[felixb.neto]

I'm not a specialist but I have been working a lot with the electrical system of this vehicle. The "Built-in System Interface" (BSI1) shown to be losing connection is that fusebox to the left, below the steering wheel. That fusebox is connected to the ignition system, lights, radio... So, if it goes down, all the things that you mentioned are non-functional anymore. Ultimately, the cause of those issues seem to be that BSI1 loses power, in other terms, voltage gets to 0v. A short-circuit, a broken line that connects intermittently or a relay could be at fault. So let me start from the battery:

1. You told that the battery is fine. Do you need to recharge after the car dies?
   1. If yes, we have to take a look at the alternator. Maybe it is stopping to work at some point.
   2. If not, which I think is the case, then we move on.
2. Are the battery terminals corroded? Are they positioned correctly? Are all the bolts correctly placed on the terminals (tightened)?
   1. Just checking
3. Above the battery, there is a fuse box. On that fusebox 2 wires go to the BSI1 connected to the F2 fuse and the F4 fuse. So, F2 goes straight from the positive of the battery to BSI1. It is used for all the things that need power all the time (regardless if the car is on or not). The F4 has a relay before it and is only powered when you turn on the car with the key.
   1. We could check a few things here. First, check fuses.
   2. Second, if you have a multimeter, next time it shuts off, check the voltage at F2 and F4. That will help us determine where's the fault. I believe the problem is here and it will be F2 = 12 to 13v and F4 = 0v. If that happens, the relay could be the issue. Relays are sensitive devices and break with time. If you are lucky, you may be able to put it back to the fight by giving it some taps, and trying to turn on the car again.
   3. If both F4 and F2 don't have voltage, something is wrong with the battery fusebox or the battery cables and terminals.
   4. If both F4 and F2 has voltage, then it means that the problem is after the battery fusebox. That could be caused by a failing wire (shorting to ground or severed), something that is shorting on the car, or a relay on BSI1. Then we need to investigate way more stuff.
4. There is also a device in the car that has the task to manage the battery. But on that, I don't know its inner workings. If we reach this point, I can study a little bit more.

Let me know the results of your investigation and we keep brainstorming.

 

[felixb.neto]

Thanks for the reply.
1: No need to charge the battery, just disconnecting/reconnecting.
I have cleaned both terminals of the battery, but it's strange as it does the same with engine on, so the alternator should take the load.

3: I will try to replicate the fault and start measuring, i do have a range of voltmeters as i am an electrician myself, but automotive it's so different lol

You say f2 would be 12-13v and F4 0v, wouldn't that mean that i will have power delivered to something even tho f4 is 0v?
I repeat, nothing has power, i have tryed every single button and function.
I will measure everything you mentioned and hopefully i will find something useful.
I will comeback after the measurements to update

Yes, there are things that are connected straight to the battery positive terminal. Those things are not apparent though (like lights or buttons) and its hard to determine what should function after a failure like that. There is a lot of electronics on every device, so even if something is powered, it does not mean it is going to function.
If the problem is not on that relay, then it could be on the voltage retaining device. I would need some time to investigate that myself in order to help

 

[felixb.neto]

That's brilliant! Awesome job with the investigation and good to know that the fix was for free. You could bet that if the car landed on a shop they would charge big money for this.