Why does the Chinese robot vacuum cleaner electric cool m 288 consider the battery to be low?

Question

Robotics

The question sounds strange, but here are the details:
I bought a new Chinese electric cool m 288 robot vacuum cleaner at a flea market (sort of like a clone of iRobot roomba 1st generation). The robot passes the test, all systems work normally (brush, chassis, dust suction, rotary brush).
When charging is connected, the indicator first lights up green, then red (charging is in progress). And that's it, the green light of the indicator, symbolizing the completion of the charge, no longer exists, no matter how much I charge it (I kept it for 24 hours once).
At the same time, if you turn on the test mode, the systems work normally, the same brush and wheels spin for more than an hour (i.e. the battery is not dead). There is a suspicion that the battery (Ni-Cd) has been without recharging for a long time.
Can anyone suggest what could be the problem? I bought it very cheaply, so I don’t feel sorry for the money, but I want to bring it to working condition. The robot is new, never used.