Gotta be one of those 3. The battery isnt holding (or is loose), the alt isnt charging it, or the car is bleeding it out.
How did it test good? (Which test method, I mean) With 1 bad cell it may test right but not provide cranking amperage, as they are compiled from a series (6) of cells. Nevermind, you replaced the batt already.
What is it doing? Slow crank, no crank, intermittent crank, jumpable, etc?
If I remember right, Benjamin Franklin actually coined the term battery for electricity after an artillery battery, because of the resemblence of the cannons and of the cells lined up to amass the needed force. Neat, huh?
“Coming of age in a fascist police state will not be a barrel of fun for anybody, much less for people like me, who are not inclined to suffer Nazis gladly and feel only contempt for the cowardly flag-suckers who would gladly give up their outdated freedom to live for the mess of pottage they have been conned into believing will be freedom from fear.”