the battery is good and the alt is also good battery is not charging on 2003 Hyundai Sonata

old alt was at 8v with engine running, i replaced the alt

Asked by for the 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.
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check for power going to the alt if you have power you might have a defective alt