As my auto shop experience reflects, the instructor said,
"Charging system at battery should read 13.2-14.1 volts."
This is under load or no load conditions. Funny, what applies 30 years ago still applies today.
Schucks or Kragens will check it for free and pull load test on battery.
I bought an alt for my 190E for 75.00 with core charge included
and free shipping this summer. It cost more to send core back than core charge so I kept it for parts.
After battery hassles, I will only buy Optimina Batteries. You can get them at Costco.