Can't switch Karcher window vac off - Karcher WV50 Window Vacuum

I had the same problem and I let the battery run down to stop it cycling on and off. My husband investigated it and found that there is a fault with the on board battery charge sensing circuit. (when the charge is about 10 to 15 % remaining the onboard ic controller cycles the unit on and off, irrespective of the switch. He checked it by cutting wires to the switch and it still cycled off and on so the switch was not faulty.
He put it all back together and said to charge it before it gets too low. If it cycles on and off it probably needs a re charge at that point.
Unfortunately there is no battery level indicator on the tool and nothing to suggest in the manual of what may happen when battery is flattening.
I am taking this up with Karcher as I only used it three times so far and it it was not a cheap gadget. Hope this helps