I had a problem with my Gen 4 12V battery too, and in the end the dealer replaced it.
I keep an eye on it with this:
https://www.ebay.co.uk/itm/12V-USB-Charger-Car-Cigarette-Lighter-Socket-Power-Digital-Voltmeter-Thermometer/282786775448?epid=791786015&hash=item41d767c198:g:1YAAAOSwSQFaQ2d6
If I put it in the rear cigar lighter socket and put the car in ACC mode, I can see the voltage. By comparing the reading when just parked after driving for a while, with a reading when the car has stood unused for a day or two, and then watch to see if the reading drops while leaving the car in ACC for several minutes, I can judge whether I'm heading for further problems.
In READY mode, it will show near 14V, 12-13 when just parked, and around 12V after a day or two. If it drops below 11, I will start to worry.