I replaced my battery last week. Like you, for the last couple of years I've been living with 'low battery' messages and the unavailability of the stop/start system due to 'electrical system' issues that can be fixed by leaving the car on an overnight charge. I figured that entering winter with a near-seven year old battery was pushing it, hence jumping in and buying the new battery.
Since replacing it, the stop/start has become permanently available, once warmed up.
If your car is still on the original battery then I'd recommend replacing it in the next month or so. I got mine from a Volvo dealer for £99 + delivery...
https://www.volvoforums.org.uk/showp...1&postcount=12