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There I was running out of range my GOM gone green to burnt amber and eventually multicoat red and I wondered what phrase you would use that is equivalent to the ice phrase "running on fumes" when the tank is empty?
Technically you are always "running on Electrons" so you might be better with running on Protons or running on neutrons but that's a bit geeky.
So I like AnthonyLR's "Running on Static" and if you are you probably need a quick "Halt and volt"
For a lot of electrical equipment, you could say "running on what's left in the caps" (capacitors), but as a Tesla's inverter(s) run directly from the DC batteries, I'm guessing they don't have smoothing caps.