Just seen this on the internet is it true? Yes or no?

[John77]

No, you generally cannot drive a Mercedes-Benz petrol PHEV (Plug-in Hybrid Electric Vehicle) if the main high-voltage (HV) battery has completely failed or requires replacement. [1]
While you can drive with a depleted battery (where the car reverts to using the petrol engine and regenerative braking), a dead or malfunctioning high-voltage battery triggers a few critical issues: [1, 2, 3]

• No Starter Motor: Modern Mercedes PHEVs do not have traditional 12V starter motors. The petrol engine is cranked by the high-voltage electric motor. Without the HV battery, the internal combustion engine cannot start. [1, 2, 3]
• Safety Lockout: When the vehicle's diagnostic systems detect a catastrophic failure or zero voltage in the traction battery, the car will prevent itself from starting or going into gear to protect the sensitive electrical components from further damage. [1, 2]
• 12V System Drain: The high-voltage battery is responsible for keeping the conventional 12V battery charged via a DC-DC converter. If the main battery is dead, the 12V system will eventually drain, leading to the loss of dashboard displays, power steering, and braking assist. [1]

 

[SL222]

It sounds very plausible based on my wife's Toyota which was a self charging hybrid but it was rubbish at charging the 12v battery. That meant it went flat frequently and the whole thing ground to a halt. Not exactly the same but the electronics are so complex and once you get "computer says no" you're stuck.

We sold it by the way

 

[rf065]

Not sure about this in all cases?
Watched a you tube video where they bought a non running PHEV (Audi) and it had a high voltage starter/generator and a normal starter motor. The system worked using the starter/generator most of the time, when the car was warm and air temp warm etc, but on cold starts when the car had not been used for some time, the normal 12v starter was used.
Regardless of that, the system was explained in great detail and the only take away was, why make things so complicated and hard to work on when they are only going to fail at some point anyway?