I have a diesel truck with 2 batteries. There is also on this farm numerous diesel tractors that do not have a high rpm speed. All vehicles work fine. Most times the tractors aren't used at the highest rpms.
While optimal charging speed may be 1200-2500 rpm I still think that battery charging will happen at idle. Also, if it didn't, the alternator light on the dash would be lit until the rpm of the engine was raised.
Bupkis mit Kuduchas!
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