Charging coils come in a few variations.
This to accomodate the different electrical loads applied by the mower.
If you motor was changed out & you have one from a mower with a manual PTO (Lever not a switch ) the charging coils will not be able to cope with the load applied by the electic PTO so as you are mowing the battery will slowly drain.
Next, some lower output charging coils will require any thing up to 2 hours of continious full speed running to recover the power the starter used starting the mower.
Another thing to remember is the mower engine must always be running at WFO speeds to effectively recharge the battery.
I had a customer who was gobbleing up a battery every season. Reason was the mower went too fast for him so he ran it at 1/2 throttle so it was never fully recharging the battery.
I adjusted the seat so he was more solidly seated then adjusted the control levers to their least sensative position and problem solved.
Seems a common predelivery around here to set the control levers to the most sensitive position & adjust the mower to go maximum speed, perhaps the mechanics do drags at lunchtime.
This is quite daunting especially to owners new to ZTR's or that machine, They also seem to set the seat height for Harlem Gloetrotters so the owner is sitting there with 10" between his feet and the foot board.