We Tear Down an eBike Battery: What’s Inside?

Knowledge · Maintenance ·Aug 13, 2024

Electric bike batteries come in a few different types, with lithium-ion, nickel-metal hydride (NiMH), and lead-acid being the most common.

Among these, lithium-ion batteries dominate the market, powering over 85% of ebikes due to their excellent balance of weight, capacity, and durability.

But what’s inside these batteries that makes them so effective? Let’s take a closer look at the inner workings of an ebike battery to understand why it’s the heart of your ride.

Look What’s Inside the Eelctric Bike Battery

At the core of most ebike batteries are lithium-ion cells.

These cells are popular because they pack a lot of energy into a small, lightweight package, making them perfect for ebikes.

The most common type you'll find is the 18650 cell, a small cylindrical powerhouse that measures just 18mm across and 65mm tall.

The name "18650" comes directly from these dimensions: 18mm in diameter and 65mm in length.

These cells have become the industry standard for electric bike batteries and are widely used not only in ebikes but also in laptops, power tools, and even electric vehicles like Teslas.

Why the 18650❗❗

Energy Density: 18650 cells are known for their high energy density, meaning they can store a significant amount of energy in a small volume. This makes them ideal for applications like ebikes, where space and weight are critical.
Reliability: These cells have a proven track record for reliability and durability. They can handle hundreds of charge and discharge cycles without significant degradation, which is crucial for ebike users who rely on their batteries for daily commutes or long rides.
Availability: 18650 cells are mass-produced and widely available, which makes them a cost-effective choice for manufacturers. This also means that replacements are easier to find if you ever need to service your ebike battery.

So, what’s inside these little cylinders

Anode: This is the part of the battery where the energy is stored when you charge it. It’s usually made from graphite.
Cathode: During use, the stored energy flows from the anode to the cathode, which is made from a lithium compound.
Electrolyte: This is the medium that allows lithium ions to move between the anode and cathode.
Separator: A safety feature that keeps the anode and cathode from touching each other directly, which could cause a short circuit.

When these cells are linked together, they create a battery pack that powers your electric bicycle.

Lithium battery cylinders form a large ebike battery

Battery’s BMS Is Crucial

Now, it’s not just the cells doing all the work. The Battery Management System (BMS) is like the brain of the operation, making sure everything runs smoothly. Here’s what it does:

Voltage Monitoring: The BMS keeps an eye on the voltage of each cell and the whole battery pack to make sure they’re within safe limits. If the voltage goes too high or too low, it could damage the battery or even cause it to fail.
Temperature Control: Lithium-ion batteries don’t like extreme temperatures. The BMS helps manage the temperature, so the battery doesn’t get too hot or too cold.
State of Charge (SoC): The BMS calculates how much charge is left in the battery, giving you an accurate reading of how much juice you have left.
Balancing: Over time, cells can become imbalanced, meaning some hold more charge than others. The BMS balances the cells, so they all stay at the same charge level, extending the life of your battery.
Safety Protections: The BMS also acts as a safety net, preventing things like overcharging, over-discharging, and short circuits.

How It’s All Put Together: Battery Pack Construction

Inside the battery, the individual cells are connected to each other using spot welding. This process uses metal strips, often made of nickel, to join the cells together. There are two main ways the cells are connected:

Series Connection: Cells are linked end-to-end to increase the voltage of the battery. For example, linking ten 3.7V cells in series gives you a 37V pack.
Parallel Connection: Cells are linked side-by-side to increase the battery’s capacity (measured in amp-hours). If each cell has a capacity of 2.5Ah, connecting four cells in parallel gives you a 10Ah battery.

This combination of series and parallel connections allows the battery to deliver the right amount of power for your ebike.

The Outer Shell: Keeping It All Safe

All these cells and the BMS are housed in a protective enclosure. This casing is more than just a box—it’s designed to:

Protect the battery from physical damage, such as bumps or drops.
Keep out moisture and dust, which could cause short circuits or corrode the components.
Dissipate heat to prevent the battery from overheating.

The casing is typically made from tough materials like aluminum or high-impact plastics. In some cases, it might also include a thermal management system to keep things cool in extreme conditions.

It’s Not OK to Leave an E-Bike Battery Plugged In

In recent years, concerns over the safety of eBike batteries have increased, particularly due to a rising number of fires associated with these devices.

According to the National Fire Protection Association (NFPA), lithium-ion battery fires, including those involving eBikes, have surged in recent years.

In 2022 alone, eBike-related fires accounted for over 200 incidents in major cities like New York, resulting in numerous injuries and even fatalities.

For instance, a 2023 fire in New York City, attributed to an eBike battery left charging overnight, resulted in the tragic loss of four lives.

The city's fire department reported that the fire started in a small apartment where the battery was left plugged in and unattended, eventually igniting and spreading rapidly through the building.

This incident highlights the significant risks associated with improper charging practices.

Leaving your eBike battery plugged in continuously, especially overnight, can be hazardous if the battery’s BMS or charger lacks proper safety mechanisms.

Although most modern eBike batteries come with a BMS designed to prevent overcharging, it’s still a good idea to unplug the battery once it’s fully charged.

Doing so not only minimizes the risk of fire but also extends the battery's lifespan by avoiding prolonged exposure to maximum charge levels, which can degrade the cells over time.

Warning that ebike batteries can cause fires

Safety Tips for Handling Electric Bike Batteries

While eBike batteries are generally safe, there are a few things you should always keep in mind:

Handle with Care: Avoid dropping or damaging your battery. Physical damage can lead to dangerous chemical reactions.
Dispose Responsibly: Don’t just toss old batteries in the trash. They contain materials that need to be recycled properly to avoid environmental harm.
Follow the Rules: Always follow the manufacturer’s instructions for charging, storage, and use to keep your battery in good shape and ensure it lasts as long as possible.

Conclusion

Taking apart an ebike battery is like peeling back the layers of a complex and finely-tuned machine.

From the lithium-ion cells to the Battery Management System, each component plays a crucial role in delivering the power needed for your ebike adventures.

Understanding what’s inside your battery not only satisfies curiosity but also empowers you to take better care of your ebike and possibly explore new ways to upgrade or customize it.

FAQs

How long does a typical eBike battery last before it needs to be replaced?

A typical eBike battery lasts between 3 to 5 years or approximately 500 to 1,000 full charge cycles, depending on usage, maintenance, and the quality of the battery.

Can I upgrade my eBike battery to increase its range?

Yes, you can upgrade your eBike battery to a higher capacity model to increase its range. However, it's essential to ensure the new battery is compatible with your eBike's motor and electronics.

What’s the best way to store an eBike battery when not in use?

It’s best to store your eBike battery in a cool, dry place, ideally at a 40-60% charge level. Avoid storing it in extreme temperatures or leaving it fully charged or fully drained for long periods.