Can You Overcharge Your Electric Car? Battery Management System 101


I had just purchased our electric car and the first question that came to mind was, can I overcharge my electric car? The battery is the most expensive part of the car and I already had a used electric car so the lifespan was ticking away. Would overcharging further degrade the battery?

Electric car manufacturers have programmed precautions to prevent battery overcharge, deep-discharge, and overheating. The battery management system protects the battery from overcharge by slowing down the charging process as the battery nears 100% and then stops charging. Making it so you can’t overcharge your electric car.

If you are concerned with overcharging your electric car, you may have questions on improving the lifespan of your electric car’s battery. I want my 2017 used Nissan Leaf to last as long as possible without getting to 60% of original capacity where I will need a new battery. 

Here is why overcharging isn’t possible and also why you can charge to 100% with confidence.

Why You Can’t Overcharge Your Electric Car

Overcharging a battery is the concept of leaving your electric car plugged in to 100% charge and then electrons continue to be pumped into the battery pack. This could lead to overheating and eventually damage. The worst case being a battery fire. 

This will not happen with your electric vehicle.

Electric car manufacturers know safety is paramount. Therefore, they have system interlocks and controls to prevent the electric car’s battery pack from overcharging. 

You do not need to worry about overcharging your electric car. 

If safety is the first priority for a car manufacturer, longevity should be in the top 5 of design goals. Overcharging degrades the battery’s lifespan. Manufacturers want to maximize the lifespan of their battery pack because they are mandated by a federal regulation to warranty the battery for 100,000-miles or a minimum of 8-years. EV manufacturers also want to maintain their reputation for quality and craftsmanship. 

Thanks to the battery management system (BMS) on your EV, you can sleep soundly knowing your car is stopping an overcharge event. 

Overcharging Your Electric Car is Prevented by the Battery Management System

Car manufacturers have software and hardware installed to improve the life of the battery pack. The battery management system (BMS) does far more than prevent overcharging. 

The BMS, in its fundamental design, manages the stored chemical energy in an on-board high-voltage battery and distributes the power to the rest of the vehicle. But it does far more than that. The BMS also includes the following functions:

  • cell balancing 
  • cell health and wear leveling
  • charge and discharge monitoring
  • safety assurance

The battery management system achieves these functions by monitoring the temperature, voltage, current, and wear leveling of the battery cells. The battery pack is made up of hundreds of 3-4 volt individual batteries. They are connected in series to deliver higher voltage and in parallel to increase capacity.

It is fair to have concerns about overcharging the battery and the reliability of the battery management system. One does not want to pay for a battery replacement. The battery is by far the most expensive part of an EV. A new battery pack can cost $7,600 for the 2020 Chevy Bolt to $12,000 for the Tesla Model X (at $137 per kWh).

Your EV has a brain with one thought, keeping the battery operating safely and efficiently for thousands of miles. But there is some operator guidance that the car manufacturers provide. 

What Happens When the EV Is Kept On the Charger for Too Long?

When you leave your electric vehicle on the charger for an extended period of time, the car will charge to 100%. As the state of charge nears 100%, the battery management system will reduce the amps flowing into the battery to prevent an overcharge.

At 100% charge, the BMS will start trickle charging the car. This means the BMS will accept electrons periodically to match the self-discharge rate. 

The battery management system maintains a 100% charge without overcharging the battery.

First generation Nissan Leafs had a programmable charging threshold. For example, an owner could set a threshold to charge to 80%. The BMS would stop changing at 80%. Tesla still offers this feature.

With second generation Nissan Leafs, the engineers removed this feature. Nissan states they have no issue with charging a Leaf to 100% with a Level 1 or Level 2 charger.

This is yet one manufacturer though. There are strong opinions when it comes to charging to 100% on the regular. But if changing to 100% is so detrimental to the battery, one would assume all manufacturers would have this programmable threshold limit or would prevent charging to 100% in the first place with a BMS buffer.

Related article: Facts on Charging Your Electric Car Every Night

Ideal Level of Battery Charge to Prevent Overcharge

There is no hard rule on the ideal state of charge (SOC) or level to charge an electric vehicle battery. The forums are full of opinions and manufacturers don’t specify. Tesla, namely Elon, have recommended charging a Tesla to 50% to 90% for daily driving. For long road trips, charge to 100%. 

A 100% charge will give you the extra miles you may need to get to the next charging station. But also charging to 100% is not efficient as regenerative braking won’t be activated at 100% charge. Regenerative braking is automatically reduced when the Li-ion battery is fully charged to prevent an overcharge.

Other manufacturers do not provide specific charging guidance as Tesla. Best practices for charging electric devices, like a smartphone, is 30% to 80%. The same guidance gets pushed to electric car batteries that are also lithium-ion batteries. 

Many people are adamant about not charging regularly to 100%. Based on my research and understanding of battery management systems, it is cautious advice to not charge to 100% regularly. 

For one, manufacturers have installed a buffer on the lower and upper ends of the battery charging level. The buffer is to avoid a deep-discharge and an overcharge on either end of battery operation. 

To maximize the battery life, the manufacturer would not depend on user operation to prevent overcharging, for example. They may have the BMS stop charging at 95% but report the car’s state of charge to by 100%. On the other end, they may shut-off the car at 2% battery capacity to prevent a deep-discharge. 

We know the manufacturers have buffers on the range of charge. The EPA-rated range of a fully charged RWD Long Range Model 3 was 310 miles. This was evident as the reported range on the dashboard when at 100% charge. Tesla released a software update that increased the range to 325 miles. The forums were abuzz with the new, longer-range of their battery pack. What happened was Tesla is learning they have more capacity without degrading performance so they reduced the upper-range buffer, perhaps from 5% buffer to 2% buffer.

Therefore, I wouldn’t stress about charging your vehicle to 100% if you need the range for your commute. There is no hard data on EV battery degradation due to 100% charging. 

Related article: Do Electric Cars Lose Range Over Time? Here is the Data

If you don’t need the full range of the electric car for your commute, then error on the side of caution and keep your state of charge between 50% and 80%. 

Electric cars have lithium-ion battery packs. Research on lithium-ion battery packs show they prefer a partial cycle rather than a deep-discharge or full-charge. The management of charging to optimize the life of the lithium-ion battery pack is built into the battery management system. 

Therefore, you can do partial cycles to optimize the longevity of the battery pack but most of the concern of battery degradation is being prevented by the BMS.

Note: Best practice is to consult your owner’s manual for recommended battery maintenance and operation best practices. Each car is unique and each BMS is programmed differently.

Electric Car Battery Warranties are Improving

As electric cars hit the road, manufacturers are collecting real-life data on battery performance. With this data, they are improving the algorithms in their battery management systems and extending their battery warranties. 

With the start of the electric car industry, the federal government sought to protect consumers and boost consumer confidence in electric cars. Federal regulations mandate at least 8 years and/or 100,000-miles warranty on an EV battery pack.

Manufacturers offer this as standard while some manufacturers have extended their warranties.

Hyundai has extended their battery warranty from 100,000-miles to a lifetime warranty starting with its 2019 Kona Electric.

Tesla’s Model X and S have 8-years and unlimited mile warranty on their battery packs. 

The Kia Niro EV and Soul EV have 10-year warranties and/or 100,000-miles. 

The BMS protects the car from overcharging but also reduces wear on the individual battery cells. This technology is improving the lifespan of the battery pack to the point manufacturers are putting their money on the line. 

Don’t Worry About Overcharging an Electric Car

The battery management system is protecting your car from overcharging. So, plugin everynight with peace of mind your car will not be a smoldering wreck in the morning. The BMS is protecting the integrity of the battery pack while also working to extend its life. 

Manufacturers don’t leave complicated battery operation to the layperson. They are continuously improving the BMS to get the most life out of your electric car. I expect to see more manufacturers offer lifetime warranties on their battery packs in the coming years.

Related articles:

Jordan Fromholz

I'm an electric car owner, enthusiast, and founder of the Plugin Report. As a Chemical Engineer with over 14-years in the energy industry, I've made my passions be renewable energy, batteries, and electric cars. My family lives on Maui where we drive our 2017 Nissan Leaf and share everything there is to know about electric cars.

Recent Posts