Best charging behavior for LFP batteries

BYD Blade Battery prismatic cells
BYD Blade Battery prismatic cells

I have been writing about the importance of LFP (LiFePO4) batteries for a while now, but I haven’t written about the best charging behavior for this battery chemistry yet.

It’s time to write a small, but important article about the subject, especially because I have seen some confusion on online forums.

 

First, let’s compare the nominal and maximum voltages of LFP with other popular battery chemistries.

 

LTO anode + LMO/NCM cathode

  • Nominal voltage: 2,3 V
  • Max voltage: 2,9 V
  • Cycle life: up to 100.000 cycles

 

Graphite anode + LFP cathode

  • Nominal voltage: 3,2 V
  • Max voltage: 3,65 V
  • Cycle life: up to 12.000 cycles

 

Graphite anode + NCM cathode

  • Nominal voltage: 3,7 V
  • Max voltage: 4,2 V
  • Cycle life: up to 4.000 cycles

 

Battery chemistries comparison by Tesla

Battery chemistries comparison by Tesla

 

Did you notice a pattern? The higher the voltage, the lower the cycle life is.

Common liquid electrolytes are chemically more stable at lower voltages, that’s partly why LTO (2,3 V) and LFP (3,2 V) battery cells have better cycle life than most NCM/NCA battery cells that work at higher voltages (3,7 V).

When you charge a battery you’re increasing its voltage and at higher voltages the liquid electrolyte starts slowly corroding the current collectors inside the battery cells.

This is also why we still don’t have high-voltage spinel (5 V) battery cells, which require new electrolytes and better coating of the electrodes to prevent extreme corrosion from occurring.

 

Therefore, it’s obvious that keeping the batteries at lower SoC (State of Charge) is better for their cycle life.

 

Now let’s talk about the confusion generated by a recommendation from Tesla…

 

If your vehicle is equipped with an LFP Battery, Tesla recommends that you keep your charge limit set to 100%, even for daily use, and that you also fully charge to 100% at least once per week. Ifย Model 3ย has been parked for longer than a week, Tesla recommends driving as you normally would and charge to 100% at your earliest convenience.

Following the above guidance maximizes available range and improves the vehicle’s ability to accurately determine the state of charge and estimated range.

Tesla

 

Does it mean that LFP batteries are immune to extra battery degradation associated with frequent full charges? No…

 

The only reason why Tesla recommends to fully charge the Model 3 cars that have LFP batteries is because it helps the BMS (Battery Management System) to estimate the available battery capacity more precisely.

Compared to other battery chemistries, LFP’s charge/discharge voltage curves are extremely flat. Basically, the voltage only rises when the battery is almost full and drops when it’s almost empty. This is why it’s much harder for a BMS to estimate correctly the SoC of a LFP battery.

 

charge-discharge voltage curves of different battery chemistries

charge/discharge voltage curves of different battery chemistries

 

Imagine looking at the GoM (Guess-o-Meter) on your dashboard and seeing 20 % battery remaining, but a minute later only 5 %โ€ฆ

Regarding customer service, for Tesla is more important to have a reliable GoM than to have a battery with a fenomenal cycle life – since one feature becomes obvious to the customer much sooner than the other.

Nonetheless, even if you always fully charge a LFP battery, its average voltage will still remain lower than it would be in a half charged NCM or NCA battery…

 

LFP is not perfect, there are no perfect batteries, batteries are either not good enough or good enough for a task. Fortunately, the LFP battery chemistry is good enough for many tasks. I mean, this is pretty obvious since its inventor was John B. Goodenough (this sounded way funnier in my head).

 

Summing up…

 

If you care about keeping a lithium-ion battery healthy, my advice is to cycle it between 20 and 70 %, as the data in my most popular article suggests. On the other hand, if you care more about having a reliable GoM, that estimates your remaining range more accurately, fully charge the battery more often.

Like many things in life, maybe the best is to compromise and reach a middle ground. You can usually charge your electric car with LFP battery up to 70 %, but then once a week charge it fully – before taking it for a ride (avoid let it stay on 100 % for a long period).

Anyway, unless the GoM is acting crazy by frequently showing sudden drops of estimated range, you shouldn’t fully charge your electric car – even if powered by a LFP battery – very often if you care about the cycle life.

 

I hope that you find this article helpful and clarifying.

 

 

More info:

https://www.researchgate.net/publication/261135878_Optimized_Operating_Range_for_Large-Format_LiFePO4Graphite_Batteries

Pedro Lima

My interest in electric transportation is mostly political. Iโ€™m tired of coups and wars for oil. My expectation is that the adoption of electric transportation will be a factor for peace and democracy all over the world.

15 Responses

  1. Europe says:

    Pedro, Samsung NCM 622 cells in the BMW i3 are rated for 4500 cycles so clearly they can be way above 2000 cycles. As you increase energy density the cycle life goes down. This applies to NCM, NCA, LFP and all other chemistries sadly.

    • Pedro Lima says:

      Indeed, but the Samsung NCM 622 battery cells are not vanilla NCM, they are probably the most expensive battery cells in the auto industry, with optimized electrolyte and tick coating to prevent corrosion.

      Modern Panasonic prismatic cells (used to allow a one million kilometre warranty) are also the exception, not optimized for cost nor energy density as most EV battery cells.

      Automakers are not particularly interested in optimizing cycle life beyond the warranty period. This cycle life thing is just interesting for us.

      • Carlos says:

        Hmmm, how about the SK NCM622 in the Kia e-Niro? That car has been around for 3 years and nobody is complaining about degradation.

        I believe that 800 cycles is already a very good figure for a car with a decent battery. Let’s again take the e-Niro. It has a real world range of 400 kilometers and with 800 cycles it would do 320k km. That’s what the average person drives in 20 years.

        High cycle life becomes important with V2G or for robotaxis. For ordinary car owners, 2000-3000 cycles is already way more than they will ever need.

  2. Frederico Matias says:

    Very instructive…. as always :)…

    Loved the Goodenough joke…not only good in your head ๐Ÿ™‚

    Now I would just like to add a 60kwh net LFP battery pack to my 40kwh leaf, add the CCS2 instead of Chademo and adjust BMS not to rapidgate a lot…. hummmm… the old 40kwh NCM pack would be good enough for expansion of my current 1,5kw home solar storage

    • Pedro Lima says:

      That would make the Nissan Leaf almost perfect, CCS and LFP battery with TMS and V2G.

      I guess that planned obsolescence plays a part on Nissan’s decision to refuse to improve its electric car.

  3. f1geek says:

    Pedro,
    Are you listing 70% to 20% cycles?

    I have been tracking my e-Golf 35.8 kWh battery pack usable capacity for two years: from 29.000 to 65.000 km I have observed no chance in usable capacity – it is still at 30 +/- 0.6 kWh usable energy. I usually charge to 80 or 90% SoC and drive until 20 to 40% before charging. I have 575 charge cycles on the pack. We shall see what happens.

    • Pedro Lima says:

      Are you estimating the usable battery by measuring the power consumption of a full charge from the plug? Or do you check the SOH (State of Health) with a OBD?

      Anyway, if you keep avoiding full charges your battery will be good for a lot of cycles.

      • f1geek says:

        I read SoC using the OBD Dongle, drive more than 154 km (at least 70%) of pack and calculate usable energy using trip efficiency data.

  4. Shovon Goutam says:

    LTO is not a cathode material. LTO is an anode material. LFP, NMC, NCA cathode materials can be used with LTO anode material. Toshiba’s SCiB long cycle life battery is with LTO anode and NMC…usually >10,000 cycle…

  5. Yorkshire Le Tour says:

    So Tesla says always charge to 100% and this says something else. I also understand the battery operates a variable buffer and if it doesnโ€™t see 100% it can enlarge the buffer thus reducing range.
    What if you are going on vacation for 2-3 weeks and leaving it plugged in?

    • Pedro Lima says:

      I don’t think that leaving the car plugged in for long time is healthy for its battery. On vacations I would leave the battery charged at around 50 percent.

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