[neilgorin]

I assume this is the normal car dealer nonsense where they've done everything other than the obvious - i.e. replace the battery.

Even from the same batch one battery might last 5 years and another fail in 6 months.

I've got a car (not a Volvo) that needs a new battery every autumn without fail for example, the batteries are an odd size/shape and spec and are just not very good quality - or I can pay 4x the price for the maker's own battery, which was replaced under warranty at 18 months and failed 19 months later - so I just buy the cheap ones each autumn. When they start to fail they show as fully charged after a run yet are down to sub 12v the next morning. I put a new one on and it's fine for 12 months or so, then it happens again. Just bad batteries.

My first move would be to replace the battery in the car. Dealer of course won't do that if it's in warranty but I would bet if it was out of warranty and you were paying it would be the first thing suggested...