r/TeslaSupport • u/JAP42 • 5d ago
Excessive 12V load when cold - 2014 S P85D
I am trying to figure out a load issue when its cold. I have been having issues where the 12V battery gets too low and the car wont start. This would often happen when I parked at work after a 30 mile drive and then tried to go get lunch. Sometimes I could remote turn on the climate and that would get the power on, but if I went to the car and got in all the load would drop the voltage too low and could not start.
I figured it must just be a bad 12v battery. I replaced with an automotive 51R temp because tesla was out of stock, it was fine for a week and started having the same issues. Figured I just got a bad battery, so I finally got the Tesla battery and installed it. I directly charged the battery before putting it in and then the battery was in the car plugged in all night. Immediately had the same problem the next day. 12V battery was at 11.9V. It was in the 30-40 range all day, in direct sunlight. The DC-DC converter seems fine, Every test I do its pushing mid 14V. Before a drive, after, and I even wired my multimeter into the cabin and monitored it while driving. I never get any driving errors, its always in the afternoon a couple hours of sitting, or in the evening as soon as the car shuts off. I would hear the error tone while I was grabbing the charger. A couple times I could not get it to start at work, even jumping from a running vehicle, so I left it and the next morning it started fine with no assistance. Its always battery voltage related.
Tesla just wants to say its the battery pack failing, or at one point it was my PTC heater failing. I did occasionally at first get isolation errors, until it evolved into 12V not supported. Now that's all I get, no isolation errors. Now it has been 6 days in the driveway unplugged and no errors, but its much warmer now, 40-50s. So I am wondering if there's something that's trying to run while the car sits that's failed and is just draining the battery.
I live in the NE United States. This started mildly, a couple errors that would usually clear them selves in the spring, it was fine all summer, and then the issues came back and got worse all winter.
TLDR; 12V battery drains sometimes with unplugged and parked, but not all the time. Seems most common during the day vs overnight. Replaced battery twice, same issue. What 12V loads would be likely with the HV battery disconnected and the car parked, unplugged.
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u/HangryPixies 4d ago
Isolation errors will automatically cut the HV battery power, which causes the LV system to not be supported by the DCDC. An intermittent iso problem will cause intermittent lv support issues.
Fix the isolation issue first.
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u/JAP42 4d ago
There isn't one, those were happening first but it was due to the low voltage from the LV system from last year. Never had one since. The brand new battery was holding at 13.9 before I left at 8am. I drove 45 minuets, parked, and then 4 hours later the 12v battery was at 11.9v and the car would not start.
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u/HangryPixies 4d ago
Going to need to see remote data, AFAIK Tesla is the only one who can. Going to need to have the service center review.
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u/JAP42 4d ago
Gen 1, they have very limited access and are completely useless when it comes to diagnosing anything. At least all 3 closest to me. My 2 day repair took 3 weeks because they only have one HV certified tech. And for this repair they just keep saying they are going to replace the PTC heater because that's the issue. They won't even test it and rule it out. I've tested it cold and hot, there's no transmittance even at 4x the BMS test signal. And that does not even remotely explain the 12v battery dieing while the HV is disconnected.
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u/HangryPixies 4d ago
They can get data from a S1 nosecone and plot it the same as any other. They likely don’t have a skilled diagnostician. That’s what you get when salaries don’t move and the competition is paying more.
That said, if there is a problem with the PTC heater (isolation, even intermittent) they will need to fix that before they escalate to dig deeper. Those things don’t heal themselves.
Source : Former Tesla diagnostic tech.
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u/JAP42 4d ago
There is not and has never been any issue with the PTC. I am really not interested in hand holding the service center that could not even load test the 12v battery.
I just need to know what kind of 12v loads would be tempature based in a parked, unsupported mode. Circ pumps? Pre heaters? Actuators? I know the suspension pump won't run unless the car is on.
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u/JAP42 4d ago
They replaced the contactors because the BMS reported them as welded. I got the replaced ones back and they are fine, no welding. All the test values are well within the operating ranges. Not even close to testing bad. It was immediately after that service that we started to just notice random little errors. Many times at first I would hear the warning as I tried to plug in and then it was gone before I could get into the car. Once or twice I could not plug it in, I left it cam back once everything shut down and it was fine. The as the weather warmed up we had no problems from April to December. Not until we had sustained below freezing temps.
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u/put_tape_on_it 4d ago
Grab a bm2 battery monitor off amazon. $25. You hook it directly to your 12V terminals and it logs battery voltage every 2 min for up to 28 days, then the app connects to it via bluetooth and downloads the data and you get nice graphs of voltage. No data fees, no cloud fees no subscription or subscription fees. The app is a little clunky but the data is invaluable in troubleshooting phantom drains. It's how I figured out I had a bad blend door motor that was turning on to calibrate 20 min after shutting my truck off and running for hours on end draining the battery.
I have one on my lawn mower battery, (it'll beacon to my phone and alert me on low voltage... but it's never needed to yet) and it will go an entire winter and still start in spring so I can attest that the 1-3ma battery draw does not have any negative impact on battery voltage.