Best charging behavior for LFP batteries

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.

The other thing that damages battery cells is high temperatures, since charging isn’t 100 % efficient, the more you charge the battery the more you heat it. Especially if you’re using fast chargers and charging at high C-rates…

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. If Model 3 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

 

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

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…

Degradation over time due to the chemical reactions that occur during charge and discharge cycles are inevitable, but more detrimental and pronounced at extreme DoD (depth of discharge) levels.

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

https://www.bravabatteries.com/lifepo4-battery-discharge-and-charge-curve/