A standard 18650 lithium-ion cell is commonly described as 3.6V or 3.7V, but that is its nominal voltage rather than its maximum voltage. The actual voltage changes during charging and discharging, and the final battery-pack voltage depends mainly on the number of cells connected in series.
For OEM buyers, understanding these voltage differences is important when choosing cells, designing a BMS and matching the battery to the equipment and charger.
One supplier lists:
3.6V
Another lists:
3.7V
Both may be selling standard lithium-ion 18650 cells.
So which one is correct?
In many cases, the difference is simply how the manufacturer defines and presents the nominal voltage.
A conventional lithium-ion 18650 cell commonly operates around this nominal voltage range, while its voltage during actual use moves considerably above and below that value.
For practical battery-pack calculations, 3.6V or 3.7V nominal may therefore appear depending on the cell specification.
The important number isn’t only the nominal voltage.
You also need to know:
Maximum charge voltage
Discharge cutoff voltage
Nominal voltage
These three numbers tell you much more about how the cell behaves inside a real battery.
It is not the voltage you will measure from the cell at every moment.
For many conventional lithium-ion 18650 cells, you will see:
3.6V nominal
or
3.7V nominal
This value is used when calculating battery-pack specifications and energy.
For example:
3.7V × 3Ah = 11.1Wh
This doesn’t mean the cell stays at exactly 3.7V throughout its discharge.
In actual use, the voltage gradually changes.
That’s why a battery-powered device doesn’t simply receive a perfectly constant 3.7V from a single 18650 cell.
4.2V
This is significantly higher than the nominal 3.6V or 3.7V figure.
So a single cell may be described as:
3.7V nominal
4.2V full charge
There is no contradiction here.
The battery operates across a voltage range.
When the cell is fully charged, its terminal voltage can reach approximately 4.2V under the specified charging conditions.
The exact maximum voltage should always come from the cell manufacturer’s datasheet.
Different lithium-ion chemistries can have different voltage characteristics, so it is not safe to assume that every cylindrical lithium cell uses exactly the same charging voltage.
The actual discharge curve isn’t a perfectly straight line.
There can be a relatively stable voltage region followed by a more noticeable decline as the cell approaches its lower voltage limit.
The exact curve depends on:
Imagine a power tool starting its motor.
The cell voltage may temporarily drop because of the high current demand and internal resistance.
The battery may still have considerable capacity remaining, but the device can see a lower instantaneous voltage.
This is one reason engineers look at both capacity and discharge performance when selecting an 18650 cell.
A typical lithium-ion cell may have a cutoff around:
2.5V–3.0V
depending on the cell specification and application.
But this is not a universal number.
You should always follow the actual manufacturer’s datasheet.
The cutoff voltage affects usable capacity.
If you stop the discharge earlier, you may leave some energy unused.
If you discharge too far, you can damage the cell and create safety concerns.
This is one of the jobs handled by the battery-management system in a properly designed multi-cell battery pack.
When cells are connected in series, their voltages add together.
For a typical 3.7V nominal cell:
1S = 3.7V
2S = 7.4V
3S = 11.1V
4S = 14.8V
5S = 18.5V
6S = 22.2V
7S = 25.9V
8S = 29.6V
10S = 37V
13S = 48.1V
15S = 55.5V
These are nominal voltages.
If the cell has a 4.2V maximum charge voltage, the full-charge voltage is:
Series count × 4.2V
So:
4S = 16.8V full charge
10S = 42V full charge
15S = 63V full charge
This distinction becomes very important when choosing the charger.
*Typical values based on a 3.7V nominal, 4.2V maximum-charge lithium-ion cell. Actual specifications depend on the selected cell.
This chart is useful when you are doing an initial battery design.
For the final specification, however, use the exact cell datasheet.
Parallel connection primarily increases capacity and current capability while keeping the nominal voltage approximately the same.
For example:
4S1P
and
4S2P
both have approximately:
14.8V nominal
The difference is capacity.
If each cell is 3000mAh:
4S1P = 3Ah
4S2P = 6Ah
4S3P = 9Ah
The voltage remains approximately 14.8V.
This is why the S/P notation is so useful.
S tells you the voltage configuration.
P tells you the parallel capacity.
A customer asks for:
10S2P 18650 battery
This means:
10 series groups
2 cells in parallel per group
Total cell count:
10 × 2 = 20 cells
Using 3000mAh cells:
Nominal voltage:
10 × 3.7V = 37V
Capacity:
2 × 3Ah = 6Ah
Full-charge voltage:
10 × 4.2V = 42V
Nominal energy:
37V × 6Ah = 222Wh
Now the battery specification starts to make sense.
Instead of saying “20 pieces of 3000mAh cells,” the finished battery can be described as approximately:
37V 6Ah / 222Wh
This is much easier for an equipment manufacturer to use.
Using a typical 3.7V nominal cell:
15 × 3.7V = 55.5V
At full charge:
15 × 4.2V = 63V
So a 15S lithium-ion battery may be specified approximately as:
55.5V nominal
63V full charge
If you are developing a 15S battery, the charger and BMS need to be designed around this voltage range.
This is particularly important for equipment with a nominal voltage around 48V–56V.
For OEM buyers, understanding these voltage differences is important when choosing cells, designing a BMS and matching the battery to the equipment and charger.
Why Does 18650 Say 3.6V or 3.7V?
If you’ve ever compared 18650 lithium-ion cells, you may have noticed something slightly confusing.One supplier lists:
3.6V
Another lists:
3.7V
Both may be selling standard lithium-ion 18650 cells.
So which one is correct?
In many cases, the difference is simply how the manufacturer defines and presents the nominal voltage.
A conventional lithium-ion 18650 cell commonly operates around this nominal voltage range, while its voltage during actual use moves considerably above and below that value.
For practical battery-pack calculations, 3.6V or 3.7V nominal may therefore appear depending on the cell specification.
The important number isn’t only the nominal voltage.
You also need to know:
Maximum charge voltage
Discharge cutoff voltage
Nominal voltage
These three numbers tell you much more about how the cell behaves inside a real battery.
What Is the Nominal Voltage of an 18650?
The nominal voltage is a convenient reference value.It is not the voltage you will measure from the cell at every moment.
For many conventional lithium-ion 18650 cells, you will see:
3.6V nominal
or
3.7V nominal
This value is used when calculating battery-pack specifications and energy.
For example:
3.7V × 3Ah = 11.1Wh
This doesn’t mean the cell stays at exactly 3.7V throughout its discharge.
In actual use, the voltage gradually changes.
That’s why a battery-powered device doesn’t simply receive a perfectly constant 3.7V from a single 18650 cell.
What Is the Full-Charge Voltage?
For many standard lithium-ion 18650 cells, the maximum charging voltage is:4.2V
This is significantly higher than the nominal 3.6V or 3.7V figure.
So a single cell may be described as:
3.7V nominal
4.2V full charge
There is no contradiction here.
The battery operates across a voltage range.
When the cell is fully charged, its terminal voltage can reach approximately 4.2V under the specified charging conditions.
The exact maximum voltage should always come from the cell manufacturer’s datasheet.
Different lithium-ion chemistries can have different voltage characteristics, so it is not safe to assume that every cylindrical lithium cell uses exactly the same charging voltage.
Why Does Battery Voltage Drop During Use?
As the cell discharges, its voltage decreases.The actual discharge curve isn’t a perfectly straight line.
There can be a relatively stable voltage region followed by a more noticeable decline as the cell approaches its lower voltage limit.
The exact curve depends on:
- Cell chemistry
- Discharge current
- Temperature
- Cell age
- Internal resistance
- Cutoff voltage
Imagine a power tool starting its motor.
The cell voltage may temporarily drop because of the high current demand and internal resistance.
The battery may still have considerable capacity remaining, but the device can see a lower instantaneous voltage.
This is one reason engineers look at both capacity and discharge performance when selecting an 18650 cell.
What Is the Discharge Cutoff Voltage?
The cutoff voltage is the lower voltage limit used during discharge.A typical lithium-ion cell may have a cutoff around:
2.5V–3.0V
depending on the cell specification and application.
But this is not a universal number.
You should always follow the actual manufacturer’s datasheet.
The cutoff voltage affects usable capacity.
If you stop the discharge earlier, you may leave some energy unused.
If you discharge too far, you can damage the cell and create safety concerns.
This is one of the jobs handled by the battery-management system in a properly designed multi-cell battery pack.
How Does 18650 Series Connection Change Voltage?
This is probably the most important calculation when designing a battery pack.When cells are connected in series, their voltages add together.
For a typical 3.7V nominal cell:
1S = 3.7V
2S = 7.4V
3S = 11.1V
4S = 14.8V
5S = 18.5V
6S = 22.2V
7S = 25.9V
8S = 29.6V
10S = 37V
13S = 48.1V
15S = 55.5V
These are nominal voltages.
If the cell has a 4.2V maximum charge voltage, the full-charge voltage is:
Series count × 4.2V
So:
4S = 16.8V full charge
10S = 42V full charge
15S = 63V full charge
This distinction becomes very important when choosing the charger.
18650 Battery Voltage Chart
| Configuration | Nominal Voltage* | Full Charge* |
|---|---|---|
| 1S | 3.7V | 4.2V |
| 2S | 7.4V | 8.4V |
| 3S | 11.1V | 12.6V |
| 4S | 14.8V | 16.8V |
| 5S | 18.5V | 21.0V |
| 6S | 22.2V | 25.2V |
| 7S | 25.9V | 29.4V |
| 8S | 29.6V | 33.6V |
| 10S | 37.0V | 42.0V |
| 13S | 48.1V | 54.6V |
| 15S | 55.5V | 63.0V |
This chart is useful when you are doing an initial battery design.
For the final specification, however, use the exact cell datasheet.
Does Parallel Connection Increase Voltage?
No.Parallel connection primarily increases capacity and current capability while keeping the nominal voltage approximately the same.
For example:
4S1P
and
4S2P
both have approximately:
14.8V nominal
The difference is capacity.
If each cell is 3000mAh:
4S1P = 3Ah
4S2P = 6Ah
4S3P = 9Ah
The voltage remains approximately 14.8V.
This is why the S/P notation is so useful.
S tells you the voltage configuration.
P tells you the parallel capacity.
What Does 10S2P Mean?
Let’s take a practical example.A customer asks for:
10S2P 18650 battery
This means:
10 series groups
2 cells in parallel per group
Total cell count:
10 × 2 = 20 cells
Using 3000mAh cells:
Nominal voltage:
10 × 3.7V = 37V
Capacity:
2 × 3Ah = 6Ah
Full-charge voltage:
10 × 4.2V = 42V
Nominal energy:
37V × 6Ah = 222Wh
Now the battery specification starts to make sense.
Instead of saying “20 pieces of 3000mAh cells,” the finished battery can be described as approximately:
37V 6Ah / 222Wh
This is much easier for an equipment manufacturer to use.
What About a 15S Battery?
15S configurations are also used in higher-voltage applications.Using a typical 3.7V nominal cell:
15 × 3.7V = 55.5V
At full charge:
15 × 4.2V = 63V
So a 15S lithium-ion battery may be specified approximately as:
55.5V nominal
63V full charge
If you are developing a 15S battery, the charger and BMS need to be designed around this voltage range.
This is particularly important for equipment with a nominal voltage around 48V–56V.