Battery has been replaced twice once last year and again 3 weeks ago. I am still having battery problem. Whatcould be the problem as the mechanic can't seem to find the problem. Thanks.

Try this: turn everything in the car off--that means everything, all accessories, lights, etc. Remove all items in cigarette lighter sockets or power ports. Key off, car as dead as you can make it. Then take the negative battery terminal clamp off the negative post. Hook up a 12v test light between the terminal clamp and the post. If the light glows brightly, you have a parasitic drain going on which is slowly draining your battery down. The light may glow dimly to power the clock and some computer memory and that's okay. But a bright glow means a short. Now, with the lamp glowing brightly, start pulling fuses one at a time until the light goes out. The short is on one of the circuits of the last fuse pulled.