2015 Santa Fe Sport completely dead. Even connected to charger does not register.

What exactly are you saying here? That the battery is completely dead and not registering on the charger, or that the car is completely dead and shows no sign of power when hooked to the battery and/or charger and you turn the key or try to use any of the electrical accessories in the car? Please clarify so we can properly help you.

Battery is fine. Santa Fe completely dead. Nothing works. I know it's under warranty. Bought new three months ago and have 3100 miles on it.

OK, Rick, no problem. Open the hood and locate the underhood power supply, like a big fuse box. Open the lid, and first check ALL the fuses in there. There should be some that are quite big. There may also be a fusible link or 2 located in there, one of which may have two threaded studs with hex nuts on them or just hex bolts. There may be what appears to be 2 battery cables on the studs. If there are, one will come from the battery and the other will go out to the starter system. I believe there is a piece of metal joining the 2 studs & that could have melted during an electrical over load. I am not certain if your car has them, but a lot of cars do. If there, put positive lead of a test light ot meter on the one from the battery and the ground lead to any ground and see if voltage. Do the same for the other side. That will tell you if you have power into the box and power out of it to the starter. If you have power in but not out, the link has melted. If you have power out but not at starter your power loss is obviously somewhere between the supply and the starter. This is just a place to start. You could also have a fault in the ignition system itself, but if you did, at least the accessories in the car should work, at least all of them that are not run through the ignition switch.

I just replace the battery in my 2016 Hyundai Santa Fe Sport. I went to start it and it was totally dead. There were no lights on the dashboard nothing it was as if there was no battery in it I checked the battery and it was charged since it was brand new. I thought it had to be something electrical and the ECM or some computer chip of some sort just out of frustration I looked at the fuse box and I saw an on off switch. Maybe this is something that is standard with a Kia or a Hyundai but I had never seen an on-off switch before I turned it on and everything was fine. I hope this helps somebody in the same situation. I was just lucky to find it