The alternator current output capability is dependant on RPM. However, the alternator is fully capable of producing enough current at engine idle RPM to charge your battery. The reason people rev the donor engine during a jumpstart is to make the donor alternator capable of supporting both the running car AND the high current draw of the dead car. Under normal conditions (ex: the battery was able to start the engine), there is no need to rev your engine in neutral or drive around to recharge your battery. It would be far more efficient and economical to buy a battery charger that plugs into the wall than to run your engine…