you don't need to discharge nimh, just charge it to the max.
wait, it's really not clear in this thread

did you change the spring for a stronger one? in that case your battery's too small.
Use
at least 2000 mAh.And also the cell size matters.Some >2000mah batteries with super small cells WONT pull that spring.
I don't know how much is 1.5 joule. if it is over 400 fps@.20 your battery's way too weak.