Alfa Romeo voltage

[Rickjames5087]

So I have a 2018 Alfa Romeo Giulia running 1500rp skar amp and EVL 12”sub. I have an aftermarket OBD digital gauge and my voltage is weird. Running full tilt sometimes it’ll stay up at 13.9v others it’ll drop down to 12.8v and then as I decelerate the volts with go back to normal until I completely stop then back down to 12.8 and revving the engine doesn’t help. Is it some weird ass computer controlled charging on the car? I’ve double and tripled checked all wiring nothing loose. And even with the amp unhooked it’ll fluctuate the same as above just a little less range. With a second battery help or change the alternator or am I screwed?

[Kyblack76]

If you are seeing 12.8 min, you are beyond fine. Car audio is 12vdc mate.