Let's assume we have a good, chargeable battery.
My truck idles at around 650 rpm. The battery does charge at that rpm. So, for my truck with my alternator, the answer is 650 rpm.
As each model of alternator is different, and is built for each model of vehicle, I think the answer would vary. BUT - I also believe that the minimum rpm for charging would be the idle state of the engine. ELSE - running the engine at low rpm would kill the battery. Most modern engines cannot run by alternator alone.
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