just a recent experience with our Passat CC
Our 2.5 year old Varta E39 battery kept dying which was purchased from Tayna - battery comes with a 4 year warranty. Had it tested at a local garage which confirmed the battery was faulty - gave me a print out etc.
Contacted Tayna who asked me to send the printout. Once they checked the data on the printout, they asked me to send the battery to the, for further testing. Now, although the car was on the drive as we were in lock down, i still may have needed it so didn't want to have a car with no battery. Tayna gave me the option to buy a replacement battery which would be refunded once my old battery was confirmed as faulty.
I ordered the replacement battery and had the garage fit it as it needed coding. Sent off the faulty battery to Tayna for testing (so glad they were still open through lock down).
A few days later, they emailed back to say the battery was tested and confirmed as faulty. Full refund issued on the replacement battery.
The only thing i couldn't claim back was the cost of postage - £11 with DHL (via parcel 2 go) as the battery was purchased over 6 months ago.
Although it worked out ok in the end, it was a ballache having to buy a replacement first (as i needed the car) and sending the battery back for testing whilst incurring the courier costs.
I have the Bosch S5 fitted in the ED30 and of course, a Varta fitted in the CC. I dont think theres much between the brands to be honest.
I would probably be more cautious with buying a battery online in case of issues like mine.
Hope this helps
Rich