When charging batteries there is no need for alternator to produce electrical power at 60 cycles per second, so engine speed is not as limited. An alternator can charge with engine at 1000 or 3000 RPMs. It’s similar to inverter generators where speed can be varied in order to meet load at higher efficiency. With systems like Volta engine RPMs are even more flexible than an inverter generator because batteries are a buffer to handle surges.
Battery, charging, and inverter technology has existed for years in electric and hybrid autos — high cost remains major issue. If viewed as a luxury not requiring justification, then you can buy one now.
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