Best Battery Usage Strategy for a low mileage user.

[DonaldBecker]

Here are two references that seem opposite to the above comment:

• Tesla provides the following statement in their EPA applications: “To maintain service life, the battery pack should be stored at a state of charge (SOC) of 15% to 50%.”

• This graph shows a significant improvement in Li-ion battery capacity retention when stored at 15% instead of 90%.

Source: https://res.mdpi.com/d_attachment/applsci/applsci-08-01825/article_deploy/applsci-08-01825-v2.pdf

That paper isn't showing what you are suggesting, for multiple reasons.

The clearest point is that the '90%' graph line is based on battery capacity, not the charge displayed to the user. As they state in page 6 of that paper:

"In vehicles, the full capacity of a battery pack is normally not utilised, in order to extend the lifetime of the battery. When 100% SOC is displayed in the vehicle, i.e., fully charged, this could typically correspond to the single cells being charged to about 90% of the upper SOC limit given by the manufacturer."

This specific graph is showing that always charging to 90% actual, which would be 100% indicated on the display, isn't good for longevity. That is unsurprising, and aligned with pretty much every paper on the topic. It is why Tesla explicitly does not recommend charging to 100% (indicated, 90%-ish for raw cell capacity), labeling the upper 10% 'Trip'. Tesla labels 70%-90% as the 'Daily' upper limit. That indirectly suggests small depth-of-discharge use should be centered on 70%.

I did read the whole paper. It reinforces that studies like this are time consuming, difficult to match real-life use, obsolete when published, and often don't end up with good enough data to show a nice curve that produces a good conclusion. The bulk of the data in the paper reinforces things that are generally known: don't cycle at either end of the range, and don't run batteries hot. In the middle of the range they needed to discharge at 2C and 4C to see the effects, and that data was extremely inconsistent.

If there is any conclusion to be drawn from this paper, it indirectly says always buy the big battery. Once the discharge rate drops below 1C, and especially below 0.5C (it takes 2 hours or more to discharge the battery), you can operate over most of the SoC range with only minor and difficult to consistently measure wear.

 

[Zoomit]

That paper isn't showing what you are suggesting, for multiple reasons.

The clearest point is that the '90%' graph line is based on battery capacity, not the charge displayed to the user. As they state in page 6 of that paper:

"In vehicles, the full capacity of a battery pack is normally not utilised, in order to extend the lifetime of the battery. When 100% SOC is displayed in the vehicle, i.e., fully charged, this could typically correspond to the single cells being charged to about 90% of the upper SOC limit given by the manufacturer."

This specific graph is showing that always charging to 90% actual, which would be 100% indicated on the display, isn't good for longevity.

That is not a correct interpretation of their statement. The data in the paper is from 90% SOC, not 100%, and of course 90% in (most) Tesla's is actually close to 90% not 100%.

Tesla labels 70%-90% as the 'Daily' upper limit. That indirectly suggests small depth-of-discharge use should be centered on 70%.

They label 50-90% as a "Daily" charge limit. But I don't follow your logic. It is not reasonable to infer that DOD should be centered on 70% given just those charge limits by Tesla. Clearly it's reasonable for Tesla to expect people to discharge below 50%. We would need to know what Tesla expects for a "Daily" low charge state to make any inferences about where DOD should be centered; even then, the upper range is between 50 and 90%.

 

[DonaldBecker]

That is not a correct interpretation of their statement. The data in the paper is from 90% SOC, not 100%, and of course 90% in (most) Tesla's is actually close to 90% not 100%.
They label 50-90% as a "Daily" charge limit. But I don't follow your logic. It is not reasonable to infer that DOD should be centered on 70% given just those charge limits by Tesla. Clearly it's reasonable for Tesla to expect people to discharge below 50%. We would need to know what Tesla expects for a "Daily" low charge state to make any inferences about where DOD should be centered; even then, the upper range is between 50 and 90%.

I acknowledge your point about the app and in-vehicle UI. Tesla is not directly making a recommendation about the preferred daily SoC. They are just recommending that the 90%-100% range only be used when charging for a long trip.

I don't agree that Tesla's indicated (the number presented on the screen) SoC is close to the cell SoC. We don't have access to their engineering decisions, but it's likely they are similar to others in the industry: 100% indicated is closer to 90%.

I think we can both agree that the longevity difference between centering your typical cycling at 50% (e.g 60%-40%-60%) or 70% (60%-80%) is going to be minor. As that paper hinted, it's going to be overwhelmed by other factors and the noisy results will be difficult to interpret, let alone draw a conclusion from. Just stay away from either extreme, and minimize storage at the extremes.

 

[Zoomit]

I don't agree that Tesla's indicated (the number presented on the screen) SoC is close to the cell SoC. We don't have access to their engineering decisions, but it's likely they are similar to others in the industry: 100% indicated is closer to 90%.

100% is 100% for most Tesla batteries. This has been widely seen and reported. With the exception of the 10% top locked SR and recent shenanigans with the new packs in EU cars, the batteries charge up to 4.2V per cell, which for Li-ion chemistry is 100%. Here’s an example at 100% SOC showing 403V at the pack and 4.20V at the cell level. Start at about 2:40.

I think we can both agree that the longevity difference between centering your typical cycling at 50% (e.g 60%-40%-60%) or 70% (60%-80%) is going to be minor. As that paper hinted, it's going to be overwhelmed by other factors and the noisy results will be difficult to interpret, let alone draw a conclusion from. Just stay away from either extreme, and minimize storage at the extremes.

Yes, totally agree. This graph is a qualitative version of that thought. I made it for this thread:
Battery Degradation Scientifically Explained

 

[rrolsbe]

I wonder if there is any freight recommendation when shipping a large amount of batteries oversea
using containers full of batteries in a boat or by air?

Should the batteries been depleted to lower fire risk?

What the SoC used by Tesla when shipping 7,000 cars in a boat?

Is there a special 'Shipping' mode for the Tesla cars during the two months of shipping so the cars reach a very deep sleeping state?
- I noticed that my 12 V battery get recharged by the DC/DC internal charger for about two hours every two days.
- Also the pumps are running even when the car is sleeping.​

Since Tesla recommends the cars be put in "Storage Mode" when the cars can't be plugged into shore power for an extended period, it seems there must be some provision for the potential long shipping times to overseas markets. The cars being shipped overseas might also be exposed to temperatures below -22F for more than 24 hours (could that possibly do permanent damage?). Maybe someone on this forum has some insight as to the overseas shipping protocols.

If the you hear the pumps running, I do not consider that sleeping. In fact, if you do NOT hear the pumps but DO hear the clicking sound at the charge port door the car is not in what I would consider a sleep state. if the high voltage contactors are closed for any reason, I consider the car to NOT be sleeping.

 

[Rocky_H]

The cars being shipped overseas might also be exposed to temperatures below -22F for more than 24 hours (could that possibly do permanent damage?).

It can't get to that cold over the ocean.

 

[rrolsbe]

It can't get to that cold over the ocean.

Good point, must of had a senior moment. Wonder what the coldest air temp that was ever recorded from an ocean going shipping vessel?

 

[Dave EV]

I don't agree that Tesla's indicated (the number presented on the screen) SoC is close to the cell SoC. We don't have access to their engineering decisions, but it's likely they are similar to others in the industry: 100% indicated is closer to 90%.

This is false.

4.2 V is widely recognized as 100% SOC and as already mentioned, what Tesla charges the cells up to when you select 100%. 90% around 4.1V and even 80% is around 4.06-4.07 V.

0% appears to be right around 3.0 V. 3.0 to 4.2 V is a very standard 0% to 100% SOC range.

I would argue that voltages above 4.0 V is a bit higher than ideal to retain maximum capacity over time, so would suggest 70-75% maximum for maximum life if you don't need the range.

Storing cells around 3.7-3.8 V is widely regarded as the optimal storage voltage for your typical li-ion cell. Some say that lower voltages can be better, down to a point (perhaps not any lower than 3.3 V or so).

The biggest downside to running a narrow SOC range appears to be that the BMS allows the pack to get out of balance over time. Occasionally charging to 90% should help provide enough signal to the BMS to minimize this. Of course, if you don't really need all the range and are interested in maximizing battery life, you don't have to worry about this.