charging system problem high output

I went t Olds dealer because antil lock brake lights where coming on and air bag light and sometimes alt light would flicker they done Diag scan detected 2 history codes .p1630,37 monitor alternator output to high faulty alternater showing over 16 volts so stoped at auto zone after left there picked up alternator install it all lights on dash stayed lit checked voltage and new alterator was showing at almost 18 volt took my old one and new one back to auto zone they tested both showed me both tested ok
I cannot figure out why the over charging , it passed there michine test all ok Yet when I put a meter on, the battery shows very high voltage at battery when cars not running. 12.11 car shut off is this ok or is battery fried, and when put new alterator on showed to high output around 18 volt . alternater that i got Olds sevice tech after scan said thought regulater in alterator but new one was even higher voltage I wonder if auto zones tester is giving right reading on old and new alternator can bad or weak cell battery cause over charging to a alternator and high output when install new one . is there something else i can check or should i try another new alterator and new battery , should i a try diffrent one from another parts store even if they said both passed sorry to ramble on just frustrated
I would appreciate any info you can give me. searched in this forum for overcharging found some things i could check what i finally tried is removing hot cable and wires from the starter cleaning them putting them back after i started and checked alternator was puting a steady 14.45 seem to have fixed the over charging have to drive it a while so the 2alternators 2 scans, must have been poor contact thanks to all

If you do a search in this forum for overcharging you will find your answers. :thumbsup: