I have spent about a week and several hours trying to figure out why a 50 amp hour Lithium Iron Phosphate battery, even when fully charged at install time, gets a low voltage condition and pops up the "12 volt battery must be replaced" error within a few hours of installation, whether you are driving or not, and even if the car is parked AND plugged in!!!
After going through the full cycle of three times of fully charging the battery, shutting the car down, disconnecting the high voltage, and then reconnecting the fully charge lithium battery - and then watching carefully what happens, I can say with 100% certainty that this is a Tesla software problem - and it really looks like they are purposely draining the battery to cause the error condition!!
Here's the details.
1) I purchased a 12.8 volt 50 amp hour battery. I have fully drained it and fully recharged it three times. I've recharged it with two old standard 12 volt battery chargers, and in with both chargers the battery ends up fully charged and at around 13.8 volts.
2) If you power down the car, disconnect the high voltage, connect the 12v battery and turn on the high voltage again, the error message will be cleared and it looks like everything is good.
HOWEVER, the first time I did this, I went on a two and half hour drive and about two hours into the drive the same "12 volt battery error came up" . I finished the last 30 minutes of the drive and immediately turned on service mode on the car so I could observe what was going on - and I was surprised to see that the car was pulling 20 amps continuously out of the battery, even when parked, and the voltage was still at 12.8 volts. HMMM, why is it pulling such a high current out of the 12 volt battery when there's still lots of power in the main battery. So while watching what's going on, I then plugged in the Tesla charger and continued to watch. AND EVEN WITH THE CHARGER PLUGGED IN --- IT CONTINUED TO DRAW 20 amps from the 12v for about another hour (at which point it had been pulling 20 amps for about 2.5 hours). Since the battery is 50 amp hours, drawing 20 amps for 2.5 hours is all you're going to get from the battery, at which point the voltage dropped to 12.3 volts, and thankfully the Tesla stopped pulling 20 amps and the battery just sat there at 12.3 volts. Now this is with the CAR PLUGGED IN AND CHARGING. It WILL NOT CHARGE THE 12 volt battery, even though I left it overnight charging. The high voltage completely charges, but it refuses to charge the 12 volt battery. THIS IS CLEARLY INSANITY.
3) So, being stubborn, it was about 4:30 in the afternoon, I disconnected the 12volt battery from the Tesla and I plugged in a standard 12 volt battery charger that charged at 5 amps. Starting at 12.3 volts and 6 hours of charging, the battery was back to 13.3 volts after about 6 hours and I left it charging overnight. In the morning, the charger was reporting it was fully charged and with the charger disconnected the voltage showed 13.85 volts. So I went through the routine of reinstalling it and power cycling the Tesla, and just like the first time I did it, the Tesla powered up and the error cleared.
4) This time I just sat and watched what the Tesla did. The battery is at 13.8 volts. The Tesla pretty quickly tries to charge the 12v battery, and it quickly climbs for 13.8 volts to 14.8 volts (which is the maximum charge voltage allowed by the 12v battery BMS). At which point the charging current goes to zero and the charging stops. A few seconds later the Tesla puts a 17 amp load on the battery and the voltage quickly drops down to 13.8 volts, and then the Tesla stops discharging the battery and tries to charge it up again.
5) Same pattern repeats.... and repeats.... and repeats. I'm estimating about 20 times the battery is cycled between 13.8 and 14.8 volts. After which it quits trying to charge it higher. After watching it another 5 minutes or so I quit watching it. And started doing some other things. The car was just sitting in the garage doing nothing and it was plugged into the charger, but not charging because it was fully charged to the limit.
6) A couple hours later I came back to check on the car. And even though the car was sitting there plugged in - when got in the car -- the error message had reappeared again!!! And when I looked at what going on with the battery the Tesla was doing the 20 amp draw down again (even though plugged in to the Tesla charger) but the voltage was still a healthy 13.0 volts. After another 45 minutes of 20 amp drain the battery is still holding at 12.8 volts. But I know it's going to keep pulling a constant 20 amps until the battery is at 12.3 volts. THIS IS WHILE THE CAR IS PLUGGED IN. THIS IS INSANITY.
This is clearly either a really poorly written software, or they are purposely killing 12 volt lithium batteries. The behavior of pulling 20 amps for 2.5 hours EVEN WHILE PLUGGED IN just makes zero logical sense that this is just a bug. It's 2024, the battery I'm using is an awesome battery and is most definitely 50 amp hours of which 95% of the time the voltage stays above 12.8 volts, and a Tesla Model 3 doesn't handle it - even though every other car I own and have switched to Lithium works just fine??? Tesla - this is clearly a software problem that needs to be fixed!!! The way the car just one day pops and error message and is immediately degraded and you can't charge your phone battery is AWFUL PRODUCT DESIGN.
Any other thoughts on this?
