You say the alternator 'seems OK'. Has the charging rate been checked by the garage to make sure it actually is OK? Is the fan belt tight? Have you got a light that's staying on anywhere, such as in the boot or luggage compartment or even the glove box? Even though it's only a small bulb, it can still drain the battery over a period of time. Has the garage checked the cold start relay? This sometimes causes problems by staying 'connected' and means the preheat plugs are on for too long, draining the battery. Is the radio/CD player turning off OK? Is it left on when the ignition's turned off? Again, even though it takes only a small current, it can still flatten the battery. Are all the lights turning off properly? There could be a short somewhere which keeps maybe one or two rear lights on, again draining the battery. An electrical fault like this is sometimes very, very difficult to track down. You say you've taken the car to a garage. They may not have the expertise or equipment to find the fault. Try a proper car electrician (in the Yellow Pages). He'll have proper test gear to find any drain in the circuitry. Best of luck!!
Denis Langley, August 2005