Charging left battery

Hello! Have a Phaeton now since Friday. I am very satisfied, only the left battery seems to weaken. Time, consumption and further settings are reset after each start. It also shows system errors, testing chassis, stopping brakes and checking level… The car cannot be locked by radio, the tailgate only goes on when the ignition is on. All control units have been read out, no error has been found. The batteries in the lock It sounds like a voltage problem, doesn’t it? Is this emergency start function? I only have to turn the key to the left so it starts… Sorry for the maybe doozy questions, I hope you can help me. I tried to charge the battery, but it shuts down after a short time through an audible “crack” and is not charged any further. How can I charge the left battery (AGM)? The right battery is fine. I am grateful for every hint!