Weird 12v/ Electrical Issues

[stutech]

Hey everyone I am having kinda a weird set of issues with my 2019 Sr+. So at first I thought my 12v battery was bad as I got the typical 12v failure messages such as can't support all electrical features etc. I replaced the 12v with a brand new agm type battery from O'Reillys that has the same cca as the OEM and same size. I was out of warranty by 100 miles and the Tesla service center is 140 miles away, so didn't want to drive there to get an OEM one. Everything seemed fine and working great for about a month then weirdly I noticed software updates kept failing. I tried about 5 times and it just kept failing, checking service mode it showed some odd errors after the failed updates doing a software reinstall from the service menu fixed it back to normal, but new updates kept failing. Interestingly the system would show a 12v failure after the failed updates, however testing the battery it was fine each time. Doing a vcleft front hard reset from the service menu and doing a software reinstall of brought it back to life and then all errors went away. I later discovered it was my sub woofer install causing high startup current and triggering the 12v errors during update, so I left it disconnected and then disconnected anything aftermarket to rule out issues from accessories. After doing this software reinstalls then worked without the 12v death errors, but regular updates continued to fail. At one point it logged some error about contactors not opening intermittently on wake up after one of the failed updates and also a mysterious coolant level low, but only showing in service screen not regular ui. I measured the pcs output and it was charging the 12v just fine as everything else was working as expected, so it was odd and I just kept driving the car as normal. Fast forward another month or so and yesterday my car said dead 12v randomly and then I put a booster on the 12v battery and it then said it was opening the contactors in service menu but no click or anything. It did this a few times then said bms err hv. Interestingly it also showed the pcs as offline when the contactors were stuck open. I had contacted Tesla about the issues with updates previously and they had said something about the pcs not communicating intermittently, so it made me wonder if that's why the contactors weren't opening. I then just removed the 12v and had disconnected the penthouse connector and let the car sit for a couple hours. I came back put a new 12v in after reconnecting the penthouse connector and to my surprise I heard the contractor clunk. The car came to life and drives and operates with no errors. I'm not sure what to do now as the car terrifies me that it will randomly do stuff that could strand me somewhere. Tesla service is quoting me 3k to replace the pcs, but it's acting more like the contactors are the issue and randomly not opening. Has anyone has similar experiences or any idea what could be happening?

 

[ATPMSD]

A couple of thoughts. First nothing, absolutely noting, should be connected directly to the 12v battery. This can cause all kinds of problems. Second, is the replacement battery the same voltage as the original 12v? For example, if you replaced it with a Lithium perhaps it is running at 16v, this is also something that will cause problems.

 

[Birdsfan]

It may have something to do with the AGM battery vs the standard lead acid battery your car came with. It also could be tied to a software update that changed something in the car and now the new battery is confusing it. The li io replacement batteries you can buy struggle with this. Mobile service can replace your battery for a little over $100, I might start there.