Ditto bad regulator/brush assembly. Bad brushes can result in a repeated heavy drain and under-charge condition on the battery - just happened to mine. What is the voltage on the battery with the engine off? Also, worth checking the battery ground cable for corrosion at attachements. You will also want to check that there is no electrical drain on the battery while the engine is off? The rear dome light switch was accidently left on, and since the rear dome bulb was burned out, we couldn't see that there was a power draw (from the passenger lower door courtesy lights). This and the marginal brushes killed the battery, apparently suddenly, but in actuality over several weeks.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E