Watchdog

Hi all, confused about this. 1982 Toy pickup 22 r. Doesn't seem to be charging. Battery good, maintains a charge after slow charging. New alternator. Charge light comes on as supposed to before turning key, goes off after engine starts. Doesn't come back on when running. BUT when running the battery discharges. any ideas what to look at? Have checked battery connections, tight. Checked belt , about 1/2 in deflection. What else should I look into?