2021 MIC Model 3 SR+ with LFP battery pack — my thoughts on charging best practice

This is for those who received a Model 3 with the LFP battery (2021 SR+ from Shanghai).

As is well known, the ‘daily’ and ‘trip’ segments from the charge limit settings have been removed and there is that one Chinese social media Tesla statement going around with a recommendation to charge the car to 100% at least once a week. This is a surprise for EV owners who are used to NMC/NCA batteries that experience high degradation at 100% state of charge (SOC). Charging this high would obviously benefit the available depth of discharge (DOD) at a given minimum charge of say 20% and positively impact the overall range.

Among many sources regarding LFP, many from the solar community, this 2020 paper was particularly useful in contrasting the three relevant battery chemistries.

Thoughts for discussion

1: LFP batteries outperform NMC and NCA in the full cycles available until the battery is declared as ‘done’ at 80% of its original capacity.

The LFP cells exhibit substantially longer cycle life spans under the examined conditions: 2500 to 9000 EFC vs 250 to 1500 EFC for NCA cells and 200 to 2500 EFC for NMC cells. Most of the LFP cells had not reached 80% capacity by the conclusion of this study for the NCA and NMC cells, and their longer-term degradation will be reported in a later work.

If LFP batteries can go for 2500+ full cycles (using 100% of battery capacity available from full to empty) until they reach 80% of their original performance, they will outlast the life of the car for practical purposes. This is great news for people worrying about battery health throughout ownership and for resale to discerning buyers. It would be good to know more about the rated life of the specific packs and battery management combination used in the SR+ as this number will impact all of the points below.

2: LFP batteries still degrade faster at a higher DOD and when charged to 100% SOC, however, less so than NCA/NMC. In combination with more available cycles, it may therefore be fine to utilise a wider percentage range of the battery and frequently charge LFP to full.

For all cells in this study, the rate of capacity fade increased with an increasing depth of discharge (…) Compared to LFP cells, the NCA and NMC cells experienced a more dramatic transition in capacity fade from partial to complete DOD and this result is consistent with previous studies (…) As in cycle aging studies, NCA and NMC cells exhibited particularly rapid capacity fade at 100% SOC.

The accelerated degradation of NCA/NMC at full charge is obviously what is behind the ‘don’t charge to 100% unless you need to’ rule for most EVs. Since LFP does not suffer as much of this and should in theory have a lot more cycles in reserve, it may be fine to frequently charge an LFP Model 3 all the way to 100% without excessive worries about battery health, particularly if we run the battery at a lower DOD that increases the available cycles before the battery is considered done even further.

If we assume a minimum charge of 20% for the sake of a range buffer and battery protection, running a Model 3 between 100% and 20% with 80% DOD at our disposal would in theory still result in such a high cycle lifespan that the added range over running between 80%-90% max. charge and 20% min. charge would far outweigh the benefits of irrelevant additional decades of battery lifespan in such a vehicle. This is particularly true for those who can’t charge at home and benefit from a fuller available capacity utilisation.

3: Given the above, charging LFP regularly to 100% should not be problematic and may assist with battery management and SOC/range accuracy.

Since LFP batteries have very flat voltages, it appears to be difficult to exactly determine the accurate SOC. Charging the battery full may therefore in theory provide a reference point for the battery management system to base further calculations on. I suspect this is behind the recommendation, with the understanding that this outweighs the degradation argument given the above factors. Until we learn more about this from Tesla or manufacturer CATL specifically, it is probably best to assume this is the case.

Interim conclusions

Scenario 1: Charge at home

In this case, I would personally charge my Model 3 to 100% once a week before driving it off to give the battery management system its reference point but whilst avoiding to have it sit at 100% unnecessarily. On the other days, I would set it at a charge limit that ensures I will never go below a 20% buffer whilst only using the maximum DOD required for my driving needs. This will in theory ensure that that battery will far outlast the life of the car.

Scenario 2: Reliance on public chargers

In this case, if I could get away with charging the car once a week, I would always charge it to 100% and utilise 80% DOD until a 20% buffer is reached before recharging. At this DOD, the impact of a 100% charge should not be relevant and offset by the additional range. If I had to charge the car twice or more a week to cover the range required, I would charge it to 100% for battery management once and then to a lower charge limit for subsequent charges in the same week, however low it can be, to reduce charging times and DOD stress just in case, for now.

The above would consolidate the battery management argument with reasonable battery protection. Should data become available that confirms that this specific setup handles many thousands of full cycles even at 100% DOD, I would then move to a ‘set and forget’ at 100% philosophy and drive the car down to 10% if needed. Until then, I still think it is prudent to limit the DOD where feasible and not unnecessarily charge to full more than once a week, just in case the setup does not perform as well as individual LFP cells do under lab conditions.

I hope this helps some of you who received as little useful charging information as I did (being none) when I picked up the car, and have since tried to make sense of what is different about our LFP vehicles. On a side note, I had no issues whatsoever with my LFP Model 3 with 2021.4.12 software in a warm climate, it charges at expected speeds and is very predictable in terms of % used for trips. My findings to date make me rather happy to have an LFP battery as the car otherwise performs very well. I am looking forward to a discussion of my tentative conclusions as I am certainly no battery expert and a lot of factors could play into this theory.

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