V60 D2 Business Edition Batteries

[Europa Jon]

I have a 2013 1.6 litre diesel with start-stop, as per D2 Business Edition spec. Lately I keep getting a low battery voltage message on the dashboard instrument cluster. Also the start-stop no longer works - probably because the battery voltage is too low. I've done a bit of research and found that the car has two 12V batteries. Why didn't they just use one larger battery? The main battery box is a lot bigger than the original battery anyway. After normal use yesterday and the car 'resting' overnight as usual, I measured the little hidden battery at 12.65V and the main one at 12.45V. All is OK apart from the start-stop non-function at present, but I think I should replace the battery or batteries before winter comes.
My questions are a: which battery is causing the low voltage warning and start-stop inactivity, and b) is it usual to replace both batteries as a pair?
I realise both original batteries are now 7 years old, so assume they have each had their rated lifespan. I intend to keep the car another 3-4 years so will probably replace both together, but just wondered if anyone has first-hand experience of the Volvo double battery thing. Thanks in advance.

[DVWR]

I’ve just has the exact same issue in my 2013 d3 v60. Different engine but same twin battery set up.
Stop start is all down to the main starter battery. I had this tested at kwik fit (for free) and it was “unserviceable”. Got it changed at Volvo, which was cheaper than kwik fit (but still around £170) and got a free inspection and valet.


Could get it a bit cheaper online and fit yourself as well.

Stop start now works as it should and no low battery warnings.