Try this…. The displayed charge level of
@EdG's 2014 i3 BEV with ~45k miles whose 12V battery was replaced within the past 6 months frequently decreases 10% when parked overnight. The 12V battery clamps and terminals are tight. It is set to charge immediately with no departure time or preconditioning set. Daytime highs are ~80 ºF and nighttime lows are ~70 ºF, so not much day to night temperature decrease. With the HV disconnect disconnected, the displayed charge level decreased ~1% per night possibly suggesting that when the HV disconnect is connected, a HV component is turning on the HV system which is somehow consuming considerable power. However, ISTA reports no stored DTC's. During one charging session in which the energy added was recorded, the displayed 55% charge level increase was appropriate for a battery pack with ~14 kWh of usable capacity. This i3 hasn't been driven much in the past few months which might explain the low resting voltage of its 12V battery, 12.0V. If the 12V battery were bad, I would think that spurious DTC's would be stored and that the displayed charge level would not be affected.
Your thoughts?…
I love that you believed my tongue in cheek claim to know everything and happy to comment to your very clear question.
I don't know the answer to your problem, nor to my UK car REX problem either. Our REX does not perform as I believe it should either. It's incapable of Holding Charge on any decent journey and slowly withers down towards zero so I give in and plug in on the Motorway service
The London BMW Service people think I'm just a misguided nuisance. I'm not. I recall it used to perform very different a couple of years ago.
However this is not your problem. Forgive me.
I recall the i3 Battery we have to change so often on our cars usually reads 13v to14volts. I wonder if yours might be inherently faulty and therefore affecting the HV.
We have suffered bad charge for wrongly setting the vehicle own dash controls to delayed charge, to heater, to alarm etc all of which work just they please the i3 and not the owner.
So far we have suffered Alzheimer key on low battery i8 which lost key to car connectivity
We have lost 12v on i3's so many times usually without warning. Won't start, won't run etc
The HV has not been a trouble save the distance calculator will give a totally different range from last night to this morning.
We just use this as a guessing guide. So i3S off the charger might say 220km range but by the next traffic lights it is suddenly just 170km
Yet my 2014 i3 has max of 105km but rolls home from town with still 55km on board.... everyday
SO SORRY, I GUESS I DON'T KNOW EVERYTHING BUT WE MANAGE WITOUT THE RIDICULOUS ******** THE DEALERS SEEM TRAINED TO RESPOND
After trying everything that I could think of, I thought that I had finally found the solution. 
