First - great forum

Second - until today my 300E has never left me stranded (unlike most of the 70± other cars I've owned in the past 25 years). I was driving around town today in stop and go traffic with the A/C on max (it was 100° out)and the stereo about half cranked (150 watt system). After shutting the motor off the car would not restart. All I got was a solenoid click. Fortunately I was at a gas station and the attendant gave me a just and it fired right up.

When I got home I checked the battery. 13.3 volts at idle and 12.8 with the key off. Starts just fine now.

Does this appear to be an isolated incident (high heat, A/C and stereo on, low alt output from stop & go traffic) or is something else going on (battery, alternator, starter solenoid)?