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Recurrent always recommends keeping lithium ion batteries between 30% and 80% state of charge, regardless of their specific battery chemistry.
But, Tesla recommends that LFP drivers charge to 100%. Why are these recommendations different?
Why the discrepancy? Two things:
- LFP chemistries are known to hold up better to high charges than NCA or NMC batteries. This means that an LFP battery charged regularly to 100% will, over time, degrade less than a different battery treated the same way. Since LFP batteries have lower energy density, the extra bit of charge means an equivalent range for cars with these packs. However, for all lithium ion batteries, avoiding a full charge will prolong life. It’s just that it may not be as noticeable a change with LFP packs.
- Tesla’s battery management system (BMS) will be more accurate if it can recalibrate at 100% state of charge. This is true for all Teslas, but particularly true for LFP models. Why? LFP batteries have a very flat voltage curve, which means that it’s harder for them to calibrate at 80% than it is to calibrate an NCA battery at 80%. Read on to understand more about this.
Nerdy aside: Why a flat voltage curve makes the BMS job’s harder
With NCA batteries, the voltage increases pretty linearly as the state of charge increases, and decreases as the state of charge decreases. That means the BMS can use voltage information, which is relatively easy to check, to estimate the state of charge. However, in an LFP battery, the voltage does not vary as much with state of charge. This makes it harder to use voltage as a predictor, especially when it comes to individual cells. By charging to 100% periodically, the BMS has a “set point” to recalibrate its capacity prediction, and hence the accuracy of its range estimates.
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