Overcharging

Im gonna check the bulk head connections today. They have never been removed. Maybe if I go through the series of events something will jump out at you guys.
I accidently left the key on about a month ago. Didnt kill the battery. Drove it after that. Then a few day later and battery was dead. It was very old. I charged it and got it running. Thats when I noticed the overcharging started. So, new battery, vr, alternator tested and new cm. 15.8, 15.9 volts. Spikes at start up. Wiring has never been touched. Bone stock. I have checked connections br, vr, cm, alt. Any help will be greatly appreciated.