18650 battery pack is built by connecting individual cells in series, parallel, or a combination of both. Understanding how these connections affect voltage, capacity, energy and current is essential when developing a custom battery pack for an OEM product.
If you’re still getting familiar with 18650 cells, it helps to start with the basics before calculating a complete pack. Our [complete 18650 lithium battery guide] covers cell specifications, applications, capacity, voltage and the main considerations when sourcing 18650 cells for commercial projects.
How much capacity?
What current?
What size?
How many cells?
What BMS?
And perhaps most importantly, how should the cells be connected?
This is where series and parallel configuration comes in.
A single 18650 cell usually has a nominal voltage around 3.6–3.7V. If you need a higher voltage, you connect cells in series.
If you need more capacity or current capability, you normally connect cells in parallel.
It sounds straightforward.
The tricky part is combining both to meet the actual requirements of the product.
For example, a 24V-class battery and a 24V 10Ah battery are not the same specification. One describes voltage; the other gives you voltage plus capacity.
For OEM battery buyers, getting this distinction right at the quotation stage can save quite a bit of back-and-forth later.
When cells are connected in series, their voltage adds together.
A typical lithium-ion 18650 cell may have a nominal voltage of approximately 3.6V or 3.7V.
So, roughly:
Full-charge voltage is different.
For a conventional 4.2V lithium-ion cell, a 4S pack reaches approximately:
4 × 4.2V = 16.8V
So when someone says “14.8V battery,” they are generally talking about nominal voltage, not the voltage immediately after a full charge.
For a detailed explanation, see our [18650 battery voltage guide].
Parallel cells increase capacity.
Imagine one 18650 cell rated at:
3000mAh
If you connect two identical cells in parallel:
3000mAh + 3000mAh = 6000mAh
So:
1P = 3000mAh
2P = 6000mAh
3P = 9000mAh
The voltage stays approximately the same.
This is the key difference:
Series → increases voltage
Parallel → increases capacity
That simple rule is the foundation of most cylindrical lithium-ion battery-pack configurations.
A 4S2P battery pack contains:
4 groups connected in series
with:
2 cells in parallel in each group
So the total cell count is:
4 × 2 = 8 cells
If each cell is 3000mAh and approximately 3.7V nominal:
So an 8-cell pack can provide approximately:
14.8V / 6Ah / 88.8Wh
The actual usable energy will depend on the cell, discharge conditions, BMS settings and application.
A 10S2P configuration contains:
10 × 2 = 20 cells
Nominal voltage:
10 × 3.7V = 37V
Capacity:
2 × 3Ah = 6Ah
Approximate energy:
37V × 6Ah = 222Wh
This configuration could therefore be described as approximately:
37V 6Ah 222Wh
This kind of calculation is useful when discussing requirements with a battery manufacturer.
Instead of saying:
The number depends on three main things:
Required voltage
Required capacity
Selected cell
For example, suppose the project needs around 36V and 10Ah.
Using a 3.7V, 3000mAh cell:
You need approximately:
10S for voltage
and
4P for capacity.
That gives:
10S4P
Total cells:
10 × 4 = 40 cells
Approximate pack:
37V / 12Ah
Actually, 4 × 3Ah = 12Ah, which is above the 10Ah target.
You could potentially select another cell capacity or another configuration depending on the exact requirements.
This is why battery configuration is usually an optimization problem rather than simply multiplying numbers.
But there are practical limits.
More cells mean:
A 10S4P pack contains 40 cells.
A 10S8P pack contains 80 cells.
The second pack has twice the parallel capacity, but it also requires significantly more physical space and materials.
If the customer says, “I need twice the runtime,” simply doubling the number of cells may not be the best solution.
Sometimes a higher-capacity cell is worth considering.
This is one reason we recommend comparing actual 18650 cell specifications before fixing the final configuration.
Suppose one cell has a continuous discharge rating of:
10A
Two identical cells in parallel could theoretically provide around:
20A
under appropriate conditions.
Three parallel cells:
30A
Again, this is a simplified calculation.
The actual pack current capability depends on cell characteristics, temperature, connections, BMS limits and the manufacturer’s specified operating conditions.
You should not simply multiply the current rating and assume the finished pack will always deliver that number.
Still, the basic principle is useful:
More parallel cells can share the load.
For power tools, robotics and motor-driven equipment, this can become particularly important.
For a commercial battery pack, cells should normally be matched according to relevant electrical characteristics.
Mixing cells with different:
Imagine assembling a large pack from leftover cells from different suppliers.
One cell group may reach full charge earlier than another.
Another group may discharge faster.
The BMS has to manage those differences, but it cannot turn mismatched cells into identical cells.
For OEM production, consistent cell sourcing and production testing are therefore important.
If you’re still getting familiar with 18650 cells, it helps to start with the basics before calculating a complete pack. Our [complete 18650 lithium battery guide] covers cell specifications, applications, capacity, voltage and the main considerations when sourcing 18650 cells for commercial projects.
18650 Battery Pack Configuration: Series or Parallel?
When a customer asks for a “24V 18650 battery,” there is still quite a lot we don’t know.How much capacity?
What current?
What size?
How many cells?
What BMS?
And perhaps most importantly, how should the cells be connected?
This is where series and parallel configuration comes in.
A single 18650 cell usually has a nominal voltage around 3.6–3.7V. If you need a higher voltage, you connect cells in series.
If you need more capacity or current capability, you normally connect cells in parallel.
It sounds straightforward.
The tricky part is combining both to meet the actual requirements of the product.
For example, a 24V-class battery and a 24V 10Ah battery are not the same specification. One describes voltage; the other gives you voltage plus capacity.
For OEM battery buyers, getting this distinction right at the quotation stage can save quite a bit of back-and-forth later.
What Does “S” Mean in an 18650 Battery Pack?
The S stands for series.When cells are connected in series, their voltage adds together.
A typical lithium-ion 18650 cell may have a nominal voltage of approximately 3.6V or 3.7V.
So, roughly:
- 1S = 3.6–3.7V
- 2S = 7.2–7.4V
- 3S = 10.8–11.1V
- 4S = 14.4–14.8V
- 5S = 18–18.5V
- 6S = 21.6–22.2V
- 7S = 25.2–25.9V
- 10S = 36–37V
Full-charge voltage is different.
For a conventional 4.2V lithium-ion cell, a 4S pack reaches approximately:
4 × 4.2V = 16.8V
So when someone says “14.8V battery,” they are generally talking about nominal voltage, not the voltage immediately after a full charge.
For a detailed explanation, see our [18650 battery voltage guide].
What Does “P” Mean?
The P stands for parallel.Parallel cells increase capacity.
Imagine one 18650 cell rated at:
3000mAh
If you connect two identical cells in parallel:
3000mAh + 3000mAh = 6000mAh
So:
1P = 3000mAh
2P = 6000mAh
3P = 9000mAh
The voltage stays approximately the same.
This is the key difference:
Series → increases voltage
Parallel → increases capacity
That simple rule is the foundation of most cylindrical lithium-ion battery-pack configurations.
What Does 4S2P Mean?
This is one of the most common questions.A 4S2P battery pack contains:
4 groups connected in series
with:
2 cells in parallel in each group
So the total cell count is:
4 × 2 = 8 cells
If each cell is 3000mAh and approximately 3.7V nominal:
Voltage
4 × 3.7V = 14.8V nominalCapacity
2 × 3000mAh = 6000mAhEnergy
14.8V × 6Ah ≈ 88.8WhSo an 8-cell pack can provide approximately:
14.8V / 6Ah / 88.8Wh
The actual usable energy will depend on the cell, discharge conditions, BMS settings and application.
What About 10S2P?
Let’s use the same 3000mAh cell.A 10S2P configuration contains:
10 × 2 = 20 cells
Nominal voltage:
10 × 3.7V = 37V
Capacity:
2 × 3Ah = 6Ah
Approximate energy:
37V × 6Ah = 222Wh
This configuration could therefore be described as approximately:
37V 6Ah 222Wh
This kind of calculation is useful when discussing requirements with a battery manufacturer.
Instead of saying:
You can provide:
That’s much easier for the engineering team to work with.
How Many 18650 Cells Do You Actually Need?
There is no single answer.The number depends on three main things:
Required voltage
Required capacity
Selected cell
For example, suppose the project needs around 36V and 10Ah.
Using a 3.7V, 3000mAh cell:
You need approximately:
10S for voltage
and
4P for capacity.
That gives:
10S4P
Total cells:
10 × 4 = 40 cells
Approximate pack:
37V / 12Ah
Actually, 4 × 3Ah = 12Ah, which is above the 10Ah target.
You could potentially select another cell capacity or another configuration depending on the exact requirements.
This is why battery configuration is usually an optimization problem rather than simply multiplying numbers.
Why Not Just Add More Cells in Parallel?
You can increase capacity by adding parallel cells.But there are practical limits.
More cells mean:
- More space
- More weight
- More welding points
- More connections
- Higher material cost
- Larger pack dimensions
A 10S4P pack contains 40 cells.
A 10S8P pack contains 80 cells.
The second pack has twice the parallel capacity, but it also requires significantly more physical space and materials.
If the customer says, “I need twice the runtime,” simply doubling the number of cells may not be the best solution.
Sometimes a higher-capacity cell is worth considering.
This is one reason we recommend comparing actual 18650 cell specifications before fixing the final configuration.
Series and Parallel Affect Current Too
Parallel configuration can also increase the battery’s current capability.Suppose one cell has a continuous discharge rating of:
10A
Two identical cells in parallel could theoretically provide around:
20A
under appropriate conditions.
Three parallel cells:
30A
Again, this is a simplified calculation.
The actual pack current capability depends on cell characteristics, temperature, connections, BMS limits and the manufacturer’s specified operating conditions.
You should not simply multiply the current rating and assume the finished pack will always deliver that number.
Still, the basic principle is useful:
More parallel cells can share the load.
For power tools, robotics and motor-driven equipment, this can become particularly important.
Matching Cells Is Extremely Important
You should not randomly combine different 18650 cells inside the same pack.For a commercial battery pack, cells should normally be matched according to relevant electrical characteristics.
Mixing cells with different:
- Capacity
- Internal resistance
- Chemistry
- Age
- Discharge capability
Imagine assembling a large pack from leftover cells from different suppliers.
One cell group may reach full charge earlier than another.
Another group may discharge faster.
The BMS has to manage those differences, but it cannot turn mismatched cells into identical cells.
For OEM production, consistent cell sourcing and production testing are therefore important.