I have a Bafang BBS02 installed on my Cannondale Quick 4 hybrid with 9-speed cassette. It works really well, EXCEPT the the shifter cannot raise the chain to the largest (lowest gear) sprocket. The drive sprocket is the standard 44-tooth one that was supplied with motor. It is not a great hinderance unless I ride without the motor where I want to use the lowest gear.
Is there a solution for this, such as a concave 44-tooth drive sprocket that would allow the shifter/chain to travel the the whole range of 9 gears at the back?
Check your chainline with the chain sitting on the 4 gear. It should be a straight line between that gear and the front sprocket. Also, check the position of your derailleur cage in that position. It should stick straight down. If it is positioned forward, the 44t chainring was larger than your stock bike and the chain is effectively too short. By the time you are in the lowest gear, the derailleur may be too stretched forward to shift to the largest gear.