Understanding the science behind “full” & “empty” is key to unlocking Tesla battery best practices.
Beyond Tesla’s implementation of how you charge and maintain vehicle batteries, there’s a complex system of control, variables, and chemistry. This chemistry is widely researched and implemented in lots of everyday devices in a myriad of ways. If you haven’t already, we suggest you take a look at our post on Tesla charging best practices. In this post, things might get a little complicated (spoiler: it does).
Reader-be-warned, kilowatt-hours, voltages, and provisioning lie ahead.
Background: Lithium Batteries
Lithium cells, like most batteries, produce a voltage using a combination of chemistry. This is between the anode and cathode, negative and positive terminals, respectively. The state of charge, which could also be described as the relative ability of the battery to provide power, can be determined by measuring this voltage. If there is a large amount of potential energy stored in the chemistry of the battery, the voltage is high. Respectively, the voltage across the terminals drops when the battery is drained.
Now that that’s out of the way, why is this important? Because the chemistry doesn’t have a beginning and endpoint. It just has steep declines of diminishing return. Going above a specific voltage will produce more heat and won’t really store any energy. Going below a specific voltage will damage the electrolyte and turn it into unusable, inert compounds that do not readily store or release energy.
Defining “Full”
As you approach these high and low limits of a battery’s chemistry, you generate waste heat. This can damage your battery. The magic lies in knowing at what voltage to stop, on both the high and low ends. Setting more conservative voltages reduces wear and tear. This can also increase longevity, but gives you less leeway between empty and full to actually draw power. Setting aggressive voltages increases usable capacity, but leads to shorter cell life. The takeaway here is that manufacturers of battery management systems (BMS’) and battery-operated devices (in this case, Tesla) decide what voltage to set as “empty” and “full”. This is done behind the scenes and is not communicated to the customer/end-user, ever.
You can find out this range by reading cell voltages with an OBD2 reader. Measurements are when the Tesla State of Charge (SoC) reads 0% and 100%.
We can assume “empty” is somewhere around 3.4v per cell, while full is around 4.2v per cell. We trust Tesla to set these voltages to what is good for the user. It determines what the real result of being at a “20%” or “80%” charge actually means for the chemistry. Knowing that these numbers are interpretations and approximations based on chemistry, research and testing inform how we can understand when to utilize, and when to avoid the highest and lowest reaches of a Tesla battery.
Tesla: Cells, Packs, and Batteries
Now that we’re out of the nitty-gritty of cell chemistry, we can focus more on the macro side of things. Things like how every “battery” is made up of multiple “cells”.
Cells in a Tesla pack are monitored and maintained in groups. One or more cells can fail, while the rest will continue to function. Usually, if a cell has a catastrophic failure, a small wire will melt and disconnect it. It will then cease to charge or discharge with the rest of the pack. If a large enough set of cells is out of whack where the BMS can detect that a cell group is not behaving properly, the BMS can safely disconnect that group. It may also throw an error to the main control unit (MCU.) This is to flag the vehicle for further testing or maintenance.
Maintenance most often includes replacing that battery module (~440 cells) and has been performed by third-party repair services. Less information is available on what Tesla chooses to do when faced with a warranty or elective battery replacements.
Provisioned Capacity
There is a concept called provisioning, used in wear items such as batteries or computer hardware. This includes hard drives, RAM, and CPUs. Here’s an analogy to help explain the concept:
Say you have two friends over for dinner. You own five plates and know you will need to use three. You tell your friends you are limiting use to three plates at a time, knowing that even if you break one or two, you will still have enough to each have your own plate. Under-reporting the total is done so that when capacity is lost, you can still perform at your advertised capacity.
The same goes with having an 82kWh battery but reporting it as 75kWh. The same goes for a 128 GB hard drive, but systems report it as 120 GB. As the storage inside the hard drive begins to fail, the hard drive can utilize its spare capacity. This helps to avoid any noticeable effect for the end-user.
There is another benefit to provisioning. It enables something called “wear levelling”, where you might swap out your five plates amongst yourself and two guests over time so that each plate doesn’t get as scratched as it would, were they the only three plates you had. Including more storage, or provisioning a battery pack in the case of a Tesla, allows the rated (and advertised) capacity to degrade more slowly, given the wear is spread across more cells.
Cell Temperatures
Cell temps are much like voltages, where there are “happy” mediums, but outside those norms, in this case, in particularly high temperatures, damage to the cells can occur. Cold temperatures do not overtly damage lithium polymer batteries, but attempting to charge a cold battery, can.
Tesla has gone ahead and programmed minimum charging temperatures into their vehicles that correspond to certain charge rates (measured in amps or kilowatts). This is abundantly clear in two ways: • A cold Tesla will not engage regenerative braking. ‘Regen’ can charge at rates upward of 60A or ~24kW. (For reference, a Tesla motor can output an excess of 300kW during full acceleration). Charging at 24kW when a battery is less than 18º C will almost certainly cause damage, and for that reason, the software prevents you from charging the cold battery by lowering ‘regen’ rates proportionally when battery temperatures are below ~18º C.
When it comes to supercharging, if you do not allow your Tesla battery to preheat beforehand, your charging rates will be severely stunted compared to the maximum capacity of the supercharger.
Preventing Damage
The normal driving temperature for a Tesla battery is somewhere between 15-30ºC, while a supercharger preheat takes it to 50-55ºC. Any colder than that, and the software will limit your charge rate until the temperature improves.
There are basics that apply to most lithium batteries:
- Don’t leave batteries full.
- Don’t leave batteries dead.
Doing either for an extended period of time (more than a few hours) will damage the electrolyte and reduce the battery’s usable capacity.
- Don’t charge batteries quickly (>1C) if you don’t have to.
You may wonder, what is one “C”? The easiest way to understand a C rating is how many amps would be required to charge the battery to full in one hour. On a 100kWh Tesla Model X, the maximum charge setting of the Tesla Wall Connector (240V @ 48A = 11.52kW) would be 0.12C (11,520W / 100,000Wh = 0.1152C). This would take 8hrs 41mins, theoretically, to charge the Model X 100D from empty to full. This is nice and slow — just the way the LiPos like it.
Supercharging at 250kWh, on the other hand, would be simply 2.5C for that Model X 100D. A 53.6kWh Model 3 SR, would be approaching <5C! Charging LiPo batteries at 5C is certainly possible, but generates a lot of waste heat and is not recommended. Supercharging a Model 3 SR like this can be regarded as a way to slowly degrade the longevity and long term performance of its battery.
Finding a Balance
In the end, you did buy your Tesla to use it. As such, don’t feel TOO bad if you need to supercharge! Fill ‘er up, and get where you’re going! Just try not to do that every day. What good is a perfect Tesla battery ten years from now if you never had any fun? You can now go forth understanding the stresses and limits of Lithium-ion chemistry. You can also understand why the Tesla Owners’ Manual says the things it does. Thankfully, the BMS and closely monitored temperatures of Tesla batteries enable a pretty seamless experience. This protects the battery from most harmful situations.
Leave Batteries at Storage Voltage When Not in Use
As discussed in “Battery Best Practices”, we recommend plugging your Tesla into a charger and setting the charge limit between 55 and 60% if you won’t be using it for more than a few days. This has to do with another commonality between lithium-based battery cells. A happy medium exists in the electrolyte around 3.8-3.85v per cell. If the voltage is higher, the cathode tends to throw electrons faster. This produces minuscule amounts of waste heat and possibly contributing to the buildup of harmful gases inside the cell. If the voltage is low, the depleted electrolyte can be lost. It is converted to inert chemicals that block the regular flow of electrons, reducing the potency of the cells.
Tesla Recommendations
Tesla recommends these practices in a user-friendly way whenever possible:
- limit supercharging to only when needed
- Tesla calls >90% SoC “Trip” and 50-90% “Daily”
- Tesla will preheat batteries for supercharging when given a chance
On top of that, Tesla implements the following in the background without indicating it:
- Tesla will constantly warm and store batteries around 8º C or warmer in most conditions when plugged in
- Tesla will limit regen (which requires battery charging) when the batteries are cold (less than 18º C) and even limit acceleration in extreme temperatures.
When the news of larger battery packs breaks (on the Model Y, for instance, where Tesla quietly changed their manufacturing specs from 75 to 82kWh), this does not usually correspond with increased EPA range on the vehicle. It is safe to assume that this extra capacity is provisioned to spread out the wear and drain on the cells. Use five dinner plates, instead of just three.
Wrapping Up
It’s important to remember that battery voltages are a result of molecular chemistry, and not simply a matter of “empty” or “full”. Interpreting this information is subjective and how we go on to interact with this chemistry alters how it behaves. Battery and device manufacturers like Tesla make certain decisions designed to increase ease of use. Knowing what’s behind the scenes can help us understand why, and cater our interpretations of this information based on a desired outcome, either simplicity or longevity.
Tesla battery systems are resilient and forgiving, often allowing for years of use with zero maintenance. Understanding the conditions, voltages and charge rates that affect these systems beyond their own control can help us move toward being what some might call a “power user”, squeezing that extra bit of performance and longevity out of this family of electronics.
If you are thinking about placing a Model Y order, be sure to use my referral code and get 1,500 Supercharger kilometres!
*As of Sept 2021, Tesla has put a pause on referrals 😞 I will update if/when they reinstate it.
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