Lousy customer support

We bought a battery and had it fitted at the Randpark Ridge store for my wife's Mini Cooper S. We have had endless trouble with it running flat. Every time we went back we were told of different problems caused by the car, from the sound system, which we had totally removed, no rubber mat under the battery which we have had installed. This has been going on for at least 8 months. Again yesterday my wife got stuck and the car was jump started, I had the battery on charge the whole night and this morning it was still flat, yes the charger works as it was bought especially for this car. My wife is scared to use her air con or drive at night. we are now at our wit's end. I have had the charging rate checked at 3 different service suppliers and all have told me this is OK. We have bought tires from them and which we have had no problems at all. its not like we are a once off customer.