Replaced the aux battery and everything seems to be working fine. Was able to charge main battery to 95% capacity. Voltage while driving is a stable 14.3 volts. But I'll keep on eye on it. It is the original alternator, after all.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife