Varta Battery Failed in 20 Months, Warranty Was Rejected

The Varta battery I purchased online stopped working after about 20 months, and I was stranded on the road. Despite my car being stored in a closed garage at all times and never exposed to temperature changes, the battery failed. I contacted the seller, who directed me to the service center. Although I have the invoice and the battery is still within the warranty period, they refused to assist me without the warranty certificate. I also reached out to Varta, but I received no response. The battery in my car, which is parked in a closed garage and used daily, became unusable in just 20 months, and despite having the invoice, I could not find any solution because I lost the warranty certificate. The company does not stand behind its product.