Current loss in Renault Scenic 1.6 gasoline.

hi there. The problem I have with my car is that it runs out of running but the strange thing is that it does it from time to time, even when I drive if I stop in a ceda or stop the car stops and I have to be starting again and again, and the worst thing is when I leave it in the garage the next day the battery is downloaded. Take it to the mechanic and I start it with a starter I check the battery and alternator and they are fine, and on top of failure in the machine. Some could help me ... Thank you.