What's the power consumption of the UMC not charging?

[jjrandorin]

Incorrect-- brick balancing occurs independent of the car being plugged in. It occurs based on SOC above ~85%. Here's the section from the Model 3 Service Manual with the key sentence in bold:

Brick Balancing

Note that the capacity of a pack is limited by the brick with the lowest capacity. When that brick is charging, it will gain voltage faster than other bricks. The HVBMS will stop charge when any brick reaches its ceiling voltage (~4.2V). If one brick has a significantly lower capacity that others, the pack will be limited by that brick which will get to 4.2V faster than the other ones. We refer to the brick with the lowest capacity as: min CAC. In periscope, its value can be seen by viewing the signal: 'BMS_cacMin'
Another limitation could come from bricks being imbalanced, or some bricks with a voltage higher/lower than others. This would limit ability to charge the pack as the brick with a higher voltage than others would reach the ceiling voltage early. Same idea when discharging, the brick with the lowest voltage would hit the floor voltage early which would cause the HVBMS to open contactors from low power

To mitigate this imbalance, Batman has some bleed resistor that can be placed and removed in parallel of each brick via a FET relay. Batman can put that resistor across the brick with the highest voltage which would slightly discharge that brick and bring it back to the level of the other bricks. Batman closes a FET which puts that resistor across the brick. The HVBMS will order Batman to put that bleed resistor across the brick with the highest voltage when Delta V is > 5mv MinBrickV > 4.0v (~85% SOC) && HVBMS State == STAND BY.

Note that Batman can also do balancing when the HVBMS is asleep.

The best way to balance the Model 3 pack is to set charge limit to 90% or higher and let the vehicle sit idle for hours (plugged in or not). 24 hours of balancing can reduce imbalance by 1mV.​

Ok, fair enough, but in practice most people are not going to go out and plug and unplug their car when its 90% to 85%, and that will take a couple of days. So, factually accurate, but someone who sits with their car unplugged is not going to go back out and plug it in every time it drops below 85% to balance the pack.

 

[Zoomit]

when its 90% to 85%, and that will take a couple of days.

My LR battery is draining at 1% per week, over the last 3 weeks of storage, so it could take a month to drain that 5%. During all that time the plug status is immaterial. I agree that most would just leave it plugged in, but it’s not required.

 

[Silicon Desert]

And yet despite your not having "seen" this, I assure you it for certain is happening.

I'm not sure by what criteria you think you will "see" this battery heating. But Tesla cars absolutely will not charge the battery while it is below freezing. That would cause irreversible damage, so they just don't do it. So when it is time to begin a charging cycle, it will instead, start pulling energy from the house circuit to run warming systems for a while to heat up the interior of the battery pack to get it up to a safe enough temperature that it can then start sending some of that energy toward recharging the battery cells. It will still continue splitting that energy, further warming up the pack and gradually increasing the amps of the charging rate as the temperature in the pack continues to rise.

Thanks Rocky. When I mean I haven't seen it. I have looked at all of the car's logs and the battery heater has never been on when charging at home. In fact, never on at all even when not charging. You are probably right, but maybe due to the fact that I am not charging at a very high rate and maybe never really cold in the garage. It can get to about 30 degrees F, but typically not lower than that.

 

[Rocky_H]

Thanks Rocky. When I mean I haven't seen it. I have looked at all of the car's logs and the battery heater has never been on when charging at home. In fact, never on at all even when not charging. You are probably right, but maybe due to the fact that I am not charging at a very high rate and maybe never really cold in the garage. It can get to about 30 degrees F, but typically not lower than that.

I'm wondering if this is a deficiency of the programs you are using to look at the car's logging or how it is reporting the data in some way. This is the Model 3 forum, after all. When you say the logs do not show the "battery heater" being on. I wonder if those logging tools were made years ago for the Model S and X, which did have a battery heater, so that is the variable flag it is looking to see indicated. Since the Model 3 does not have an actual separate device that is a battery heater, maybe it's always reporting not active for the battery heater, even while it is using the motor coils to generate the heat for the battery pack system.

 

[Silicon Desert]

I'm wondering if this is a deficiency of the programs you are using to look at the car's logging or how it is reporting the data in some way. This is the Model 3 forum, after all. When you say the logs do not show the "battery heater" being on. I wonder if those logging tools were made years ago for the Model S and X, which did have a battery heater, so that is the variable flag it is looking to see indicated. Since the Model 3 does not have an actual separate device that is a battery heater, maybe it's always reporting not active for the battery heater, even while it is using the motor coils to generate the heat for the battery pack system.

All great points Rocky. I realize this is the model 3 forum and I thought I was talking about my X, but now I see I did not mention it. Apologies for any confusion. I have access to the same tools used by technicians, so they are accurate, but good thought.