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15.06.2020 | 4 minutes

How to keep your battery in good shape during periods of inactivity

Extended periods of inactivity are not good for electric car batteries, but sometimes they just can’t be avoided. The current situation with COVID-19 is a good example, as you spend more time working from home. But you may also use your car less in the summer, when you’re on vacation or opt for your bicycle instead to enjoy the beautiful weather. The right charging behaviour can keep your battery in good shape.

How do you keep your electric car battery in good shape if you don’t use it for weeks at a time? While some drivers neglect their electric car for longer periods, others fully charge their vehicle on a daily basis. But none of them are doing the right thing. In fact, all of them are damaging their electric car battery.

In this article, you can learn more about the basics of battery technology and find tips for increasing the service life of your electric car battery.

Should you charge your vehicle during extended periods of inactivity? What effect does that have on the electric car battery?

The self-discharge rate for lithium-ion batteries is usually around two percent per month. But the battery in an electric car tends to discharge faster due to vehicle electronics.

For example: A Nissan Leaf is not driven for an extended period of time, the battery loses approximately 15 kilometers of range per month. A Tesla Model S is equipped with a good deal more electronics, which is why the vehicle can lose up to 15 kilometers of range per day! (provided the Tesla Model S is parked in the standard standby mode and is not fully switched off).

While you shouldn’t neglect your electric car during periods of inactivity, you shouldn’t charge it to 100 percent either, as this subjects the cell to high voltage. And the electrolytes could also degrade the cathode or positive terminal inside the battery.

What is the optimal state of charge for the battery?

Just over half: the ideal state of charge for electric car batteries is between 80 and 50 percent (not to be confused with the ideal range for the best driving range, which is between 80 and 20 percent). A state of charge of 80 percent maintains such a low voltage that the electrolyte will not degrade over a period of several weeks. At the same time, a state of charge of 50 percent is still high enough to reduce the risk of excessive discharge when the battery is not used for an extended period of time.

For certain electric cars, you can easily configure the charging limits on the dashboard or via online portals. But what about cars that don’t have a function like this? You then stop the battery charging process manually once the desired state of charge has been achieved.

But there’s no need to stand next to your car and stare at the battery display. Using the formula described below, you can determine when to start and end the charging process:

Take, for example, a 40 kWh battery pack, with the electric car battery currently charged to 30 percent (i.e. 12 kWh). You want to charge it to 80 percent (i.e. 32 kWh). Let’s assume that your electric car is equipped with an 11 kW charger and you’ve connected it to an 11 kW charging station.

Proper charging can increase the capacity of the electric vehicle battery enormously.

Proper charging can increase the capacity of the electric vehicle battery enormously. Photo: Bloomberg via Getty Images

The formula for calculating is:

Charging time (hours) = required power / charging capacity

As a result, [32 – 12 kWh] / 11 kW = 1.81 (1 hour 48 minutes)

If you connect the 11 kW charger when the battery’s state of charge is 30 percent and, after one hour and 48.6 minutes, disconnect it from the charging station, the electric car battery will be charged to around 80 percent.

Please note that the formula above merely provides a rough estimate of the required charging time. The formula does not guarantee a state of charge of exactly 80 percent, as the charging time is dependent on additional factors such as temperature, the state of charge (SoC), the state of health (SoH), the age of the electric car battery, and the type of technology.

What effect does it have on the battery technology if you simply leave your electric car connected at all times (including above an SoC of 80 percent)?

There are currently several discussions about the right charging strategy, if you want to leave the charger connected for an extended period of time. It’s a good idea to check the car battery’s state of charge at least once a week and, if necessary, charge the vehicle (if the SoC falls below 50 percent).

There’s nothing wrong with leaving your car connected at all times, provided you follow the 80 percent rule. That means your vehicle needs to be able to configure a charging limit of 80 percent. The battery management system (BMS) of your electric vehicle should regulate charging behavior, so that the battery is not continuously charged at all times, but only as needed.

It’s also important to note that the car’s 12-volt auxiliary battery, which is also used in gas and diesel-powered vehicles, for example, to power headlights, windshield wipers, and infotainment systems, is usually only charged when the electric vehicle battery is active. In other words, it’s only charged when the electric vehicle is fully switched on and ready to drive or when the vehicle is charging. It’s therefore important to charge the car or switch on the motor on a regular basis.

The conclusion regarding the optimal process for charging the electric car battery:

What should you do based on the battery’s state of charge?

  • 100–80 percent: Avoid charging the electric car battery to over 80 percent to prevent electrolyte degradation and capacity loss.
  • 80–50 percent: During an extended period of inactivity, for example when on vacation, try to maintain a state of charge between 50 and 80 percent. This is the best way to keep the battery in good shape when you don’t use the vehicle for an extended period of time.
  • 50–20 percent: Try to maintain your vehicle charge within the recommended SoC range.
  • 20–0 percent: If the state of charge falls below 20 percent, you will need to prevent additional discharge. You should connect the vehicle to a charging station immediately (but not at full charging capacity!). Simply connect the charger, and the car’s battery management system will gradually increase the charging current in a way that is easy on the battery.


This article was originally written by Pon Paulraj in response to the COVID-19 lockdown: How to maintain your EV’s battery health during Covid-19 Lock down



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