06 E320 CDI. Both batteries replaced by PO at dealer 3.5 years ago. Charging system in good working order.

As it's gotten colder, I would periodically get the "convenience features temporarily unavailable"....no big deal.

Three days ago, drove 140 highway miles and let car sit for three days. Tested main battery and it was only reading 700 CCA. So I figure battery is marginal at best and should probably be replaced.

Start the car and get the red visit workshop battery message. That usually points to a bad aux battery from what I understand. But since the main battery was pretty weak I thought that the likely culprit.

Ran a scan and was told the the aux battery was bad. Measured voltage and it's under 10v. So I figure at least that one's bad.

My question: could a bad aux battery prevent the main battery from charging properly? Or should I simply conclude that both batteries are bad?

I'm happy to throw a couple of AGMs in there and be done with it. But I am curious if perhaps the main battery may have significant life left.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife