Adding AC charging to replace the last third of Supercharging

Keep in mind that the superchargers already are pushing the extra un-used capacity of one stall over to the other stall if the other stall can use it. Superchargers are just multiplexed 10KW chargers, the same ones that are already in the car. Having the supercharger switch over to dispensing A/C and using the cars onboard charger would simply add complexity. The overall supercharger site capacity is still a limiting factor regardless if it is using the superchargers' chargers or the ones onboard the car.

wcalvin said:

When it falls to 10 kW (or 20 kW with dual chargers), why not switch to AC and use the car's built-in chargers? This would free up the 120 kW capacity of the fast DC inverters route to use for another charging stall. So instead of one charger splitting its feed between two stalls, it could do 3-4 instead.

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You can do it, but it's a lot more complexity for not much gain. Inside the superchargers you now need a switch between the AC and DC supply to the connector and the superchargers/Tesla's charging protocol will need to handle mode switching on the fly. This is further complicated by the fact superchargers take high voltage (380-480V) 3 phase power which is not what the on-board chargers can handle (European cars excepted).

The things limiting the usefulness of this is that there's a limited amount of AC supply that's supplying the superchargers, so switching to AC does not necessarily make the station as a whole be able to provide more power. The other thing is that for people without dual chargers, falling to 10kW might happen only for a few minutes (when it's near 100%), even the 20kW might still only be for a few minutes. Many people don't wait until it reaches that point.

For those that really want to charge to 100%, yes it would make sense to have them move their car, free up the Supercharger for the next person and finish off charging at a normal AC charger. In reality no one wants to do that. It's already inconvenient enough having to stop and wait. It would add another inconvenience having to come back to your car and move it. The best solution is to have more Superchargers so no one ever has to wait. The whole point of these is to charge as quick as possible. No lines, no waiting, no moving cars.