Views: 0 Author: ZERNE Battery Technical Content Team Publish Time: 2026-07-15 Origin: Site
18650 batteries can be connected in series, in parallel, or in a combination of both to achieve the required voltage, capacity, and current capability.
A series connection increases the battery pack voltage. A parallel connection increases the capacity and theoretical current capability. For example:
3S1P means three 18650 cells connected in series.
2P means two cells connected in parallel within each group.
3S2P means three series groups with two parallel cells in each group.
4S2P means four series groups with two parallel cells in each group.
The total cell count is calculated as:
Total cells = Series count × Parallel count
A 3S2P battery pack therefore contains six cells, while a 4S2P battery pack contains eight cells.
The connection structure should be selected according to the equipment’s voltage, capacity, current, dimensions, and protection requirements. A suitable BMS is also required for most rechargeable lithium-ion battery packs.
In a series connection, the positive terminal of one cell is connected to the negative terminal of the next cell.
The voltage of each cell adds together, while the ampere-hour capacity remains approximately the same as one cell or one parallel group.
For identical 3.7V, 3000mAh cells:
1S1P: 3.7V, 3Ah
2S1P: 7.4V, 3Ah
3S1P: 11.1V, 3Ah
4S1P: 14.8V, 3Ah
The series count is selected according to the required voltage range of the equipment.
In a parallel connection, the positive terminals are connected together and the negative terminals are connected together.
The voltage remains approximately the same as one cell, while the capacity increases.
Using identical 3.7V, 3000mAh cells:
1S1P: 3.7V, 3Ah
1S2P: 3.7V, 6Ah
1S3P: 3.7V, 9Ah
The parallel count is selected according to the required capacity, runtime, and current capability.
Most practical battery packs require both a specific voltage and enough capacity. A series-parallel configuration combines the two connection methods.
For example, a 3S2P pack:
Uses three series groups
Uses two parallel cells in each group
Contains six cells in total
Provides approximately twice the capacity of a 3S1P pack
Maintains the same nominal voltage class as a 3S1P pack
The letter “S” means the number of cell groups connected in series.
A 1S pack contains one series group.
It may contain:
1S1P: one cell
1S2P: two parallel cells
1S3P: three parallel cells
The nominal voltage remains close to the voltage of one cell. Adding parallel cells increases capacity without changing the nominal voltage.
A 2S pack contains two series groups.
With standard 3.7V lithium-ion cells, the nominal voltage is approximately 7.4V. The full-charge voltage depends on the cell specification and charging system.
A 2S configuration may be used in compact portable equipment, handheld products, and small battery packs that require a higher voltage than a single cell can provide.
A 3S pack contains three series groups.
Using standard 3.7V cells, the nominal voltage is approximately 11.1V. The 3S structure is commonly used in 12V-class lithium-ion battery packs. The 12V 18650 battery pack example shows how a 3S configuration translates into cell count, voltage, and capacity.
For a concise example:
3S1P = three cells
3S2P = six cells
3S3P = nine cells
The 18650 battery voltage guide explains how nominal, full-charge, and cutoff voltage change with series count.
A 4S pack contains four series groups.
Using 3.7V cells, the nominal voltage is approximately 14.8V. It is generally considered a higher-voltage pack than a standard 12V-class 3S pack.
A 4S configuration may be used when the equipment, charger, and protection system are all designed for that voltage range.
The letter “P” means the number of cells connected in parallel within each series group.
A 1P configuration has one cell in each parallel group.
Examples:
1S1P: one cell
3S1P: three cells
4S1P: four cells
The capacity of each series group is equal to the capacity of one cell.
A 2P configuration has two cells connected in parallel in each group.
Examples:
1S2P: two cells
3S2P: six cells
4S2P: eight cells
The capacity of each parallel group is approximately twice the capacity of one cell.
A 3P configuration has three parallel cells in each group.
Examples:
1S3P: three cells
3S3P: nine cells
4S3P: twelve cells
A higher parallel count generally provides greater capacity and current capability, but it also increases the battery pack’s size, weight, cost, and heat-management requirements. The effect of parallel count on pack capacity, energy, and runtime can be estimated with 18650 battery pack capacity and runtime calculations.
The basic voltage formula is:
Pack nominal voltage = Cell nominal voltage × Series count
Using a 3.7V cell:
Configuration | Calculation | Nominal Voltage |
|---|---|---|
1S | 3.7V × 1 | 3.7V |
2S | 3.7V × 2 | 7.4V |
3S | 3.7V × 3 | 11.1V |
4S | 3.7V × 4 | 14.8V |
The actual voltage range changes during charging and discharging. The nominal voltage is used for battery pack calculations, while the full-charge and cutoff voltages must be matched to the selected cell and equipment.
When selecting a series count, check:
Required device input voltage
Maximum permitted input voltage
Minimum operating voltage
Charger output voltage
BMS series count
DC-DC converter input range
A series count should not be selected solely because the pack is labeled “12V” or “24V.”
The basic capacity formula is:
Pack capacity (Ah) = Cell capacity (Ah) × Parallel count
Using 3000mAh, or 3Ah, cells:
Configuration | Cell Capacity | Parallel Count | Pack Capacity |
3S1P | 3Ah | 1P | 3Ah |
3S2P | 3Ah | 2P | 6Ah |
3S3P | 3Ah | 3P | 9Ah |
4S2P | 3Ah | 2P | 6Ah |
The 3S2P and 4S2P examples have the same theoretical capacity when the same cells are used, but their voltage levels are different because their series counts are different.
Capacity should be calculated from the actual cell capacity rather than the maximum capacity printed on a general product label. Cell capacity can vary according to discharge current, temperature, cutoff voltage, and test conditions.
Parallel cells can also increase the theoretical current capability of a battery pack.
The simplified formula is:
Parallel current capability = Cell continuous current × Parallel count
If one cell has a continuous discharge rating of 10A:
1P: approximately 10A
2P: approximately 20A
3P: approximately 30A
These figures are theoretical estimates. Actual pack current capability may be limited by:
BMS rating
Nickel strips or busbars
Weld quality
Wire gauge
Connector rating
Cell temperature
Cell internal resistance
Enclosure heat dissipation
Peak current and continuous current should be calculated separately. A cell that supports a short pulse may not be suitable for continuous operation at the same current. For high-power equipment, the selected cell’s continuous and peak current limits should be compared using high-drain 18650 battery selection.
A 3S2P pack contains six cells:
Three series groups
Two parallel cells in each group
Total cell count: 3 × 2 = 6
Using 3.7V, 3000mAh cells:
Nominal voltage: approximately 11.1V
Capacity: approximately 6Ah
Nominal energy: approximately 66.6Wh
A 4S2P pack contains eight cells:
Four series groups
Two parallel cells in each group
Total cell count: 4 × 2 = 8
Using the same cells:
Nominal voltage: approximately 14.8V
Capacity: approximately 6Ah
Nominal energy: approximately 88.8Wh
The capacity is the same in both examples because both use 2P. The energy is different because the 4S2P pack has a higher nominal voltage.
For a quick comparison of series, parallel, capacity, and energy, use the 18650 Battery Pack Calculator.
The total cell count is:
Total cells = Series count × Parallel count
Examples:
Configuration | Calculation | Total Cells |
2S1P | 2 × 1 | 2 |
3S1P | 3 × 1 | 3 |
3S2P | 3 × 2 | 6 |
3S3P | 3 × 3 | 9 |
4S2P | 4 × 2 | 8 |
4S3P | 4 × 3 | 12 |
This formula provides the electrical cell count. The physical battery pack may require additional space for:
Cell holders
Insulation
BMS
Temperature sensors
Wires
Connectors
Housing
Mechanical supports
Cells in the same pack should have:
The same model
The same chemistry
Similar capacity
Similar internal resistance
Similar age and cycle history
Similar state of charge before assembly
Mixing cells with different characteristics can result in uneven voltage, temperature, and current distribution. For production packs, these checks form the basis of cell matching in an 18650 battery pack, where capacity, internal resistance, voltage, age, and batch consistency are evaluated together.
For production battery packs, cylindrical cells are normally connected using spot-welded nickel strips or suitable busbars.
Directly soldering wires to the cell body can transfer excessive heat into the cell and may damage the internal structure. The connection method should be selected according to the pack current, production process, and safety requirements.
The assembly should include appropriate:
Cell holders
Insulation paper
Terminal protection
Fish paper or insulating sheets
Protective sleeves
Spacing between cells
Housing support
The positive terminal and the outer negative casing must be protected from accidental contact with conductive materials. If a cell includes an integrated protection circuit, its dimensions and use case may differ from an unprotected cell; protected and unprotected 18650 batteries compares these two formats.
A short circuit can cause very high current and rapid heating. During assembly:
Keep metal tools away from exposed terminals.
Do not place loose cells together with conductive objects.
Protect unfinished packs during handling.
Check the polarity of every cell group.
Inspect nickel strips and weld points before applying the final insulation.
Each series group should contain cells with similar performance. A weak group can limit the entire battery pack and may cause early protection activation. Capacity, internal resistance, and load behavior should be checked before cells are grouped; 18650 battery capacity and health testing outlines a practical way to verify them.
A BMS, or battery management system, monitors and protects rechargeable lithium-ion battery packs.
A BMS is especially important when cells are connected in series because the individual groups may not remain at exactly the same voltage during charging and discharging.
Typical BMS functions include:
Overcharge protection
Over-discharge protection
Overcurrent protection
Short-circuit protection
Temperature monitoring
Cell voltage monitoring
Cell balancing
The BMS must match the battery pack configuration. A 3S battery pack generally requires a 3S BMS, while a 4S battery pack requires a 4S BMS.
The BMS current rating should also match the expected continuous and peak load. Selecting a BMS only according to the series count is not enough.
The BMS must be designed around the pack’s series count, current profile, balancing needs, and protection limits; 18650 battery pack BMS design brings these requirements together.
The charger must match the series count and cell chemistry.
A higher series count requires a higher charging voltage. The charger should not be selected only according to the battery pack’s marketing label. The safe 18650 battery charging guide explains how charger voltage, charging current, and charging time should be matched to the pack.
Before choosing the charger, confirm:
Battery chemistry
Series count
Maximum charging voltage
Charging current
BMS compatibility
Device charging requirements
Cell manufacturer specifications
A 3S and a 4S pack may have similar capacity when they use the same parallel count, but they require different charging voltage settings.
Confusing series count with parallel count.
Assuming that 2P means only two cells in the complete pack.
Multiplying capacity by the series count.
Ignoring the difference between nominal and full-charge voltage.
Using cells with different capacities or ages.
Connecting cells with different charge levels.
Using an incorrectly rated BMS.
Selecting a charger without checking the series count.
Directly soldering to cells without controlling heat.
Ignoring insulation around the positive terminal.
Relying on the theoretical current calculation without checking the BMS and wiring.
Failing to test the finished battery pack under the intended load.
A series connection increases voltage, while a parallel connection increases capacity and theoretical current capability.
3S2P means three series groups with two cells connected in parallel in each group. The pack contains six cells in total.
A 4S2P pack contains eight cells because:
4 × 2 = 8
No. Series connections increase voltage. The capacity in ampere-hours remains approximately the same as one cell or one parallel group.
No. Parallel connections maintain approximately the same voltage while increasing capacity and current capability.
1S3P means one series group containing three parallel cells. Using 3000mAh cells, the pack has approximately 3.7V nominal voltage and 9Ah capacity.
A 3S2P pack generally requires a 3S BMS. The BMS must also match the pack’s charging current, continuous load current, peak current, temperature requirements, and protection functions.
Cells used in the same parallel group should have the same model and similar capacity, resistance, age, and charge state. Mixing unsuitable cells can create uneven current distribution and safety risks.
Use:
Pack capacity = Cell capacity × Parallel count
The series count affects voltage, while the parallel count affects ampere-hour capacity.
A 3S2P pack usually provides approximately twice the capacity and theoretical current capability of a 3S1P pack made with the same cells. The actual output is limited by the BMS, wiring, connectors, and thermal design.
Series and parallel calculations provide the electrical foundation for a battery pack, but the finished product also requires mechanical and protection design. For OEM projects, the electrical configuration should also be evaluated alongside device power, runtime, available space, charging, temperature, and validation requirements in 18650 battery selection for an OEM device.
OEM projects may need customized:
Series and parallel configuration
Voltage
Capacity
Cell type
BMS
Connector
Wire length
Housing
Temperature sensor
Mounting structure
ZERNE’s 18650 Battery Pack solutions support customized cell selection, BMS integration, connector configuration, sample testing, and production requirements.
For projects with specific dimensions, capacity, voltage, or current requirements, custom 18650 battery solutions can be developed around the equipment’s electrical and mechanical conditions.
Connecting 18650 batteries in series and parallel allows a battery pack to meet different voltage, capacity, and current requirements.
The main principles are:
Series count determines voltage.
Parallel count determines capacity.
Parallel cells can also increase theoretical current capability.
Total cells equal series count multiplied by parallel count.
3S2P contains six cells.
4S2P contains eight cells.
The BMS must match the series count and current requirements.
Cells should be matched and properly insulated.
The charger must match the battery chemistry and series configuration.
A correct design must consider more than the connection label. Cell specifications, BMS protection, wiring, thermal conditions, enclosure space, charger requirements, and actual device loads should all be verified before production.
