What are LFP batteries, and why do Tesla use them?

Last modified: 31/08/2021
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A quick primer on the two types of batteries chemistries used in current Tesla vehicles which should help ease anxiety about how they work and should be treated. It is intentionally written to be accessible to most people and skims over detail of chemical changes within batteries and the science of how they store and release electrical energy.
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Recently Tesla has started using a different lithium-ion chemistry in their Model 3 SR+ cars and have changed their advice on how to use the car.

LFP, lithium ferro-phosphate is the alternative cell chemistry being used by Tesla in some models but has been around for a long time. It is more commonly used in China and often used with high power low range applications (buses/trucks). LFP is different to conventional li-ion but neither worse nor better. It charges/discharges very easily, has an exceptional cycle life (you can charge/discharge many times with very little degradation) but at a cost of being less energy dense, so you need more volume to fit the same capacity of battery. You can very roughly equate cycle life to total lifetime mileage of the car – more cycles is more miles before pack needs replacing.

This makes sense for SR+ model 3, it has enough space for a large li-ion battery so using less energy dense LFP Tesla can still achieve the required energy capacity that the SR+ model spec requires. i.e. SR+ with LFP has a bigger battery volume than SR+ with Li-ion.

Interestingly because LFP can discharge so rapidly, Tesla could probably make this model faster without undue harm to the battery. I expect they don’t do this purely to keep model performance differences as a marketing ploy, not that the SR+ is slow anyway.

The car has an internal buffer, i.e. unused capacity at both ends of the voltage range of the cell. So when you charge to 100% it is likely actually less than this in reality. Same on discharge, when it states 0% there is actually a little reserve capacity left, this has been demonstrated by owners driving the car for several miles after hitting 0% capacity. A large buffer helps protect the battery in early days and then can be used as the battery does degrade to maintain the stated range. So as capacity diminishes, move towards filling the battery to the real 100% capacity. On the 0% end of the capacity, by leaving some spare energy in reserve, you avoid the car damaging the cells as it continues to discharge to varying degrees when not in use. The car will completely turn off to protect the battery from over discharge which will need some additional measures to open the charge flap and reenable charging. LFP models will likely ‘supercharge’ more easily than the Li-ion cars, certainly at a cell level they absorb energy far more readily but the car software will control it. There is a relationship here though with capacity, a larger battery (LR/Perf) can charge faster than SR+ simply because it has a bigger bucket to fill, so even though the SR+ with LFP has faster charging capability, because it is a smaller bucket it may actually charge at a similar or slower rate to the bigger siblings. It should charge faster than the SR+ Li-ion but Tesla software controls this and there are many factors that drive charge rate.

80% is often cited as an optimal charge level. This is a compromise but a good one – the less energy you store in the battery the more relaxed it is and the less damage caused. Think of the cell like a balloon filled with water, you can fill it to absolutely full but it’ll be very stretched and tight and will become weaker. If you can get away with only 50% in the battery and comfortably do all your journeys, then do that and recharge to 50% each day, it will benefit the pack. 80% means you get decent range and helps not stress the battery. 100% is for the days you are doing a long journey and want decent buffer to reach the next charging stop but really only use if absolutely essential – most journeys it will not be.

Another aspect of 80% is this is about the point that the pack reaches full charge voltage and switches from a constant current to constant voltage phase. Details are easy to find on the net but in simple terms, charging slows down from about 80% capacity, you’ll see this if you watch it at a supercharger. This is why if you can charge to 80% or less and have enough range to get to the next supercharger, that will probably be quicker than charging to 100% and putting less charge in at the next charger or destination when you don’t care anyway. As chargers get busier you might hypothesise that Tesla may start nudging up the kWh price after 80% charged to encourage drivers to move off to free up pumps. Note when using a home ‘fast’ 7kW charger you won’t see this slowing of charge as the rate is already very slow compared to what the battery is capable of accepting.

Charging to 100% for LFP makes some sense, as stated it has impressive cycle life even with full charge and discharge cycles, so it will degrade the cell but by an appreciably smaller margin than for the Li-ion pack. By way of example, li-ion based cells get somewhere around 500 cycles before they are judged end of life (which in battery world is actually only 80% of original capacity). LFP will often achieve 2000 – 5000 cycles before hitting this 80% of original capacity threshold. This is why Tesla isn’t bothered about you charging to 100% to maximise range as you’ll still get more cycle life than the conventional li-ion cell type.

Life of the battery is of concern for many new owners but simple arithmetic should settle your nevers. Using conventional li-ion life of 500 cycles and assuming a 200 mile range, that equals 100,000 mile battery that’s only lost 20% of capacity at that point. With the SR+LFP model, you’re probably going past 500,000 miles before that happens.

The remaining range of the car is a very challenging prediction that the car computer makes and will constantly update. For most lithium based rechargeable systems an occasional 100% charge helps calibrate the algorithm that determines capacity remaining (and thereby range). Otherwise the car is trying to track capacity without a good known starting point. That means it is constantly trying to monitor exact energy consumption at all times (including when not in use) and estimate what is left. It must also consider temperature as this has an effect of the battery and car efficiency, as well as the parasitic drains on the battery e.g. if it is very cold tomorrow, it won’t go as far as it is currently predicting today. ICE cars are no different, they use more fuel for the same journey if it’s colder, or if the driver simply chooses to travel more quickly. Conventionally drivers didn’t give much thought to the predicted range remaining on their combustion engine cars as it made little difference – you just needed to refill a little earlier given how fuel is so readily available. The range remaining in the car is a prediction only and it has no real reflection on the actual true capacity of the battery. Charging to full and draining to nearly empty to ‘calibrate’ the range is futile and only helps the driver feel better, it doesn’t make any difference to the battery performance.

So called phantom drain is a valid contributor to range reduction – the car uses energy when it’s sat doing nothing. Especially if it actually is doing something like cabin pre-heating or Sentry Mode or downloading an update and so on. This is a different to most conventional combustion engined cars which really were doing nothing when turned off. However, it wasn’t able to get regular updates, watch for intruders, defrost the screen while you were eating your cornflakes and so on. Also remember that whilst you have ‘paid for’ the lost miles, by comparison you’ve paid a lot less for the actual miles you have completed versus a combusion engine powered car.

It’s a good idea when you’re past worrying about range to switch to percentage remaining rather than miles on your display. You’ll quickly get used to charging when you need to and relying on superchargers for longer journeys. You’ll also find after a few months of owning the car, you’ll wonder why you ever worried at all.

By: Mark Andrew

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