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OK my five cents on this topic.
If you really want see what is going on i propose to install a 12V bluetooth battery monitor device direct at the 12V battery.

Reason: The cigarette plug is only powered if you switch on ignition. By this also the DC/DC Charger of the system will start and you will only see the 14,5V from the charger and only for short time the state of the 12V battery.

Additional it is totally different of behaviour to a classical combustion car. There the 12V get charged only during engine is turning.
On the Smart 451 ED3 the only source for recharging is always the HV battery via DC/DC charger after the main switched (contactors) of the HV battery.
Means independent of the state of the 12V battery the charge and energy will always be provided by the HV battery.
Only the necessary amount of charge depend on the state of health of the 12V battery. Also the internal resistor of the 12V battery will need additional charge.
Worst case is a broken shortend 12V battery which will empty the HV battery or short repeated charging cycles which will trigger the protection function to stop the recharge and cause to trigger the kill of 12V battery which trigger a damage path inside the HV battery killing the HV battery in parallel.

So come back to the 12V battery monitor supervisor device. This even allow you to see the state in the off state as it is transmitted via bluetooth to your mobile, where you could see any changes of the state of charge and some times also the state of health. Better devices even allow to send you a trigger for check your car and battery. Means if the 12V battery is falling below 11,7V you should react fast not to run in killed HV battery case.

There are also some other nice solutions you could install on the maintenance OBD2 connector but this could cause trouble with the service center in case of malfunctions.
 
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