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A Beginner's Guide To 18650 Battery

by Genius Sky May 14, 2021 14 min read

A Beginner's Guide To 18650 Battery | SkyGenius Online

What does 18650 mean

Just like other lithium-ion cells,  18650 battery is named after dimensions.

The first two numbers refer to the diameter of the battery in millimeters.

The next two numbers stand for the height and the final number means the shape of the battery.


 What does 18650 battery mean - SkyGenius  Blog

In this case:

  • 18mm Diameter
  • 65mm Height
  • 0 Shape (Cylindrical)


What is voltage

Voltage, electrical pressure, is the difference in electric potential between two points.

We can think of the 18650 battery as a bucket of water.

Water level, indicating the height of the water, somehow is very similar to voltage.


 What is voltage - water bucket analogy - SkyGenius Blog

Because they are both used to gauge the range, difference between the lowest and highest.

When the water is consumed, the water level declines. After refilling, the water level rises.

So you know that voltage decreases when the battery is running/discharging, and increases again when the battery is being charged.

Nominal voltage

Voltage of 18650 lithium battery doesn't decrease evenly.

It starts at 4.2V, slightly decreases, and then drops rapidly to about 3.2V, until it stops working.

 Typical Lithium-ion battery discharge curves - SkyGenius Blog

From the graph, you can see that most of the time, it remains at around 3.7V midway through the discharging cycle.

Nominal voltage, which is often the midpoint of that range. 

 18650 battery 3.7V nominal voltage - water bucket analogy - SkyGenius Blog

Cell manufacturers mark the nominal voltage of their Li-ion as 3.6V or 3.7V.

For marketing advantage, some manufactures would mark it as 3.7V on the package, to make you feel it's more powerful.

Why nominal voltage is 3.7V

You may wonder, why the nominal voltage of 18650 battery has to be 3.6V or 3.7V.

Why isn't 1.1V, 2.3V, or any other arbitrary number?

The nominal voltage is a function of anode and cathode materials, as well as impedance.

This sentence sounds complicated and difficult to understand.

Let's put it this way.

Nominal voltage depends on the chemistry of the battery.


Battery chemistry and their nominal voltage

Chemistry  Nominal voltage
Six-cell lead acid 12.6V
Lithium-ion 3.7V
Alkaline or Zinc-carbon 1.5V
NiMH or NiCd 1.2V



For lithium-ion batteries, the mid-way point is between 3.6V to 3.7V.

This value varies among other types of batteries.

Why it needs to increase battery voltage

You have a 3.7V 2600 mAh 18650 battery cell.

You use it to power a 5V, 3W clip on fan.

But the 3.7 Voltage is not sufficient enough to power those devices which require 5V or more.

What does it mean?

Here comes the water bucket analogy again.

Let's suppose you want to spin a small waterwheel because you just like watching things rotating.

There is a bucket that forces water through a tap halfway down.

3.7V lithium battery to power a DC 5V device - water bucket analogy - SkyGenius Blog

A jet of water, coming out from the tap, is not high enough to push the waterwheel.

With the same amount of water, we can switch to a higher bucket. So the water level increases.


 3.7V lithium battery to power a DC 5V device - DC boost converter - water bucket analogy - SkyGenius

Then it works, the little waterwheel gets to spin as you wish.


You have a 3.7V 2600mAh battery, you are expecting it can power a 600mAh fan for 4 hours and more.

2600mAh ÷ 600mAh = 4.3 h

This calculation seems straightforward, but it's not correct.

Let's go back to the water example.

One bucket has a wider base area. The other one is smaller in diameter.


 Battery Capacity difference - mah - milliamps - SkyGenius Blog


But you can't say the first bucket holds more water depending on the size of the bottom area.

The water depth also counts, right?


 bottom area size and water level - SkyGenius Blog

What really matters is the total energy stored in the battery, in other words, how much water in the bucket.

So here comes another unit, Wh.


What is Wh

Wh is short for Watt Hours.

It describes the battery power capacity. In other words, how much energy stored in the battery.

Why battery capacity is given in mAh instead of Wh

If you are buying a replacement battery, you might see their capacity listed in mAh (milliamps hours).

"Wait. Didn't you just say Wh stands for battery capacity. How come all the battery capacity were marked in mAh."

I know you must be confused right now.

Relax, the water bucket analogy will help you sort things out over and over again.

Imagine you are looking to buy a bucket online.

The product page only tells you the radius and height. They think you don't necessarily need to know the volume.

 When indicating battery capacity, the relationship between mAh and Wh is very similar to the radius and volume of a bucket.

If you take a closer look, you will find that battery capacity was actually given in 3.7V 2600mAh.

It's like telling you how high(3.7V) and how wide(2600mAh) this product is.

How to convert mAh into Wh (Watt Hours)

 While mAh only suggests "two-dimension" of capacity, Wh is a solid "three-dimension" figure that calculates battery capacity as a whole.

Ah * V = Wh

You may have not heard of AH before.

Don't feel stressed for another unknown terminology.

Ah is just a different unit that's equal to 1000mAh

For a 3.7V 2600mAh battery, the Wh will be:

2600mAh * 3.7V

2.6 Ah * 3.7V = 9.62 Wh

So now we know the energy of a 3.7V 2600mAh battery is 9.62Wh.


Like we said before, Wh is similar to the amount of water in the bucket.

Then what does W mean? 

While Wh tends to describe the energy stored in a battery, W is more frequently used as to how much energy your device consumes.

Different appliances, of course, require different amount of W (watt, or wattage)

Assuming there are 3 small waterwheels, the whole weight of their bodies differs.

 Different Wattage - SkyGenius Blog

To spin the wheel, needless to say, the heaviest one requires a strong jet of water.

The other two wheels, come in lighter, which means less water is needed to get them continuously spin.

We can reduce water flow from the tap, to prevent any waste.

If we think the tap pushing out electricity instead of water, we could say it's delivering different numbers of watts for different water wheels.

Now the bucket was filled with 9.62 Wh of water, it can supply the heaviest waterwheel for one hour.

Then we will say this waterwheel consumes 9.62W.

Other waterwheels, however, only consume 5W and 2W per hour.


 18650 battery to power devices of different Wattage - 9W

 18650 battery to power devices of different Wattage - 5W


 18650 battery to power devices of different Wattage - 2W


To find out how long these waterwheels keep spinning, just do the math:

9.62Wh ÷ 9.62W = 1h

9.62Wh ÷ 5W = 1.9h

9.62Wh ÷ 2W =  4.8h

How to calculate the battery runtime

Now that you understand the terminologies of voltage, mAh, Wh, and wattage, it's time to get down to calculating the battery runtime.

Battery doesn't run dry

We have used some examples like 9.62Wh bucket water can supply the 2W waterwheel for 4.8 hours.

But it's not realistic.

You may have noticed that the water stops coming out when the water is level with the tap.


 Voltage drop to 3.2V - 18650 battery disconnect the device - SkyGenius  Blog

So the bucket won't completely run out of water. 

The same thing applies to the battery.

It will disconnect the device at a certain voltage.

A 18650 battery is not supposed to run dry, or it dies.

Conversion loss

As the amount of water remains the same, if you want to increase the height, you can simply move water from one bucket to another.

When it comes to batteries, you cannot literally change the "container". This is not how a battery works.

In reality, using a 3.7V battery to power a 5V small device, there is a 3.7V to 5V boost converter required for the circuit.

You can think of it as a pump.

 3.7V to 5V boost converter is like a water pump - SkyGenius Blog

As a working water pump requires energy, the 3.7V to 5V boost converter will run on battery power as well.

So, not all the battery energy is used by the device.

The boost converter also consumes battery energy.

As a result of the 3.7V to 5V conversion, you lose some extra battery energy.

The ultimate calculation

mAh does not indicate the stored energy directly. Thus the first step is to calculate the battery energy Wh.

For a 3.7V 2600mAh 18650 battery, the Wh will be:

3.7V x 2600mAh

3.7V x 2.6Ah = 9.62Wh

18650 Li-ion Battery Energy rated in Wh - Battery runtime - SkyGenius Blog

Then, we take the conversion loss into consideration.

9.62Wh x 65% = 6.253Wh

So we know the "realistic" battery energy is approximately 6.25 Wh.

Battery Energy Conversion Rate - Battery runtime - SkyGenius Blog

Using the battery to power a 3W device, the runtime will be:

6.253Wh ÷ 3W = 2.08h

That's approximately two hours of runtime.

Sometimes, you don't know the exact wattage of your device. Manufactures put something like "5V 0.6A" on the package instead.

No problem.

Based on the formula of "A*V=W", you know the wattage is:

5V x 0.6A =3W

Wattage of your device - 18650 Battery runtime - SkyGenius Blog

The below graph will help you organize the calculation process.


 Calculate the battery runtime


Working principle of lithium-ion battery

All batteries have the same three basic parts:

  • a positive electrode called a cathode
  • a negative electrode called an anode
  • and a chemical layer called an electrolyte

3 basic parts of all the batteries - SkyGenius Blog

Generally, the positive electrode of a 18650 cell is typically a metal oxide that contains lithium atoms.

The negative electrode is made from carbon, which has a layered structure, called graphite.

These layers are loosely bonded so that the separated lithium-ions can be stored very easily there.


Electrochemiry of Lithium Battery - how it works

Charge and discharge

During charging, there are lithium atoms released from the metal oxide.

Because lithium atom is highly unstable, it will instantly form a lithium-ion.

The electrolyte between the metal oxide and graphite acts as a guard which allows only lithium-ions through.

After the lithium-ions flow through the electrolyte, they reach the negative electrode and get trapped there.

How does a lithium-ion battery work - SkyGegnius Blog

As soon as the power source is removed, this transportation goes in the opposite direction. Those lithium-ions will go back to the metal oxide.

Lithium-ion battery construction and working principle - SkyGenius Blog

Each lithium atom has only one electron. When a lithium atom transforms into lithium-ion, it will lose the electron.

Lithium-ion and electron - SkyGenius Blog

Unlike lithium-ions moving back and forth through electrolyte, electrons move through the external circuit, that's how electricity being generated.

Charging and discharging 18650 lithium-ion batteries - how it works - SkyGenius Blog

Construction of lithium-ion battery

In a practical 18650 cell, the metal oxide is coated onto aluminum foils.

The graphite is coated onto copper foils.

Alluminum foil and copper foil of 18650 lithium battery - SkyGenius Blog

The foils are current collectors here, with the positive and negative tabs attached to them respectively.

Current Collectors of Lithium ion battery - SkyGenius Blog

For safety reasons, a separator sheet is used to prevent contact between positive and negative electrodes.

So there are basically three kinds of sheets - positive electrode, negative electrode, and separator.

All these three sheets are wounded onto the cylinder around a center pin, thus making the cell more compact.

Image 011

For further protection, they are inserted into a stainless battery case.

Image 012

Before sealing the case, separators and electrodes will be filled completely with electrolytes.

Image 013

Since the stainless battery case is a metal that conducts electricity, an extra insulating PVC wrapper is needed.

Image 014

So there are five major steps of the manufacturing process

  • Coating the positive and negative electrode materials on metal foils;
  • Rolling the positive electrode, negative electrode, and separator;
  • Inserting the wound center pin into the battery case;
  • Seal the battery case;
  • Wrap the battery.

Image 015

Reasons behind the safety issues with lithium-ion battery

There are many reported cases about the danger of lithium battery.

3 fires in Tesla cars and exploding power banks.

These serious safety problems were caused by an internal short circuit.

How internal short circuit happened?

It usually starts with overcharge and over-discharge.


(A) Charging: When charging a battery, the lithium-ions move from the cathode to anode.


(B) Overcharging: More and more ions fill all over the anode. They are stuck in the graphite and unable to get out. If ions were trapped, electrons can not be released. That's why the battery capacity is reducing. What's worse, there is limited room in the graphite to accommodate lithium-ions. With nowhere to go, other ions will begin to accumulate on the surface.

 Image 16

(C) Discharging: When a battery discharges, ions migrate back to cathode.


(D) Over discharging: Because the graphite contains few ions, this layered structure tends to collapse, thereby making it less capable to accommodate ions. This issue will make overcharge(B) more likely to happen.

 Image 17

On top of overcharge and over-discharge, the problem of lithium-ion batteries can be traced largely to dendrites.

Dendrites (from the Greek dendron meaning "tree"), form on the surface of an electrode.

Image 18

They grow over repeated cycles of overcharging and over-discharging.

  • As shown in the overcharge process(B), ions pile up on the surface of anode.
  • The accumulating ions begin to form metallic crystals.
  • Then, dendrites branch out from the metal crystals.
  • Dendrites pierce the separators.
  • Anode and cathode touch each other.
  • Internal short-circuit occurs.
  • The battery experiences a surge current within a few seconds.
  • Large current flow leads to overheating.

Once short-circuit happens, the damage is irreversible.

All the battery energy converts into heat. Voltage is down to zero. This means the battery won't hold a charge anymore.

If this is a single cell that's stored separately, users may not notice internal short-circuit ever happened. Cell temperature drops to normal after the internal current stops.

But what if the device contains more than one cell of 18650 lithium battery?

The overheating cell may cause the separators in adjacent cells to melt. A short circuit results in a chain reaction so the whole battery pack catches fire.

In conclusion, the separator between electrodes plays an important role in battery safety.

Always maintain the good conditions of separators. Users may need to prevent the growth of dendrites or high temperature.

  • Eliminate the growth of dendrites: Do not overcharge or over-discharge
  • Avoid high temperature: Do not place your device or battery near fire. Avoid any overheating environment.

What's the difference between flat top and button top

Flat top

The positive end of the battery is a flat surface

Button top

The positive end of the battery is protruding

Flat top and button top - 18650 lithium battery - SkyGenius Blog

Before you are trying to swapping out the battery, take a look at the + pole.

Some devices like mod only accept flat top. 


 electronic vaping devices only accept flat top 18650 lithium battery - SkyGenius Blog


As the flat top ones are slightly short, button top will not fit in the vaping devices. 

While some brands of flashlights like Nitecore & Jetbeam only work with button-top batteries. 


 some flashlight take button top 18650 battery - SkyGenius Blog



Aside from the size, it's also worth mentioning that NOT all the button top batteries are protected.

There are common opinions like "the button top comes with protected circuit, and the flat top cells are unprotected". 

This is not true.

Below will walk you through the terminologies about the protected and unprotected, and how to know which one is protected.

Unprotected battery

Unprotected battery. With this name, it sounds pretty unsafe and acts like a buyer-repellent.

If that's the case, they will be NOT allowed in the market. How come there are so many unprotected cells out there?

Actually, unprotected battery has the PTC and CID, which means they are safe if handled properly.


PTC thermistor is a small disc that is found at the top of the battery.  

When the temperature rises, the resistance increases

If there is a surge current and the battery heats up significantly, the PTC will increase its resistance to limit the current. 

And then the temperature will be back to normal, which prevents your battery from becoming burned out.


 PTC of 18650 lithium battery - SkyGenius Blog


When the battery is overcharged and the pressure inside becomes too high, the CID will disrupt the connection of the positive terminal

Imagine you are pumping air into a balloon. 

After it has reached the optimal level, you are still blowing air into the balloon. 

Guess what happen? It burst! 

CID, which is like a small hole (the pressure valve), works by releasing the gas through the hole. 

So the "balloon" will slowly leak without any explosion.



CID is built into the majority of 18650 lithium battery - SkyGenius Blog

What's the added benefit of protected cells

Now that you understand unprotected batteries are safe from burnout and explosion, you may wonder what exactly is the added benefit of a protected battery?

PTC and CID are built into the majority of 18650 batteries. They prevent the danger of fire and explosion. So users are not exposed to the potential risks associated with batteries.

However, overcharge and over-discharge still get to happen, while damage is caused to the battery itself.

PCB, the extra protection method, helps resolve the issue by detecting voltage changes.

With the help of PCB, battery will disconnect at a certain voltage. 

So it protects the battery against overcharging and over-discharging, thus maintaining longer battery life.

Aside from overcharging and over-discharging, protected battery can avoid short-circuit effectively.

Short Circuit

How short circuit occurs with 18650 battery?

It happens when the positive and negative terminal connected with a conductor (e.g.metal), like wire.

I believe most people don't use wire to do some random electrical test, but something else could cause a dangerous situation.


In these cases, short circuit will deliver extremely high current in a short amount of time. 

Battery will heat up like an open flame within just a few seconds.

So avoid doing any of the above! Or you will learn the hard way.


Limit charge voltage, maximum voltage of 18650 battery voltage, is 4.2V.

When charging the battery, the voltage keeps increasing.

Once the voltage reaches above 4.2V, it can be considered as overcharge.




Cut-off voltage, the lower limit of 18650 cell varies from 2.75V to 3V, depending on the specification.

It is recommended NOT to discharge the battery lower than 3V, just to be on the safe side.

You may know battery discharges when it connects to a device that needs power.

However, this reaction still happens when the battery is not connected to the device.

This phenomenon is called self-discharge.

Self-discharging is not a manufacturing defect.

It happens to all the batteries.

Although battery cuts out the connection with devices at around 3V.

Self-discharge still consumes the battery energy, with the voltage keeps decreasing.

Rechargeable li-ion batteries is charged and discharged by li-ion moving along.

At a very low voltage, ion stops the movement. That's usually why you find your battery doesn't hold a charge.

You can run Li-ion cell to totally flat and nothing dangerous will happen.

The thing is, once voltage drops to the minimum limit, lithium ion inside is not active anymore.

Manufacturers or sellers may ask you to recharge the battery every 3 months when it's not being used, which can help voltage remain at an optimal level.

Why manufactures don't add PCB to the unprotected battery

Don't panic if you found the battery to be the unprotected type when opening the battery compartment.

Although unprotected battery doesn't have PCB, your devices already include the built-in protection circuit.

Aside from the final products, some electrics hobbyists, rather than mainstream consumers, demand "raw" unprotected batteries to build their own battery packs. They will add the protection circuits on the battery packs as a whole.

The third case. Electronics manufacturers purchase unprotected batteries from industrial distributors. Again, these manufacturers will design the protection circuit into the OEM products.

No matter in which cases, PCB will be added to the device or the battery packs.

However, don't ever think about adding PCB on your own, if you have limited knowledge about electrical engineering.

Most people don't know what they're handling. Online sellers should use pop-up disclaimer, helping consumers making an educated purchase decision.

If you are using the unprotected cells

Modern devices are way smarter with managing power. 

Both your device and charger have built-in protection. 

They protect the battery against heavy charge and discharge conditions. 

That's why unprotected batteries are still being widely used in consumer electronics devices.

So there's not really any reason to worry about it. The only danger comes from misusing these batteries and causing a short circuit. All you need to do is to use and maintain the device and battery properly.

Protected battery

What is a protected battery

As mentioned above, a protected battery includes PCB protection.

How PCB works

1. Overcharging: When the battery voltage reaches 4.35V, PCB makes it stop charging.

2. Over-discharging: PCB limits the voltage of 2.5V to prevent battery from over-discharging.

3. Over-current: PCB controls the maximum current at 3.4Ah, which protects the battery against over-current.

4. Short circuit: PCB works by preventing the flow of current. This method is kind of like the third one. But this time, it reacts much quicker(usually within 0.007 second) to the surge current due to short circuit.

How to tell your battery is protected

Protected cell has a rim on the side (which is the shape of wire)


PCB protection - 18650 battery - SkyGenius Blog


If your battery is a protected one,  you can feel the pole connecting wire on the side of your battery. While the surface of unprotected battery appears to be all smooth.

Protected cell is also commonly longer than an unprotected version because there is a PCB board connect to the cell bottom.


 PCB board at the bottom of 18650 battery - SkyGenius Blog

The bottom of a protected battery usually differs in color from the main body.



 Length difference between protected and unprotected 18650 cells - SkyGenius Blog


If you are looking for a protected 18650 battery, check the spec. Reputable sellers should list this information on the product page.

Protected battery doesn't mean you are free of concern

If you can recall specifically, the voltage of a working 18650 battery range from 3.2V to 4.2V

But the voltage limit setting of PCB is 3V ~ 4.35V.

This control system is not very precise.

PCB doesn't cut off charging until the it reaches 4.35V, but the peak voltage of battery should be 4.2V.

For the sake of longer battery life, don't leave it charging overnight. The advised charging time of one single cell is between 2 to 3 hours.

Battery sellers should ensure that customers use them properly

There are many dangerous goods on sale. But people are aware of the potential risks of those goods and take caution.

Cars can be dangerous. This is why there are driving license and traffic laws.

Excessive alcohol use can be dangerous. This is why warning signs and education are required.

Most people on seeing rechargeable Li-ion batteries would probably assume they are normal batteries.

Few people know the danger of mishandling an unprotected battery.

One of the greatest risks with an unprotected battery is a short circuit. Placing them together with any conductive objects(e.g. keys, coins) will probably deliver a high current flow. With the battery energy nowhere to go, it turns into heat.

This is how a lithium battery can lead to fire in some extreme cases.

Sellers are supposed to make people aware of the correct use and the danger of incorrect use,when it comes to lithium batteries or devices that contain ones.

Genius Sky
Genius Sky

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