Believe me has nothing to do with the auxiliary battery, it is always the main battery the culprit . Start stop system just eats batteries too quick . If battery charge goes below 70% the S/S system gets shut down

Just replace starter battery, same problem on my 2017 S60 Cross Country, solves replacing battery with. 1000amp H7 champion battery bought at Pepboys for $120 with a
Coupon , Start stop instantly become available .

Same thing happened to my 2017 S60 cross country. No start stop, reset bms and then it worked for a day or two. Replaced battery with a Champion H7 AGM 1000 ccs battery at pep boys . Problem solved !! Start stop works every time .