Can You Overcharge an Electric Car? (With Bonus Battery Maintenance Tips)

Can You Overcharge an Electric Car? (With Bonus Battery Maintenance Tips)

The battery is usually the most expensive, and most important part of an electric car. So, is it possible to accidentally overcharge an electric vehicle and damage (or degrade) the battery?

The short answer is that you can’t overcharge an electric car’s battery. Electric vehicles (like Teslas, Chevy Bolts, Nissan Leafs) all have a built-in battery management and monitoring system which makes sure that the main battery pack doesn’t overcharge. 

Here’s how it works:

Once the system detects that the battery is nearing a 100% charge, it’ll slow down the charging process. When the battery reaches 100%, it will then start to “trickle charge,” which means that the battery will be charged periodically at the same rate of its self-discharge rate. This maintains a full charge for the battery without overcharging. 

It’s worth noting, however, that continuously charging a battery to a full charge of 100% can degrade a battery over time and slightly reduce its effective lifespan. 

We’ll talk more about the factors affecting battery health (lifespan) later on, along with other related electric vehicle (EV) questions. 

Can You Leave an Electric Vehicle Plugged-In Overnight?

It is completely safe to leave an electric vehicle charging (or plugged-in) overnight.

Tesla Electric Vehicle Charging

In fact, charging at night allows you to take advantage of off-peak electrical hours so you can get your car charged for cheaper. 

For example, electricity costs up to 13.4 cents/kWh during the middle of the day in Ontario, Canada. From 7PM to 7AM, however, the cost is only 6.5 cents/kWh. That’s a savings of over 50%!

Should You Leave Your EV Charging While You’re Gone for Long Periods of Time?

If you’re going to be leaving your electric car at home for a couple of days or weeks, you might be wondering: Should I charge my EV while I’m gone, or just let it sit unplugged?

The answer to this question depends on which model of electric vehicle that you have. However, for the most part, you can leave your electric vehicle charging without any negative effects on your battery. 

Keep in mind that it’s best to consult your owner’s manual to find the best route-of-action if you’re going to be leaving your car for an extended amount of time.

In general, if you’re going to be away for months on end, it’s probably a better idea to keep your car plugged-in to avoid your battery draining to zero. 

However, if you’re only going to be away for a couple of days or weeks, you could just leave your car unplugged (as long as it was adequate charged beforehand). Just make sure that the battery doesn’t discharge too low.

What exactly is “too low” of a charge for a battery? There are varying opinions on this matter, but a general rule of thumb is to keep your battery above 30% to maintain long-term performance. Going below that level once in a while won’t do too much damage (just make sure the battery doesn’t discharge to zero). 

Image of a silver Tesla Model S electric car
A Silver Tesla Model S (Electric Vehicle)

If you own a Tesla, it’s recommended that you keep the car plugged-in, but with a maximum charge level set at 50% (this can be done in the car’s settings).

This will allow the battery to maintain a charge of 50%, which is the most stable state for lithium-ion batteries. 

Will the Battery Continue Drawing Power after Fully Charged?

An electric vehicle does continue to draw power even after it’s fully charged. This is because there are some electrical components in the car that are always on, which will slowly discharge the battery.

This small loss of power while your electric car is supposedly “off” is known as phantom power or vampire power, and there are a variety of contributors to this loss of power.

For example, if you own a Tesla Model 3 and you use sentry mode, then the motion sensors and cameras on the car will be constantly drawing power.

In another example, if you monitor your car from an app on your phone (or via bluetooth), then your car is always drawing electricity to relay the requested information to your phone. 

Location of the Nissan Leaf Battery Pack (In Orange) / Image courtesy of Norsk Elbilforening / CC BY 2.0

The amount of phantom drain for different electric cars can vary based on different situations, but it’s usually very minor. For instance, on average, a Tesla battery self-discharges about 1-2% per day, even if the car isn’t driven at all.

Overall, phantom drain won’t have a large impact on your battery’s charge level nor will it have a large impact on the range of your car. The cost of the electricity lost by this normal battery self-discharge will be minimal as well. 

Factors Affecting Battery Health and Lifespan

Battery lifespan, or the state of health (SoC) of a battery refers to the condition of the battery compared to when it was new. 

As a battery is used over many charge cycles, it’ll slowly degrade and lose capacity relative to when it was new. In other words, the state of health of a battery slowly decreases over time, leading to a gradual decrease in range for your EV. 

Nissan Leaf Electric Car Battery Pack
Battery Pack of a Nissan Leaf EV / Image courtesy of Gereon Meyer / CC BY-SA 4.0

In an electric car battery, there are 2 main factors that affect battery health.

1. Extreme Temperatures

If exposed for long periods of time, extremely hot or extremely cold temperatures can degrade a battery. 

2. Extreme Battery Charge Levels

Keeping a battery at a very high or very low charge level for a long period of time can reduce its lifespan. 

As I’ve already talked about before, you can’t overcharge an EV battery. However, the battery can still be slightly damaged by too much charging or too much discharging. 

How to Preserve and Maintain your Electric Vehicle Battery

It’s worth noting that for the most part, electric car batteries are extremely reliable and will not degrade much over their lifespan

For example, recent reports show that on average, current batteries in Tesla vehicles still retain about 90% of their original capacity after 200,000 miles (320,000 kilometres). 

With that being said, however, here are a few tips to help you maintain your electric vehicle battery and increase its lifespan.

1. Avoid Deep Battery Discharges

Keeping your battery at a low percentage of charge for long periods of time can reduce its lifespan.

As a general rule of thumb, keep your battery charged above 30% to maintain battery health long-term.

2. Avoid Extremely Fast Speeds and Acceleration

When you suddenly accelerate your electric car or drive at super high speeds (for long periods of time), the battery has to supply more current to the motors in the car. This heats up the battery and degrades it faster.

You’ll know about this if you’ve ever switched to “Ludicrous Mode” (which unlocks better performance) in a Tesla Model S performance. You’ll get a warning that turning on “Ludicrous” will heat up the battery more than normal and will reduce its lifespan over time. 

3. Don’t Charge Your Battery After it Reaches 100%

Leaving your battery at a charge of 100% for extended periods of time can lead to faster battery degradation. 

In general, try to not let your battery sit at a 100% charge level for more than 8 hours at a time. 

Some cars (like Teslas) also have the ability to set the maximum charge level allowed for batteries. It’s recommended that set your car battery to charge to a maximum of 90% if you won’t need the range from the extra 10%. 

4. Avoid Fast Charging Whenever Possible

Fast charging heats up your EV’s battery more than normal charging. As you already know, heat can contribute to a faster decline of battery health. 

Tesla Model S at a Supercharger / Image courtesy of Jakob Härter / CC BY-SA 2.0

Whenever possible, try to avoid using Supercharging Stations or fast charging stations (unless you’re in a hurry). 

5. Be Mindful of Extreme Temperatures

If you live in an extremely hot or cold climate, it can be hard to avoid extreme temperatures sometimes.

However, you should try to do what you can to avoid extreme temperatures. For example, park in the shade instead of parking in direct sunlight.

This Post Has 2 Comments

  1. LEE ARMER

    how much power does the transformer (cable) draw when not connected to the car?

    1. Hugh

      Hi Lee,

      That’s a great question. Normally chargers for phones, laptops, etc. will drain a small amount of power when not in use so I’d assume EV chargers might as well, but I couldn’t find any specific stats online. Nevertheless, EV chargers should normally be connected to their own circuit breaker on your home’s electric panel so if you don’t use your EV charger often you can cut the electricity from there. Personally, I would assume that the vampire power drain from EV chargers is negligible due to the careful design process that they go through. They should be optimized to reduce unnecessarily drain so that it won’t greatly affect your power bill.

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