Backstory: My battery died out and I am seriously low on cash right now so I couldn't replace it. My parents dug up an old battery but it was oversized and did not fit properly. I did not want to use it but there was no other vehicle and it was almost time to pick up my son from school. I picked him up and when I got home I had company waiting and completely forgot about the battery in the car. I mainly use it to go grocery shopping and taking my kids to and from school, but this was on Friday and I ended up not driving it for 2 days until Monday. But on Monday I used it heavily, went grocery shopping and ran errands, I had completely forgot about the oversized and incorrect battery in my car. When it came time to pick him up from school that afternoon the car would not start. Only the airbag light was dimly light, I finally remembered about the oversized battery and opened the hood to find the rubber protector from the positive side of the battery was completely melted, it got hot and it was close to the hood so it melted and then it burnt through the mat on the inside of the hood until it reached the metal.

My guess is the contact with the metal and the positive side of the battery short circuited something. I thought maybe it fried the battery so I got 2 different batteries that fit and I know work (I borrowed them), and it was the exact same issue, a dimly lit airbag light only. Everything is completely dead, nothing works, when I turn the ignition on the only thing that happens is the airbag light turns on dimly. I tried to check all the possible fuses and they all seem to be fine, I have tried looking online and found nothing. Its officially out of something I can fix on my own, I am more of a tech guy, but I change my own oil and fix anything that has come up wrong with my car on my own. But at the moment I do not have money to have a mechanic look at it so I am turning here to see if anybody could have an idea of what could be wrong or what I can try on my own. I have a OBD II tuner where I regularly check anything that comes up, but I have not used it now because I didn't think it would work since the car seems completely dead besides the single dimly lit light. But any ideas or suggestions you guys can come up with would be greatly appreciated.

Car Information.
Year: 2008
Engine: Standard V6
Style: Premium Coupe
Color: Metallic Grey

 

I fear that you may have completely fried the ECU.
Might be worth using your handheld tuner to scan it and see what you get.

 

I would first have your alternator checked.

 

I hardly think the alternator's the main problem when the OP has tried two batteries that he knows to be good, and the car's still as dead as a dodo.
If he does somehow manage to get the engine running and the battery voltage still remains low, a bad alternator voltage regulator could then be considered a possibility.

 

I only suggested that because mine wacked the rectifier bridge and when that happened it took out a brand new battery with it. Totally different symtom then a bad voltage regulator.

 

I apologize for the late reply, I am a single father so weekends are very busy for me.

Is there a way for me to check wether it's the ECU or not?

i actually thought the alternator might be a problem. Before I was getting the classic symptoms of the dimming headlights and occasional flickering of the dash too. I was going to rebuild the alternator but since I've been struggling financially I put it off a couple of weeks, until this happened.

But everyone I've talked to have told me it should still start or the lights in the clusters should flip on or something with the good batteries connected. All 3 batteries (including the 2 I swapped out I know are working and good) resulted in just the dim airlight bag. That's it, no other activity whatsoever, ive tried jumping it and swapping good batteries and basically checking every fuse in that car, they are all fine.

 

Check the battery NEGATIVE cable. It had to carry all the short-circuit current back to the battery. Likely the terminal end down at the starter, where you cannot see it, is burnt nearly off. If that end seems OK, go upstairs to the battery negative terminal, check wire as it enters the lug. imp

 

thank you, I will check the negative cable and update on the findings

 

Now that you've provided us with this additional piece of information, it seems that you do indeed have an alternator problem.
However something else is going on, and I think that imp made a good call about checking the positive and negative battery cables & connections.

 

yeah I just figured the shortage maybe finished off the voltage regulator in the alternator and its why it was dead. But I did think the other cluster lights or sound system would turn on with the known good batteries. I do still need to replace or rebuild the alternator (I'm hoping rebuild).

But i think imp was right on the money. I checked the negative ground and it's completely burnt and broken, and the plastic covers in the cables all melted off and were stuck to the engine block. So I'm positive that is the current issue. Thank you guys for helping me figure this out!