The Model 3/S/X use a dedicated 12 volt battery for powering of the onboard electronics, even though have 12v DC to charge that battery from the main battery.
For high and irregular current draws, they will need some means of supplying that power on-demand at the proper voltage.
Imagine that you have a load that uses 3 cells and suddenly changes to need 200 cells. The charger cannot anticipate the load.
So my guess is, pending some ingenious solution, another battery or supercapacitor.
Which works when you have the entire grid as a sink, but taking one half of the circuit would unbalance a charger being run in reverse, no?
An bi-directional inverter charger can do voltage, current and frequency matching on the fly. In fact any solar inverter needs to do this to "follow the grid" otherwise it would be disconnected by the protection circuits. I'd imagine they'd tap the main battery, and not use another battery that can't be used for driving, for any significant loads (over 500W) and not use the 12V circuit for that. In an effort to reduce complexity and cost, I'd imagine that both the battery charger and inverter, and potentially even the solar regulator are integrated into one unit for the CT. That way all power flows can be managed as required.
As for "balancing" I'm assuming that means load balancing across the inverter circuits? That shouldn't be required with a split phase setup.
Technically, if they wanted to, which they won't, they could even integrate a MIG/TIG/MMA/Plasma along with a high current DC charger into the same system. You could then weld and cut steel straight on the CT! I think I saw someone add a CNC router/cutter/3D printer to the truck bed somewhere. Make that a MIG 3D printer with CNC so it's a car that makes a car...reproductive technology!
(Welder/alternator/generator for ICE: http://www.unipower.com.au/download/Unipower185_Brochure.pdf)
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