I took my 2012 C250 for repairs. Intermittent slow...

I took my 2012 C250 for repairs. Intermittent slow cranking after engine start. After diagnostic testing, and this took about 6hrs, the only indication was the battery was low. I forked over $372 for a new battery, and I am STILL experiencing the same issue. I know it is not the battery, so just exactly what is the problem? My car should come back for repairs, I just really don't want to spend another $372...