Low Battery Warning problem in cars Renault Megane, Model 2006?

Hello. I'm Vahid, and a car Megan automatic 2000 cc, 2006 model, I found very easy to start, after 20 to 40 minutes of moving or parked, LOW Battery Warning Warning smart screen appears. Which has once again turned the car off and removed. But after 20 minutes Low Battery warning is repeated again.
Renault went into labor service company, after visiting the alternator and battery, found no particular problems. I am Iranian, and I felt like operator may not have enough accurate information, not specific terms. I am facing a big problem.
If one of the general objection to fix it, please help me...