What I heard, but did not verify, was that the engine runs a generator, at a steady-state load/RPM (~80% Duty Cycle), which produces more power than the vehicle needs to cruise, so in the event that it's cruising and the engine is running, the excess charge goes through the motor into the batteries.
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"¿ʞɐǝɹɟ ɐ ǝɹ,noʎ uǝɥʍ 'ʇı ʇ,usı 'ʎlǝuol s,ʇı"
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