In this article, we briefly explore the history of high voltage batteries to shed light on where some of the charging confusion comes from, as well as reasons that your car’s software systems might benefit from occasional full charges. However, when it comes to lithium-ion batteries themselves, there is never a need to fully charge. If you need to do so for a long trip or other infrequent event (see below), make sure not to store the car with a full charge.
Brief + Recent Battery History
A battery cell is a chemical system made up of two electrodes and an electrolyte. The performance, cost, and safety of a battery is determined by the specific choice of electrodes and electrolyte chemistry, as well as the structural design of the cells within the battery. Given that lithium is the lightest metal and the one that loses its ions most easily, most modern batteries are based on lithium-ion technology. However, many other battery compositions exist.
Prior to the rise of lithium-ion batteries, nickel-based battery composition was commonplace. Arguably, the resurrection of modern EVs kicked off in 1997 with Toyota’s introduction of the Prius Hybrid, which contained a nickel metal hydride battery. One of the major drawbacks with nickel batteries, and one the reasons that this battery chemistry was replaced, was a phenomenon known as the memory effect. The memory effect is a battery’s ability to remember its regular usage pattern. What does this mean? If you regularly use just 30% of a nickel battery to commute to and from work, it will gradually lose its ability to use the full capacity. It “forgets” whatever capacity is not used.
The way to restore a nickel battery’s memory is by doing a complete discharge, or running the battery down to 0%. After complete discharge, the battery must be carefully recharged to full, ensuring it is not overcharged, which can cause damage to the battery.
With modern smart chargers (below), the risks associated with this process can be significantly reduced. However, as our previous battery articles have mentioned, complete discharge of any battery is a bad idea and can result in an unnecessary damage to the battery materials.
Having to reset a battery’s memory made nickel chemistries less than ideal for EVs since you might have to plan for a reset before any longer trips. In contrast, lithium-based batteries have very minimal, if any, memory effects. This property, in addition to many other positive attributes like energy density, made lithium-based batteries the preferred choice for EVs.
The short answer
Since lithium-ion batteries do not have any memory effect, the battery itself never needs to be fully discharged or charged to 100%. Both of these states can cause damage to a battery. While most manufacturers leave a buffer at the top and the bottom of the capacity range to prevent users from inadvertently damaging them with a full charge or discharge, it is still not recommended to leave your car at an extreme state of charge.
But, here’s where things get complicated.
Modern lithium-ion batteries are paired with sophisticated computers that help manage the battery health and temperature. These computers may need occasional full charges in order to calibrate the software and give accurate state of charge estimates. This may happen after a period of use or following major software updates that affect range or efficiency. Check with your owner’s manual to see if your EV’s computer system requires occasional full charges, but know that the lithium-ion battery itself never does.
Below, we dive into the role of battery management systems and why calibration is important to the software.
Smart Batteries
Although batteries seem like simple and easy-to-use technology, they can be notoriously hard to monitor. It is important to understand some limitations of the standard battery:
- The user is unaware of how much energy the battery has remaining
- The user is uncertain if the battery has sufficient power for a particular use
- The charger must match the battery size and chemistry
The solution to these unknowns is the smart battery, which integrates communication between the battery, the electronic device powered by the battery, and the user through use of a battery management system (BMS). The BMS gets data from the battery, extrapolates and computes information about its state, and communicates to the user.
Unfortunately, unlike measuring the fuel level in a gasoline fuel tank, measuring stored energy from an electrochemical device is complex. The BMS generally estimates state of charge via coulomb counting, which measures in-and-out flowing energy. While this method seems like it should be fairly accurate, it relies on having a good starting estimate for state of charge. Given that each estimate is built on previous ones, an error along the way can get compounded and lead to inaccurate measurements.
And now, we come to the heart of the matter: occasionally recharging your EV battery to 100% might help improve the BMS and the accuracy of state of charge readings.
System Reboot
If the battery management systems is no longer measuring state of charge accurately, the readings can be improved in two ways:
- Calibration of the battery management system
- Cell balancing
BMSs generally do both of these things just fine on their own, but there are simple ways to reset those that have gotten out of whack. These tips might be helpful if you've gotten a new battery pack, a software upgrade that calculates efficiency and range differently, or if you have not been using your car much recently. A sudden drop in on-board range might also signal that calibration or balancing is necessary.
Calibration
Calibration requires a variety of stable readings across a range of states of charge. To get a stable reading, the battery needs to be left for several hours without being used or charged, allowing the battery to reach equilibrium (‘resting’). By doing so, the battery can set SoC “orientation points” (see graph). Increasing or decreasing the battery capacity between these points lets the BMS to assess the energy storage capacity. Ideally, the SoC orientation points are spaced far apart, which allows the system to have a better approximation of 0% and 100%. It is important to note that each specific manufacturer designs a battery management system with its own mechanism that is not disclosed.
Given the flat discharge curve of a lithium-ion battery in mid-SoC range, the best reading locations are below 30% and above 70% SoC, where battery estimates are most likely to be off. The process outlined below allows for self-calibration, which may improve range prediction by up to 20%. It is important to note that charging to exactly 100% is not necessary during this process.
The following procedures are suggested in regards to Tesla batteries but similar procedures may work for other cars:
- Allow the battery SoC to drop below 30%
- Allow the battery to rest 4-6 hours prior to charging. If possible, repeat steps 1&2 for several SoC measurements (e.g. 30%, 20%, 10%, then proceed to step 3)
- Charge the battery to between 80% and 100% using L1 or L2 (avoid fast charging)
- Following charge, allow the battery to rest 2-4 hours before use
For optimal results, this calibration procedure may need to be repeated.
However, the way you normally drive may be easily tweaked in order to calibrate your BMS. Simply put, the goal is to maximize the number of occasions where the battery is in deep sleep for 4-6 hours and across a variety of charge levels. Low SoC orientation points will likely occur at the end of the day or week before recharging. If you can delay your next charge, it should leave adequate time for the battery to rest at the orientation point. Similarly, high SoC orientation points will occur after a full charge, but require a few hours of rest before using the EV. This could be as simple as setting a midnight departure time on your car once in a while.
A typical lithium-ion discharge voltage curve is shown below, with examples of orientation points that can help calibrate the system.
Cell Balancing
The battery for an EV is simply a combination of individual battery cells in parallel and/or series. A modern Tesla Model S battery contains over 7000 lithium-ion cells. Given the large number of cells and the complex connectivity, cell imbalances can occur over time.
Cell imbalance occurs when individual battery cells have different levels of charge. When this happens, it’s harder for the BMS to determine the true capacity of the battery while keeping the individual cells in their working range. For instance, one of the cells may reach 100% while the others are still around 95%. The BMS will stop the entire pack from recharging any further, potentially robbing you of a small percentage of your total capacity.
Although the best cell balancing happens at the battery assembly plant by choosing high quality cells with near identical capacity, the process outlined below may provide minor corrections to imbalances. Note that modern BMSs have pretty good, built-in cell balancing capabilities.
Balancing Procedure:
- Plug the car into a L1 or L2 charger and set the charge limit to 100%
- Charge to 100%, then leave the car plugged in
- Allow the car to continue charging until it indicates no energy is being added to the battery. This may take 1+ hours after the car has reached ‘100%’ but eventually the battery should stop taking current, concluding the process
Conclusions
The modern smart battery has the ability to self-calibrate, but occasionally requires some help. Thoughtful considerations when charging and discharging the battery will help this. Specifically, sufficient rest time at various states of charge are required. Some battery scientists claim that left unattended, SoC estimates can be off by as much as 30%, which can be a massive problem for EV drivers.
Written by Brandon August, a lifelong explorer of all things academic. After obtaining an undergraduate physics degree and a doctoral degree in biomedical, he began to explore various professional fields in health and wellness, rideshare work, freelance writing, and day trading. On the recreational side, he has always been involved in the automotive field, owning various vehicles across the years. After a recent move to California, he entered the EV space, purchasing both a Chevrolet Bolt EV and a Bolt EUV for his household.
