Diagnosis
This is typically a battery communication issue or the battery cells are failing and can’t deliver power under load, even if they show a full charge.
How to Fix It
First, try recalibrating the battery. 1) Run the vacuum until it completely dies and shuts off on its own. 2) Place it on the charging dock and let it charge uninterrupted until the light is solid blue (or goes off, indicating a full charge). Don’t take it off mid-charge. 3) Leave it on the dock for another hour after it signals ‘full’. This cycle can help the battery management system relearn the true capacity.
IF THAT DIDN’T WORK The problem might be with the battery’s temperature sensor or a faulty cell. Contact Tineco support. The S15 Flex has a removable battery pack. If you’re comfortable, you can try removing and reseating the battery to ensure a good connection, but if the battery itself is failing, you’ll likely need a replacement.
Heads Up
Try to avoid always doing short cleaning sessions where you put it back on the dock after a minute or two. Like any lithium-ion battery, it benefits from occasional full discharge/charge cycles to keep its ‘fuel gauge’ accurate.