Polo battery runs flat

[Supr11]

I have a 2009 VW polo with a battery that runs flat if I haven’t turned the engine over for about 4 days. It’s been doing this for about 3 years years now. In the past I didn’t drive it much and mostly short journeys but since the problem started I’ve been driving it at least twice a week for about 45 mins each time. I have RAC cover so each time it’s flat they run checks on the battery and almost every time the result is replacement battery needed. My cover provides replacement batteries so there’s no additional cost but it does mean I’m getting about 3 new batteries a year. I’ve had the car checked multiple times for a drain, none can be found and the alternator is fine. I’ve been told repeatedly I just need to drive it more often and for longer journeys but even with the extra effort it still runs flat after a few days of not being used. Any ideas?

[Volkswagen Owners Club]

It could be a bad earth connection or lead, faulty alternator, loose belt, terminal corrosion on the battery terminals, poor quality battery (maybe pay extra for Premium one with more power), more use on the road during the daytime (without lights, etc).....in fact many other issues which would need further diagnosis