Bringing Dead i3 back to life

This is similar to another post I saw but with one twist. 12v battery died and car was completely non responsive (power door locks not firing, etc). I decided that the easiest way to get the car back up and running was to buy a new 12v battery and get the car's electrical system working. I found multiple videos on youtube on how to do the change. All was going well until I went to reinstall the battery terminals on the posts. After connecting the positive terminal, I went to connect the negative and sparks came off the negative terminals. Disconnected and tried the negative terminal first and then the positive -same result. If I hold the terminal close, the car electrical system starts (fan, head lights). This was a bit scary as I was not expecting and not sure how to connect the battery? Is there a disconnect for the 12v so that the repair can be completed? Help...

I'd expect that. You could open the high voltage disconnect (probably should, if you haven't), but I don't really think that's the problem or a solution.

Put on rubber gloves if the sparks are making you nervous, and get that clamp on there with no futzing. As soon as it's making solid contact the sparks will stop and you can properly tighten the clamp.

A small amount of spark is normal, You are charging up a good amount of circuitry, including capacitors. If you have a voltmeter that also measures current (ammeter) you should see the current tapering off after you first connect it. Keep an eye on the voltage if you have reason to think it is being drained. Keep in mind your car can't charge it's 12v when your high voltage battery is disabled.

Confirming that it worked and followed the advice. I will share with others that the car will light up and turn on for a few minutes after reconnecting the battery.