Why should you only charge an EV to 80%?

Why charge to only 80 percent

Charging your EV to 80% is a battery saving measure that extends its lifespan. While charging to 100% now and again won’t cause immediate harm or noticeable degradation, it will accelerate degradation over the long-term.

To understand why, you have to understand the chemistry, which we will go into below.

In a nutshell, the battery is constantly degrading and the more often you charge it to full capacity, the faster its degradation. By charging to 80%, you limit the amount of time the battery is exposed to high levels of heat, current and voltage, which helps to preserve its life.

Charging to 80% helps to reduce stress on the battery cells and prevents them from overheating, which can also help increase battery life.

Another reason to charge to 80% is speed at public chargers, which reduce charge speeds significantly after 80% to protect the vehicle’s battery from heat.

Why do charging cycles degrade batteries?

Charging cycles can cause mechanical and chemical damage to the battery cells over time.

The process of charging causes lithium ions to move from one electrode to another, and as this happens, there is an exchange of heat and electricity. This exchange causes microscopic damage to the electrodes and electrolyte, leading to reduced capacity and cell failure over time.

Degradation is caused by a slow build-up of insoluble lithium deposits on the anode during regular use. These deposits interfere with the flow of ions between the anode and cathode, reducing the effectiveness of the electrochemical energy exchange.

Eventually, these deposits will reduce the amount of energy your battery can store and make it less effective as time passes.

Related: Next-gen battery technologies

High charge and discharge rates can also lead to increased degradation. This is why people suggest only charging from 10% or 20% up to 80% to reduce the number of charge cycles and related stress applied to the battery cells.

This is important because lithium-ion batteries in EVs also degrade for other reasons.

Factors that degrade lithium-ion batteries

Discharge and charge cycles degrade the battery cells on a microscopic level. Over time, this leads to a breakdown in the chemical energy transfer process.

Lithium-ion batteries degrade over time because of a variety of processes and factors. These include oxidation, corrosion, impact damage, mechanical stress, and charging cycles.

  • Oxidation occurs when a battery’s electrodes react with oxygen to form lithium peroxide – a compound that can decrease the battery’s storage capacity by coating the electrodes and prevent electrons from flowing.
  • Corrosion occurs when metallic ions in the battery react with water, forming a foreign material that can block current flow.
  • Impact damage occurs when an impact from dropping or banging the battery causes microscopic cracks in the battery cells, which causes an increase in internal resistance and leads to capacity loss.
  • Mechanical stress occurs when lithium ions are transported between the positive and negative electrodes during charging and discharging cycles, leading to microscopic wear and tear that can become irreversible over time.
  • Heat – batteries get hot when charging beyond 80%. When an EV battery is rapidly charged, the heat generated increases significantly, causing the battery temperature to rise. When a battery approaches its full capacity and is continuing to be charged at a high rate, additional heat generates throughout the cells of the battery.

How to extend the life of your EV battery

Here are some tips for making your EV battery last longer:

  • Store your EV in a garage. This will help keep the battery’s temperature in an optimal range.
  • Avoid charging to 100%. This can damage your battery and cause it to degrade faster. Instead, limit your charging to 80%, which is the typical safe charge limit.
  • Avoid letting your battery go completely empty. Most li-ion batteries like to have a minimum charge level of between 10-20%, so try to keep it at this level or higher.
  • Try to avoid extreme temperatures or hot and cold weather. Batteries tend to degrade faster in temperatures that are higher than 85 degrees F or lower than 65 degrees F.

How long do EV batteries last?

EV batteries are designed to last 10-20 years, but can become unusable in an electric vehicle from only five years old if the battery is abused (after which, the battery can still be used as energy storage in a home or caravan).

Generally, EV batteries are expected to last around 10 years or 150,000 miles before needing to be replaced. However, these estimates can vary widely depending on a variety of factors including driving habits, climate, and maintenance.

What happens if an EV battery goes to 0%?

If an EV battery goes to 0%, then the vehicle will no longer be able to move will need to be towed to a charging station, or you will need to call a breakdown service to give your car a small boost (some breakdown cover includes mobile charging).

While some electric cars will crawl for a few miles on 0% charge, you risk fully depleting the battery reserve if you don’t make it to a charger, rendering the vehicle useless and robbing you of the ability to run the heating or climate control (not good in winter!).

It is generally recommended to start charging a car when it reaches 20% charge or below, as this helps ensure optimal battery performance and a longer battery life.

Summing up

Charging to 80% will extend your EV battery’s lifespan versus charging to 100%.

Lithium-ion batteries have a limited number of charge cycles and overcharging them can damage the cells and reduce the overall battery lifespan.

When you charge your device to 100%, the charging current will stop and then will trickle in to offset losses from self-discharge and maintain the 100% charge level. This can put unnecessary stress on the battery cells since they are never fully discharged.

By reducing the charging rate at 80%, your battery can remain in a healthy, balanced state that preserves its capacity and lifespan.

Related: How to choose an EV home charger in 2023

James Lewis is our resident electrical head. He drives an MG ZS EV (2018, which he loves) and plans to get the new one soon. James is much more excited by the lower end of the EV market and is looking forward to the Ora Cat.