Batterty down to -5%

[David99]

Had an interesting experience today. For a video I drove the car down low to the point it would shut off. I charged it up again and arrived at 2%. I didn't plug it in right away as I wanted to wait for the cheaper off peak electricity. The car was parked for about 2 hours. When I came back I saw an error I had never seen before.

The battery showed 0% and zero miles left. It would turn on and drive, though.

I looked at the CAN bus data and it showed the state of charge was -5.38%! The car doesn't show a negative value but the BMS obviously counts negative. You can show it in the car if you use the trip graph when using the navigation. It shows -5% as the starting point!

The odd thing is that when I parked the car, it showed 1-2% battery remaining. While it was parked the BMS got somewhat confused. The battery voltage didn't drop. It was at a reasonable 3.2 Volt which is perfectly fine as a resting voltage when almost empty. I'm not worried. Looks like the BMS just got a little confused. I'm charging to 100% tonight and I'll see what it shows in the morning. My car has never done this before so I'm sure it had to do with the experiment of driving the car down to the point where it shuts down. Either that confused the BMS or it recalibrated it and tomorrow I will have 5% extra range that was recovered LOL

 

[gjunky]

Why would you let it go this low? It could get you stranded or damage the battery. Either way, not a positive outcome.
Truly curious.

 

[David99]

Why would you let it go this low? It could get you stranded or damage the battery. Either way, not a positive outcome.
Truly curious.

It was planned and I had charging available immediately. There was no risk.

 

[gjunky]

Ok. Not worried about possible battery damage with the voltage dropping that low?

 

[hiroshiy]

Had an interesting experience today. For a video I drove the car down low to the point it would shut off. I charged it up again and arrived at 2%. I didn't plug it in right away as I wanted to wait for the cheaper off peak electricity. The car was parked for about 2 hours. When I came back I saw an error I had never seen before.

View attachment 333260

The battery showed 0% and zero miles left. It would turn on and drive, though.

I looked at the CAN bus data and it showed the state of charge was -5.38%! The car doesn't show a negative value but the BMS obviously counts negative. You can show it in the car if you use the trip graph when using the navigation. It shows -5% as the starting point!

View attachment 333261

The odd thing is that when I parked the car, it showed 1-2% battery remaining. While it was parked the BMS got somewhat confused. The battery voltage didn't drop. It was at a reasonable 3.2 Volt which is perfectly fine as a resting voltage when almost empty. I'm not worried. Looks like the BMS just got a little confused. I'm charging to 100% tonight and I'll see what it shows in the morning. My car has never done this before so I'm sure it had to do with the experiment of driving the car down to the point where it shuts down. Either that confused the BMS or it recalibrated it and tomorrow I will have 5% extra range that was recovered LOL

Just a guess for why the battery level went down while parked for two hours. Basically the battery capacity changes based on temperature. I guess the battery temp was high when you arrived at home, due to some driving. After parked for 2 hours it cooled down so the capacity was reduced.

 

[David99]

Ok. Not worried about possible battery damage with the voltage dropping that low?

I watched the voltage and it was never at a critical level. But more importantly the BMS does exactly that. Protect the battery from any use that could damage it. Elon actually tweeted about it and said it’s safe as the battery has a safety buffer.

Just a guess for why the battery level went down while parked for two hours. Basically the battery capacity changes based on temperature. I guess the battery temp was high when you arrived at home, due to some driving. After parked for 2 hours it cooled down so the capacity was reduced.

Yes when it gets really cold that can happen. Battery temperature didn’t change much in those two hours though. Maybe 1 degree.

 

[Kuhz]

So you waited 2hrs for cheaper electricity to save a few cents yet risking degrading your battery. Makes sense

 

[David99]

So you waited 2hrs for cheaper electricity to save a few cents yet risking degrading your battery. Makes sense

The cell voltage was at a healthy 3.2 Volt. That's a perfect resting voltage. At this level you can store the battery for a long time and have the least amount of degradation. Storing at 100% is what is bad for a battery.

 

[Nietschy]

I have been at -4% when my car was new two years ago.

 

[Nitemare3219]

The cell voltage was at a healthy 3.2 Volt. That's a perfect resting voltage. At this level you can store the battery for a long time and have the least amount of degradation. Storing at 100% is what is bad for a battery.

3.2 volts - healthy? Please reference any source that recommends storing a lthium cell that low, I'd love to see it. Running the battery near 0 for a video seems asinine - you aren't the first person to do it, so why bother? Leaving it near 0 for hours to save a few dollars on electricity? Even moreso.

 

[N..8]

3.2 volts - healthy? Please reference any source that recommends storing a lthium cell that low, I'd love to see it. Running the battery near 0 for a video seems asinine - you aren't the first person to do it, so why bother? Leaving it near 0 for hours to save a few dollars on electricity? Even moreso.

When you buy new batteries out of the package check the voltage. I'm finding that most of them are around 3 volts, the 18650 can be brought down even lower although not recommended for storage, due to the constant drop. LiFePO4 is more affected by heat then lower voltage when dealing with short term storage. Some of our problem is people bring older Lead Acid thoughts to the newer batteries. Lead Acid voltage range of operation is really small, 12.6 - 12.2 and you really start to hurt the life of them.

 

[Candleflame]

even though lithium ion batteries have a relatively linear discharge voltage which also doesn't vary under load (unlike let's say lead acid batteries; one reason cars don't have a % gauge for the 12v battery) a lot of the % estimation is actually calculated by the computer. wouldn't be surprised if the temperature was responsible for this and after a long drive the battery might have been warm which improves range further.

I'm thinking in particular driving a car in winter through south USA desert at 35 degrees and then leave the car at 2% at nighttime when its down to 0 degrees. Of course the computer will get confused and give a reading like -5% or even less.