Volkswagen review: denial of warranty

I purchased my VW golf tds October 2017. Since I got it the battery had to be replaced twice. The first time it was replaced for free but the second time my request for a new one was denied. The reason? The dash cam caused the battery to fail according to the service personnel. My dash cam automatically turns on when the car is started and turns off when the ignition is turned off. Furthermore when it's on the screen turns off after a few minutes even though it continues to record video. I do not believe it is the cause of premature battery failure. It is a plug and play device and gets power using a cigarette lighter adapter.