charging system issue

[KC Bonne]

I am seeking help from those more experienced with Bonnevilles. I have a recently inherited 2005 SLE. My mother-in-law had the battery replaced 3 times since Jan 2018 at the local small town GM dealership. After the initial purchase, the battery was replaced twice during an 18 mo period. No diagnostic was done to determine reason for battery failures. I replaced the alternator while traveling with her. We stopped at local auto parts due to voltmeter showing charging at 18v and was told alternater was bad. Parts store had closed during the installation and it was discovered it was still overcharging. Made decision to keep traveling additional 1.5 hrs to destination. Gauge continued to show overcharging. Car was not driven for some time after arriving at destination. Battery required replacing again a short time later. Ground under seat has been checked (no rust at all, still like new). Gauge is still showing erratic overcharge. Upon start it initially showed just over 14v. Driven for a short ride. Immediately restarted after return and gauge showed 18v again. Car sat a couple more days before any further attempt to figure out what was going on. Battery was now completely dead. Jump started car and now guages were failing. Volt meter was completely pegged. Tach would not register upon start and had to be approx 2000rpm to get gauage to move. All electrical connections seem good. This a car with 41k that has always been garage kept. I am looking for help on what the issue could possibly be.

[irad]

Check with ohm meter from battery negative terminal to the spot on the frame it grounds to. Check alternator voltage with a multimeter at the alternator. Check the ohms from the battery positive(red) to the alternator.