Roomba discharges battery while on charger

[Carl E]

Hello,
I've got an older roomba. I've installed a new battery. I can charge the battery with an external (Not roomba) charger to 16.3V and the vacuum unit works fine. When I put the roomba on its own charger, the lights appear to indicate it is charging for awhile, however after a couple of hours on the charger, the battery starts yo discharge and after a days is discharges to about 8 volts. The brick charger's output is 22 volts, so it is ok. This happens both with the vacuum on the docking station, or with the brick charger plugged directly into the vacuum. I have tested both the charging station and the battery pack with a different roomba and both work OK. I've also changed out both mosfets ( U2 and U3, P/N N3PF06 ) on the motherboard. I used partial schematics that I found on the web. it's hard to see with the dark background.  Anybody had a similar issue or have a suggestion where the problem could be ? ?  Thanks !