NIU NQI Cargo

[FlyingDutchman]

Hello

The charger will also remain green if it doesn't detect a battery. It sounds like that battery wasn't charged, and as such the charger is no longer recognizing anything is connected. It will throw error 194 when turned on since that battery is not sending any data on the databus since it's dead.
The scooter uses about 5% battery charge a day if its not in use because of the alarm and GPS. The older 29Ah batteries are notorious for completely depleting themselves this way if they are not charged often. The newer 26 and 35Ah batteries have an earlier shut-off.

NIU bms don't have undervoltage protection. 35ah will also dying

Wrong.
The BMS will only fully close at about 2.7v/cell in the 29Ah battery, which means it is unable to be revived after about 3-4 months. The 35Ah already shuts down at 3.1v/cell, which means there is much more hope for survival after a few months at that state.

3.1V is still chargeable with OEM charger? Or does oem charger won't work at that low state?

It usually wont charge with the oem charger (sometimes its just close enough it will by putting in the charger and pressing the button on the battery), but it can be safely revived by a battery revival tool.