Is 25% SoC equivalent to 75% SoC for battery health?

[Yankees]

I'm trying to figure this out and haven't really ran into any specific info. Before getting into it, let me preface by saying that I am well aware of Tesla's recommendation to just plug it in whenever possible, and that a Tesla is happiest when it's plugged in. This is just an exercise.

My understanding is that 50% State of Charge (SoC) is where rechargeable batteries, such as those in Teslas, are happiest. What I want to figure out is if a 25% SoC, which is 25% under the ideal 50%, is equivalent, from a battery health perspective, to a 75% SoC, which is 25% over ideal, or if one is preferable to the other? Is 25% under or over the same, or is one better (even if marginally)? Does it change as you increase how far from ideal the SoC is? I would imagine as you approach the absolute limits (1% vs 99%) that 99% is preferable. What about 5% vs 95%? 15% vs 85%?

I can think of one practical reason why it's better to be on the high side, as soon charging stops, it starts to lose charge from various electronics and vampire drain. This will result in an ever-so-slow trend toward ideal, whereas an under-ideal-charged battery (say, 20%) will instead ever-so-slowly trend away from ideal (although in a daily use car, this is negligible).

 

[Uncle Paul]

You may be overthinking this...

 

[Troy]

Check out this chart that I found here. The cycle numbers shown here are too high. At 1000 cycles, your mileage would be around 270,000 miles. Therefore it makes sense to zoom in on the first 500 cycles. It's interesting that 75-65% is better than 75-45%. However, at 500 cycles, the difference between these two charge patterns is only about 1%.

 

[ewoodrick]

Stop worrying about the battery, enjoy the car instead.

 

[Yankees]

Guys, I am enjoying the cars! I'm on the tech board with a technical question. Doesn't mean I don't enjoy the cars. And I'm not worried.

@Troy thanks, I've seen those before, and am aware that shallow cycling is fine for these batteries.

 

[arnis]

@mastre

Most li-ion batteries should be stored in near-discharged state. The less active chemistry there is the better. Though it is very important to keep voltage well above minimum safe voltage (above 2.5V per cell !).
At 20% state of charge voltage is around 3.6V. That is near ideal state for storage.
Maximum "very safe" state of charge is 30%. At this state batteries can be shipped by air for example.
The second part of unstoppable degradation is temperature. Chemical processes (including decomposition etc) happen faster with warmer temperatures.
Maximum ideal temperature for Li-ion is 20*C (room temperature). This is the maximum ideal. To make it better than normal, reducing temperature will help.
Minimum safe is -20*C. If cells are stored in non-airtight container (pack) humidity should be kept under control in colder temperatures.

Higher voltage (state of charge) adds to degradation speed exponentially. Degradation acceleration between 25% and 50% is MUCH MUCH smaller than between 50% and 75%.
Same story with temperature. Difference between +5*C and +10*C is much much smaller than between +20*C and +25*C.

 

[remlemasi]

You may be overthinking this...

Stop worrying about the battery, enjoy the car instead.

This is a bit of a thread bump but I feel compelled to point out that there are some of us that just have a compulsive thirst for knowledge. Not necessarily for a useful purpose (maximizing useful life of our cars) but just for the sake of knowing.

@mastre thank you for asking this question as I was looking for the exact info

@Troy @arnis thank you for posting the data. Very helpful. I try to read up on batteryuniversity (again, “just because” and not because I’m paranoid about degrading my battery) but they don’t always have the answers to very specific questions like these (or they are hidden deep in the depths of the many pages of text).

I spent several hours yesterday listening to the Autonomy live stream. I have an engineering background so about 40% (just a random guess) of the material got through but I loved every minute digging into the minutia of the chip design, power consumption, neural nets, machine learning, etc. Most people don’t care about this and would rather just “enjoy” FSD (whenever that happens) and would tune out after 1 minute of boring technical talk, but not all of us.

If anything, understanding the details of how these things work INCREASES our enjoyment of the vehicles rather than feeding some sort of paranoia that leads to a poor experience.

Sorry, I went kind of long here, but just want to say that questions like these are totally valid. It is questions like these, especially from young minds, that lead to challenging the status quo and producing amazing products from crazy companies like the one we all love so much.

 

[MrTeaUSA]

Check out this chart that I found here. The cycle numbers shown here are too high. At 1000 cycles, your mileage would be around 270,000 miles. Therefore it makes sense to zoom in on the first 500 cycles. It's interesting that 75-65% is better than 75-45%. However, at 500 cycles, the difference between these two charge patterns is only about 1%.

What about recommended to charge to 90% SOC? From this graph is it best to charge from 65% to 75% max daily and not 90% for long term usage if not drive much?

 

[arnis]

10% cycle is 10x less than full cycle from 100% down to 0% up to 100%.
This graph will confuse many. 1000 cycles between 65 and 75% is 1000x 10kWh* on 100kWh battery. Therefore 10 000 kWh of energy used for driving. Same can be done with 100 cycles between 0% and 100%.
Therefore having cycle number on X-axis is pretty much useless in terms of actually using energy.
X-axis should have different units. MWh discharged. That would at least tell us something.

*not actually correct