If you have a static 12.8 V then you must have good charging system performance. The Voltage should be checked at battery, you will read lower inside car at cig lighter or fuse box.

The Voltage regulator that Volvo uses is a temp dependent type that will vary voltage as temp of regulatoe increases to lessen the evaporation rate of electrolyte in battery. If you start cold then the reading should be 13.9-14.1 V and it will go down to as low as 13.6 as it warms up after 3-5 minutes. A 70 degree F battery has to be charged at 13.92+ V to charge but the voltage necessary falls with temp increase.