A '77 will most likely have a Magnetek 6300.. This may claim to be a 35 amp converter (And it is) but it only charges the battery at like 3.5 Amps. not enough for a volt meter to really sit up and take note for several hours.
now, a nice Progressive Dynamics 4645, That's a 45 amp converter/Charger. It can put the entire 45 amps into the house batteries if nothing else is drawing power and they are HUNGRY.. then as the voltage rises the charge drops off.. all the way, eventually, to Zero.. This never happens with the Magnetek so the batteries boil dry if left on charge too long.