0  OP40601  10S2P  3.0Ah 
7.5  OP4075?  10S3P  2.5Ah 
4Ah OP40401 Battery on Ryobi OP401 Charger  


4Ah OP40401 Battery on Ryobi OP403 Charger  


4Ah OP40401 Battery on Ryobi OP406A Charger  


OP4015 contains 1.5Ah cells. 1C rate = 1.5Ah/1hr = 1.5A. 6.15A > 1.5A
PP4020 contains 2.0Ah cells. 6.15A > 2.0A
Cell capacity is half that of the battery pack, since the design is two cells in parallel.
Each string of ten cells receives half the charge current, or 6.15/2 = 3.08A.
OP4026 is rated 2.6A, so the cells are 1.3Ah. 3.08A > 1.3A
OP40261 is rated 2.6A, so the cells are 1.3Ah. 3.08A > 1.3A
OP4030 is rated 3.0A, so the cells are 1.5Ah. 3.08A > 1.5A
OP40401 is rated 4.0A, so the cells are 2.0Ah. 3.08A > 2.0A
OP40501 is rated 5.0A, so the cells are 2.5Ah. 3.08A > 2.5A
OP40601 is rated 6.0A, so the cells are 3.0Ah. 3.08A > 3.0A
Cell capacity is one third that of the battery pack, since the design is three cells in parallel.
Each string of ten cells receives one third the charge current, or 6.15/3 = 2.05A.
OP40751 is rated 7.5A, so the cells are 2.5Ah. 2.05A < 2.5A
I recommend using the OP406A sparingly or not at all on batteries under 7.5Ah in capacity. 
Battery Model(s)  Rated Capacity (Ah)  ACTUAL Capacity (Ah) / % of Actual 
Recharge Time on Charger Model  

OP400  OP401  OP403/OP404  OP406A  
OP40401  4.0  3.675 / 92%  2:23:23 (8,603 sec)  3:13:37 11,617 sec  2:35:29 9,329 sec  48:40 2,920 sec 
OP4026  2.6  2.305 / 89%  1:27:32 (5,252 sec)  1:57:45 7,065 sec  sec  sec 
4Ah OP40401 Battery on Ryobi OP403 Charger with extra power supply  

