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If the robot vacuum won't stay On for long after leaving the charging station, and if it is several years old at least (minimum about 1 1/2 years very roughly) then its battery may have failed, simply.
Rechargeable batteries of the type used in electronic devices of a portable - or mobile - type, no matter what the technology, always fail eventually, either failing to hold charge, or failing to deliver it.
(The one other possible thing wrong, is the charging station failing to actually charge - and that is much less likely, at least 5 times less likely at an approximate estimation. Also, make sure the battery really is plugged into the rest of the device, and that dust is cleaned off its terminals or connectors, and it is given one more try at charging then running the device, before replacing it.)
You may have to replace the rechargeable battery in the robot vacuum. If you do, be sure to get a replacement that is exact as to ratings for Voltage, and for mAh, at very least - and, of course, one that fits the space for it in the vacuum cleaner!
Let me know how it goes. Thanks.