When you think about rechargeable gadgets like a flashlight, a
laptop battery pack, and even an electric car, they generally depend on special
batteries inside. These are often lithium-ion batteries. They come in different
shapes and sizes. For a long time, one size was really common: the 18650.
Lately, however, another size has become very popular, especially in newer
devices and vehicles: the 21700.
You see, the size and type of battery cell used inside a device are
really important design choices. It affects:
1. How much power does the device have?
2. How long does it run?
3. And how big or heavy is it?
In simple words, picking the right battery cell matters a lot. This
point is important for anyone building and using electronics.
Let's start somewhere and compare the 18650 and 21700 battery cells
to see what's different and why both are used.
What are 18650 and 21700 Batteries?
First off, what do these numbers mean? They are not just random
codes. Engineers know it's actually a simple way to tell you the size of the
battery cell.
18650
This name breaks down into
dimensions.
• 18 means the battery is 18 millimeters (mm) in diameter.
• 65 means the
battery is 65 millimeters (mm) long.
• 0 at the end
usually means it's a cylindrical battery shape.
So, an 18650 battery is a cylinder, 18mm across and 65mm tall. For
many years, this was the standard for high-energy rechargeable cells. You know,
it's been used in everything from laptops to many early electric bikes.
21700
This follows the same
pattern.
• 21 means the battery is 21 millimeters (mm) in diameter.
• 70 means the
battery is 70 millimeters (mm) long.
• 0 again means
it's cylindrical.
So, a 21700 battery is a cylinder, 21mm across and 70mm tall. It's
basically a bit fatter and a bit longer than an 18650.
Definitely, the 21700 is physically larger than the 18650. This
difference in size, even though it seems small, has a big effect on
performance.
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Key Differences: Size, Capacity, Power, and More
Comparing these two sizes reveals why manufacturers choose one over
the other. This point is noticeable once you look at the specs.
1. Size and Dimensions
As we covered, the 21700 is 3mm wider and 5mm longer than the 18650.
While these differences are small, they mean a device designed for one cannot
fit the other without changing the battery holder and internal layout. They are
not interchangeable just because they look similar. You can say it is the most
common "mistake" beginners might assume they can just swap them, but
do not panic, it's a simple physical size problem.
2. Capacity (How much energy they store)
This is a major difference.
Because the 21700 is bigger, it has more internal volume. This extra space
allows manufacturers to put more of the active materials inside the cell that
store energy. You will see that typical 18650 cells have capacities ranging
from around 2000mAh to 3500mAh (mAh stands for milliampere-hour, a unit of
charge capacity). In contrast, 21700 cells often range from 3000mAh up to
5000mAh or even higher. This point is important: a single 21700 cell can
usually store significantly more energy than a single 18650 cell.
3. Power Output (How much current they can deliver)
The larger size of the 21700
also helps with power output. The internal parts that carry current are larger,
and the cell's design can often handle higher discharge rates. This means a
21700 cell can often provide more power to a device when it needs it, like when
a power tool is working hard or an electric car is accelerating quickly.
Simultaneously, it can often maintain a higher voltage under load compared to a
similar 18650. Benchmark test shows this power advantage in demanding
applications.
4. Energy Density
This relates to how much
energy is stored per unit of volume or weight. Because the 21700 is larger, it
has a better ratio of active material to packaging material (the metal casing,
safety features, etc.). This means that, generally, 21700 cells can achieve
higher energy density (energy per
volume or per weight) than 18650 cells. You get more energy for roughly the
same amount of material and packaging overhead per cell. Everything counts when
designing lightweight, long-lasting portable devices or EVs.
5. Cost
Per cell, a 21700 is often slightly more expensive than an 18650.
However, because one 21700 can replace maybe 1.5 or 2 smaller 18650 cells in
terms of total energy, the cost per
watt-hour (Wh) of energy stored might actually be lower for 21700 cells in
some cases.
Why Did the 21700 Emerge?
The development and widespread adoption of the 21700 size didn't
happen just because someone wanted a new size. It was largely driven by the
needs of new technologies, especially electric vehicles (EVs). Companies
building EVs needed more energy and power from their battery packs to give cars
longer range and better performance.
Using 18650 cells meant building very large battery packs with
thousands of cells. By switching to a slightly larger cell like the 21700,
which holds more energy per cell and offers higher power, they could build
battery packs with fewer cells to get
the same or more total energy and
power.
Fewer cells means simpler wiring, fewer connections, and potentially
easier manufacturing and thermal management for the whole pack. It allowed
designers to think outside the box of the standard 18650 and unlock new
possibilities for battery pack design and performance. The quest for more range
and faster charging in EVs was a significant root cause for the move to larger
formats like the 21700.
Applications: Where Are They Used?
You will see both 18650 and 21700 batteries powering a wide range of
devices.
18650
Still widely used in things like laptop battery packs (though less
common in newer, sleeker models), power banks, many LED flashlights, portable
fans, vape devices, and older electric vehicle models. It's a mature technology
with a wide variety of suppliers. PCB and PCBA companies are designing products
that need moderate power and energy, and have long used 18650s.
21700
Increasingly found in newer
electric vehicles (Tesla was an early, large-scale adopter), high-power
cordless power tools, newer and more powerful LED flashlights, some e-bikes,
and home energy storage systems. They are being designed into new prototypes
for devices needing that extra boost in energy or power without a huge jump in
size.
Customers are so right in their opinion that battery life and
performance are key features. The move to 21700 in many products directly
addresses customer demand for longer runtime and more power.
Compatibility and System Design
It's very important to remember that a 21700 battery will not fit in
a device designed for an 18650 battery, and vice versa. Their physical
dimensions are different. Battery holders, charging cradles, and the internal
layout of devices are made specifically for one size or the other.
When designers are creating a new product, they look at the power
and energy requirements. They also consider the available space and the cost
target. Based on these factors, they choose which cell size makes the most
sense. For example, a small, thin device might stick with smaller, less
powerful cells or even flat pouch-type batteries.
A large power tool or an EV will look at 18650 or 21700 (or even
larger cells) to pack in the most energy and power. Choosing the right cell
size and type is a critical part of Design for Manufacturability (DFM) for
battery-powered products. You need to find a balance between performance, size,
cost, and ease of assembly.
Which is Better?
Neither battery size is universally "better." It entirely
depends on what you need it for.
If you need the smallest possible device and moderate power/energy,
an 18650 might still be the right choice, or even smaller cells. Just because
the 21700 is newer and holds more energy doesn't mean it's always the best fit.
If you need the maximum amount of energy and power in a relatively
compact cylindrical format, the 21700 often wins. It offers a step up in
performance.
When choosing or comparing, stay focused on the requirements of the
application. Look within the specifications of the individual battery cells
(capacity, max discharge current, voltage, lifespan) and compare them against
what your device needs. Be patient when researching different cell options;
specs can vary even within the same size.
Conclusion
So the basic point is that 18650 and 21700 are two common sizes of
lithium-ion battery cells. The 21700 is slightly larger than the 18650. This
larger size means the 21700 generally offers higher capacity and can deliver
more power. You will see 18650s in many older and smaller gadgets, while 21700s
are increasingly found in high-demand applications like EVs and power tools.
Choosing between them involves balancing needs for energy, power, size, and
cost.
