old alt was at 8v with engine running, i replaced the alt
the battery is good and the alt is also good battery is not charging on 2003 Hyundai Sonata
Which post (wire) is reading 8V when running? Normally you should see battery voltage while running , if alt is not charging, 8V would tell me either bad cells in battery , or excessive corrosion in wiring/connection(s). Get circuit info so you know what to expect. Fully SLOW charge the battery and then test , before putting on more parts.
check for power going to the alt if you have power you might have a defective alt