My 12V battery on a 2017 i3 REX died in the parking lot at work after being parked for 10 hours (we work 4 X 10hr day shifts). I had gotten a warning message over the weekend with the car plugged into a 10A 110V level I charger all weekend. It was saying something about excessive discharge, but wasn't specific about which battery. It meant that the 12V battery level was decreasing while parked. Once you see that, replace your battery that very day... or else.
I had to jump start the car to get the electronics to power up. Once they came up, I had one trip until it went dead again, so I called a local dealer, and drove directly there once boosted. They had me park it in the lineup to the garage, and shut it down. They had to boost it again to move it to another location, and again to put it where they were going to work on it.
If your battery is VERY dead, even using a 12V pack to jump it may not be enough. You may have to also disconnect one of the regular battery terminals and keep it from shorting out. VERY dead batteries draw a lot of current and do not charge up to 12V.
Jump starting for me involved taking out the frunk and getting to the battery terminals in the front. I left it out when driving to the dealer, and let them put it back, as it has to come out to replace the battery anyway. Don't lose the screws. I put mine back into the holes they were in (finger tight only) after removing the frunk so I wouldn't mix them up or lose them.
The terminals in the back are easier to get to normally, but smaller and closer together, and under all the stuff I had in the hatch area. In either case, you are better off using a booster pack that has smaller size alligator clips than the standard size used on normal cars. There is very limited space if you try to get to the terminals in the front, so clips that are no more than 3 inches on a side are better, and small hands. In the back you need small light clips so they don't touch each other or anything else because the terminals are close together.
You do not need to disable the HV battery to bump an i3, in fact, you cannot bump an i3 without the HV system active. The DC-DC runs off the high voltage system to produce the aux power and recharge the battery. If the HV system is disabled it won't come on, and as soon as you disconnect the jump start pack, the car will die again. I don't think you can bump an i3 by plugging into the 12V cigarette lighter ports like you can on ICE cars. Those are powered by the DC-DC converter, and not reversible as far as I understand. It is too bad, that would be a quick, simple, and safe way to bump the car if it worked.
I mention the HV disconnect only because it was listed as a necessary safety step for changing the battery yourself. I tried it, and it is MUCH easier to disable the HV than to get it re-engaged. You just pull a tab up. There is a trick to it, you can't just push it down, you also have to push (tilt) it or pry the casing to just the right angle for it to re-engage. It took me hours to get that to re-engage (standing out in the rain in a parking lot) trying different prying points and angles and just pushing harder. Once it closed, I didn't open it again to figure out exactly where to press to close it easily. The videos I saw showed it opening, but did not show it being closed. DO NOT DISENGAGE THE HV SYSTEM UNLESS YOU KNOW HOW TO RE-ENGAGE IT FIRST. Make sure you watch the video, listen to the description and review multiple videos or writeups on this before you try it. Have them queued up on you phone before you go out to the car so you can review them while trying it. I am sure it is easy once you know exactly how to release the catch, but if you are working in limited light on a cold rainy day in a parking lot, you may end up getting frustrated enough to damage the switch, your car, or yourself.
I didn't replace the battery myself because I was 320 miles from home staying in a hotel on a business trip, and could not locate anywhere but a dealer that had a battery. They sold the one they had before I got there the day I brought it in and had to give me a loaner for a couple days until they could replace it.
The all the writeups mention pre-charging the battery to 100% before installing. That is not necessary. The dealer took a battery out of a box, installed it, and started my car and left it "running" in the parking lot so it would charge itself. Basically put the car in park, press start without your foot on the brake, turn off lights, radio, etc. and it will sit there with the DC-DC operating which charges your battery. I have done that while charging so I could listen to the radio while waiting 2 hours for a free level 2 charger to put charge on my car before. You may have to come back and re-start it as it times out eventually. The dealer left my car parked outside like that long enough for a whole lot of condensation to form on the inside of the windshield.
There are also videos on using the 12V system of an i3 with a 1200W inverter (roughly equivalent to one 110V 10A outlet) to run some AC essentials in case of a power outage, and they use this trick to keep the DC-DC going. Don't do this from the driver seat, as soon as you unbuckle the seat belt and open the door the car will stop. Lean across from the passenger side to press start so the door stays closed. You might also need to plug in the driver's seat belt.
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