the charging stops too fast

I used this vacuumer for about a month now. and its pretty nice.
but now i experience some issues with the battery.

when i vacuum it operates for about 20 mins or so. then it runs out of power. and the indicator lits up and it searches for base.
but it only charges for 10 mins. then it beeps three times for being done. and it still runs out of power again quickly ?

plzzz reply on soeren@albrechtsen.be

thanks

????