Hello to anyone with ideas....?

1991 Dodge Van 350 V8, 5.9l

Van starts but voltage drops and battery gets drained. I have to jump start or charge battery. Once it starts, it runs for a few hours but then has loss of electrical power and dies. Voltage meter on instrument panel registers, but drops when driven.

Just replaced Alternator, Starter. ECM was replaced about 2 years ago.

Any suggestions...?

Thank you kindly,
Shaun

Using a digital voltmeter, what is the exact voltage across the battery terminals when the engine is running?

How old is the battery?