DESIGN: Until I owned my 2016 SUbaru, I never knew that modern Electrical Charging Systems could be so messed up that they would not charge-at-idle (this is what killed off the older DC-Generators .. needing to be at high RPM to get out meaningful Charging Current !): FACT ! But you are right ... that is exactly what happens with a SUbaru Alternator, which will not put out full charging Voltage at Idle ! Please notice that I stated Voltage, and not Current ! Until an Alternator puts out enough Voltage to reverse-flow current thru Battery-Cells, Zero-Current flows back into the Battery. By skimping on copper-turns-of-wire inside the Alternator, SUbaru Charging Systems do very poorly until the RPM's are definately above Low-Idle-RPMs !!!! Dollar-Wise/Pound-Foolish "engineering" ... from one end to the other ... but especially in the Electrical Designs !!!!! KWHr Power Generation is totally based on (sufficient) Copper-Coils, and (sufficient) Iron-Cores ... rotating at a proper designed Magnetic-Flux-Speed (Engineers casually refer to it as "dee-fee-dee-tee"): where Low-Idle 550-RPM is the Design Criteria, you never hear of stuff like SUbaru's Dead-Battery-Issues; but they do not use that more reliable and functional Operating/Design-Specification ... theirs is more like 1100-RPM, and that is the whole mess in a nut-shell ! And that is why thousands of frustated SUbaru Owners are constantly trying to Re-Design their vehicles electrical systems ... just like in this pitiful example !!!!!!!!
PS : there are up-sizing ALTernators available in the After-Market that are properly designed ... but they are not "cheap", like SUbaru's units are. The very first thing that I noticed under the hood was the very small size, and the very high (false) "specified" output of the SUbaru Alternator ..... Size does make a difference ... as it aids in the "dee-fee-dee-tee" !!!!!