Note that newer battery technologies, such as LFP, are even more resistant to certain battery failures. 

EV batteries are composed of many hundreds (or thousands) of small, individual battery cells arranged in various configurations. 

Since the battery packs include so many cells, it is inevitable that minor differences in voltage exist between them, and that some of the batteries in the pack will experience a minor degree of overcharge or over-discharge during cycling. These everyday occurrences are typically handled by the battery management system (BMS), which is responsible for monitoring and controlling cell balance across the battery. However, intentional or persistent overcharge or over-discharge may exceed the capabilities of the BMS, jeopardizing the health and performance of the battery. We’ll break down why it’s bad to overcharge and over-discharge, and then if you still want more, check out our deep-dive companion piece on causes of battery failure.

What happens when you overcharge a lithium-ion cell?

Given the complex nature of ever-improving EV battery packs - which include thermal management, battery management, and lots of cells linked together - much of the science behind what’s best comes from laboratory experiments and computer modeling of individual battery cells, or extrapolations from other devices containing lithium-ion batteries (e.g. laptops). Here’s what science tells us about overcharging lithium-ion cells. 

• Lithium plating risk. This is when excess lithium ions accumulate on the positively-charged surface during charging and fail to be absorbed into it. This process is not specific to lithium ion batteries, but also occurs with other metals such as iron and copper. The lithium plates impede the free flow of ions and energy in the battery, ultimately resulting in short circuits and battery failure (Liu 2014). The well-known Samsung Galaxy Note 7 battery fires of 2016 were a direct result of lithium plating, which eventually led to catastrophic battery failure. Lithium plating also causes a loss of active battery material, because the lithium that is used to generate electricity is trapped in the inactive lithium plates.

• Heat risk. The process of overcharging a battery inevitably means that the battery cell will experience increased temperatures. High temperatures in the cells can accelerate the rate of secondhand chemical reactions that will use up active material and have the potential to cause breakdown of the battery chemistry.

What happens when you overcharge an EV battery?

It’s actually quite difficult to overcharge an EV battery, since there are multiple fail safes in place. 

First, the battery management system will not continue to accept electricity once the battery is full. Second, the usable capacity of most EV batteries is less than the total battery capacity, meaning that some of the battery is unavailable to the driver. This is done exactly to protect the battery from overcharging, or even from extremely high states of charge. When you charge your battery to 100%, there is still some of the physical battery that is not charged. 

Nonetheless, it’s not ideal to charge an EV to 100% and store it in such a state. A fully charged battery has a large amount of electro-potential energy, making it a relatively unstable state. Under these conditions, the changes of harmful, secondary reactions are significantly increased.

What happens when you over-discharge a lithium-ion cell?

At very low charge levels, the battery may experience rapid “voltage drop.” In general, voltage drop refers to a loss of electricity to internal resistance. This loss happens all the time, but when the state of charge is low, it can accelerate. When the voltage drop is severe, some of the battery's chemical components can break down and react with the active materials within the cells. These physical changes, which are similar to what happens at high temperatures, can lead to a gradual loss of battery capacity, since there is less active material available to generate electricity. One study indicated that irreversible capacity losses of 12-25% were noted in lithium-based batteries when over-discharging the battery across 1000 cycles (Zhang 2015). 

What happens when you over-discharge an EV battery?

Unlike overcharging, it is very possible to over-discharge an EV battery. Simply drive until the car enters low propulsion mode, and then keep on driving.

Actually – don’t do this, since you risk all the same failures as with a battery cell. 

Frequent over-discharge of an EV battery, which can be defined as dropping below 10%, may cause irreversible damage to the battery. In addition to reduced range, the internal changes to the battery may lead to problems in the future, including increased risk of battery failure. Theoretically, unsafe conditions detected by the BMS could also lead to slower charging rates and further reduced power when the battery is placed in extreme conditions (i.e. below freezing or very hot climates). 

Takeaways:

Going on a road trip and want to power up to 100% before you hit the road? Great. Going on a road trip and want to power up a week before you hit the road? Less good (unless you own a vehicle with an LFP battery).

Struggling to reach the next charger since the one you planned to use was out of service? That’s fine, but try not to make a habit out of it. Running your battery down below 10% because you like the rush? Maybe not. 

As with a lot of our advice, avoiding 100% and below 10% state of charge are nice targets to aim for, but you should not let them get in the way of enjoying and using your EV. The battery management system makes lithium ion batteries more robust than ever before. 

‍

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.