Peugeot 207 battery supply or electrical supply fault

Hi after about 5 minutes of driving I get the warning "Battery supply or electrical supply fault" flash up. If air con is on it will then turn warm and its now affecting lights making them too dim to drive at night. Other than that the car has been driving absolutely fine no issues. The warning message goes quickly but the battery indicator light stays on. Garage replaced the alternator and belt but as soon as I drove home the problem was back not resolved. They have now had it a week trying to diagnose the issue but seem to be struggling. Any ideas?