What's better...full cycle charge or supercharging

[johnny_cakes]

Great bedtime reading!
I got your point: you're so kind as always! Thank You so much!

I found the coverage of central graphite peak in the document really interesting. @AAKEE has already reported on this in the past, but I didn't realize the extent at which the central graphite peak shifted over time. The vertical blue and red dotted lines indicate the central graphite peak when new and after 9.5 months, respectively. Also the higher the storage temperature, the more the central graphite peak shifts. It would be interesting to see how much further this continues to shift over time, as it would allow us folks who like to set our charge limit on the "good side of the degradation jump" to a number above the magical 55% SoC that many currently use without much harm as our cars get older.

 

[AAKEE]

What do you consider low SoC for Supercharging. Something like 50% or 30%?

The thing driving degradation (causing lithium plating) is high currents at high voltage.
So I’d say that there is not a specific limit, at least I can not recall any specific limits in the research reports.

Always precondition and do not charge more than you need. If you need 70% to the next convienent Supercharger, then use 70% (and if needed some extra for avoiding range anxiety).

Using the low SOC strategy will help the battery regenerate after the supercharging session(-s) so it will actually reduce the energy impact of the SuC.

This research focus on the effects of supercharging in this chapter, they did not look into preheating to reduce the lithium plating, but that is covered in other research reports.

Lithium Plating Confirmed as Main Degradation Mechanism

The study on charging protocols for lithium-ion batteries has clearly revealed that high charging currents aggravate degradation. Lithium plating has been confirmed to be the dominant aging mechanism when charging high-energy lithium-ion batteries with high charging currents to high charging voltages. Lithium plating is associated with a rapid capacity fade caused by a loss of cyclable lithium owing to side reactions of the plated lithium with the electrolyte. Increasing high-frequency resistances have revealed a decomposition of the electrolyte, which decreases its conductivity. Also in the differential voltage spectra, reduced storage capabilities of the graphite anode have been observed after charging with high currents.

Results of Boost Charging and Supercharging
The BC and SC protocols apply a high charging current only at a certain SoC regime. Although it has been beneficial to avoid high charging currents at high SoC regimes, where the anode potential is lowest, the high charging currents have still caused disproportionate degradation at lower SoCs. No benefits compared to CCCV charging could be identified in this study. This demonstrates that a lithium-ion cell has to be designed for fast-charging capability. Thin anodes with high porosity are typically less susceptible to lithium plating than thick electrodes with low porosity.

https://mediatum.ub.tum.de/doc/1355829/document.pdf

 

[AAKEE]

I found the coverage of central graphite peak in the document really interesting. @AAKEE has already reported on this in the past, but I didn't realize the extent at which the central graphite peak shifted over time. The vertical blue and red dotted lines indicate the central graphite peak when new and after 9.5 months, respectively. Also the higher the storage temperature, the more the central graphite peak shifts. It would be interesting to see how much further this continues to shift over time, as it would allow us folks who like to set our charge limit on the "good side of the degradation jump" to a number above the magical 55% SoC that many currently use without much harm as our cars get older.

View attachment 924905

For calendar aging, it actually is stationary when measuring the energy from 0%.

This means that if it was at 57% for a new cell, then it will be at 57/90% (= 63% or so) if the cell lost 10% capacity.

But charging/cycles and specially fast charging like SuC reduce the capacity below the peak, which will try to keep the peak at about the same SOC.

How the peak will “move” will be decided by a combination of calendar aging and for example supercharging.
The easy way is to think that it stays at about the same SOC.

Using the low SOC strategy will reduce the upwards movement, as it reduce the calendar aging.

 

[johnny_cakes]

For calendar aging, it actually is stationary when measuring the energy from 0%.

This means that if it was at 57% for a new cell, then it will be at 57/90% (= 63% or so) if the cell lost 10% capacity.

But charging/cycles and specially fast charging like SuC reduce the capacity below the peak, which will try to keep the peak at about the same SOC.

How the peak will “move” will be decided by a combination of calendar aging and for example supercharging.
The easy way is to think that it stays at about the same SOC.

Using the low SOC strategy will reduce the upwards movement, as it reduce the calendar aging.

I see, thanks for the clarification.

As I understand it, the common 55% charge limit that many people in this forum reference is because it correlates to 57% true SoC if you factor in the 4.5% buffer (57% = 55% + (1-55%) * 4.5%). 57% true SoC happens to be where the central graphite peak is for NCA batteries. When the BMS goes out of calibration and the displayed SoC drifts lower relative to true SoC, is there risk that charging to 55% will be above the central graphite peak and thereby past the jump in the degradation curve?

 

[AAKEE]

I see, thanks for the clarification.

As I understand it, the common 55% charge limit that many people in this forum reference is because it correlates to 57% true SoC if you factor in the 4.5% buffer (57% = 55% + (1-55%) * 4.5%). 57% true SoC happens to be where the central graphite peak is for NCA batteries. When the BMS goes out of calibration and the displayed SoC drifts lower relative to true SoC, is there risk that charging to 55% will be above the central graphite peak and thereby past the jump in the degradation curve?

Not really, id say!

Calemdar aging cause the most degradation for most cars -> moving the peak upwards.
(If doing 10 supercharging session each day for a long period and letting the car sleep with low SOC, then the peak probably moves down. But in this case, the issue of letting the car stay long periods above the peak us not present.

 

[johnny_cakes]

Not really, id say!

Calemdar aging cause the most degradation for most cars -> moving the peak upwards.
(If doing 10 supercharging session each day for a long period and letting the car sleep with low SOC, then the peak probably moves down. But in this case, the issue of letting the car stay long periods above the peak us not present.

Thanks for that clarification!

 

[Hiline]

Id like to add a little to post #12, as the question is not exaclty the same:

This is an easy one.

Fast charging causes much less degradation (lithium plating) when the battery is really hot. High temperatures increase the calendar aging by the double, but the high temperature is not going to be there specially long, and cycling wears less at a little higher temps like 25-35C.

The important part is that fast charging wear much less at low SOC, so more supercharging sessions at relatively low SOC is better than few with higher SOC and bigger depth of discharge.
This document covers fast charging and supercharging and finds the low SOC region to suffer less:
https://mediatum.ub.tum.de/doc/1355829/document.pdf

I use low SOC at SUC, as it means faster traveling and also less wear. Always precondition the battery completely.

Ah so supercharging to a relatively low SOC doesn't cause much damage to the battery. This will be very helpful for my next road trip - thank you for sharing!

 

[PianoAl]

Concerning the riskiness of arriving with a SOC of 5% ...

I also prefer to arrive with 15% to 20%, but there is a way to mitigate the risk. Display your energy graph and keep an eye on it. When you're coming to the supercharger, you'll have a better feeling for whether you'll make it. Also, you can slow down if you are doing worse than predicted.

 

[Park2670]

Concerning the riskiness of arriving with a SOC of 5% ...

I also prefer to arrive with 15% to 20%, but there is a way to mitigate the risk. Display your energy graph and keep an eye on it. When you're coming to the supercharger, you'll have a better feeling for whether you'll make it. Also, you can slow down if you are doing worse than predicted.

On one route that I do often, I plan on arriving at 1-2%. For some reason, I am never worried about not making it. I guess it will bite me one day, but so far my car has been very realistic when I will or will not make it to the Supercharger.