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The battery keeps draining after a few weeks. Wondering if it’s the alternator. What is the expected alternator voltage output when the car is running? I have read its supposed to be 12v. The alternator is a correct Bosch alternator.

A 12V alternator is really nominally a 14 volt one. Charging spec is 13.6-14.5.

However it's unlikely to actually be the alternator. First off does the alt light on the cluster come on? The alt gets it's excitement current to start charging thru it, so if the bulb is burned out, it's going to be a no charge condition. Next is the regulator, while alternators very, very rarely fail, the regulators do. Mostly by the brushes wearing out, but some times they do fail. A 5 minute fix on the car. Lastly you really need to check voltage at the alternator itself if it seems to be charging, but the battery won't stay charged. Wiring and connections over the years can result in the alt charging ok, but full voltage not getting to the battery.

So, 13.6 to 14.5, measured at the alternator.