DEAD I3 $10,000 PROBLEM

First i am 75 years old. 2014 i3 electric only. 32000 miles.
Issue the car would not switch on no lights nothing so to you tube new 12v battery purchased $200 and change.
fitted the battery started then died but was able to shift to N and release hand brake.
Flat bed to BMW Lakeland Florida.
first they said possible battery pack. Then no not battery pack possible two electrical boxes that would cost $10,000 dollars
Now i am waiting for a BMW field engineer to look into the issue.
Any thoughts anyone?

without error codes hard to guess.
Yet, since the car is around for a while - you can get basically all modules (electric boxes) used from various part sources.

So even if they quote you XXX for a new part - you can get the used replacement part for a fraction of the cost.

Further - you got a still 1 year of battery warranty left - 8 years total.
I would ask for repair under warranty first. Because some of the modules - are inside of the batterie - and so they are covered by that warranty.

Was the replacement 12v battery fully charged when you did the swap? Several people who did their own 12v battery replacement received batteries that were not charged up from the factory. Mine came at 22%.
Put an almost dead 'new' battery in an i3 with an already bad 12v battery, and it really doesn't like it - and will be a real drama-queen, throwing all kinds of strange unrelated phantom/false errors. I'd ask the dealer to test the 12v battery for charge, just to make sure it isn't' the culprit.

MKH is correct... when changing the auxiliary 12V battery, it should be fully charged before installing.

Are you the original owner of your car? If so, (and particularly if you have owned other BMWs before) making contact with BMW NA and expressing your concerns may be helpful.

They are aware of an owner's history and tend to be responsive to direct (especially postally-mailed) communication.