I believe the problem is the 12V battery volts are low. On measuring the voltage on a number of occasions it is about 11.7V.
Hello Pete:
The battery is covered by the same warranty as that which applies to the rest of the vehicle. Warranty length varies by country - here in North America VW warranties are typically 4 years or 89,000 km (about 50,000 miles), with longer periods for some components such as the powertrain & corrosion protection.
Your VW dealer has a tool called a "Midtronics Battery Tester" - every VW dealer has one. Just ask them to test the battery. If it fails and you are still within warranty coverage they will install a new battery free of charge. If your warranty period has expired, I strongly recommend that you buy the new battery from your VW dealer and have them install it, because there is some software coding that needs to be done when a new battery is installed. If this coding is not done, battery performance and battery life will be compromised.
I have had one battery fail during warranty during the last 35 years of VW ownership - the dealer was happy to replace it, because if the Midtronics tester fails the battery, the dealer is assured that VW will honour the warranty claim.
Otherwise, my experience has been that batteries last about 5 or 6 years before the risk of unexpected failure to perform arises. I usually replace them after 5 years if I plan to keep the car - no point in waiting until it fails at (for example) 7 years, then getting stuck and needing to replace it in a hurry.
Michael