Battery Knowledge
Understand how series and parallel cell configurations affect 18650 battery pack voltage, capacity, current capability and physical design for OEM applications. Series connections set pack voltage; parallel connections add capacity and share current. For an OEM battery project, the required voltage, runtime, load and available space should be considered together.
This guide uses nominal 3.7V 18650 cell examples to explain the electrical logic behind common layouts. Final pack design must still be confirmed against the selected cell, protection circuit, wiring, thermal conditions and device requirements.
ASOL supplies 18650 and 21700 cylindrical lithium-ion cells and battery packs for B2B applications, including standard models and project-specific configurations.
Table of Contents
What Do Series (S) and Parallel (P) Mean in an 18650 Battery Pack?
In battery notation, S means cells connected in series and P means cells connected in parallel. A configuration such as 4S2P therefore has four series groups and two cells in each parallel group.
- Series: increases voltage while capacity in amp-hours stays at the single-cell level.
- Parallel: increases capacity and available current capability while nominal voltage stays the same.
The notation describes the electrical architecture, not the full pack specification. Cell capacity, continuous discharge capability, BMS or PCM selection, conductor size and enclosure design still affect the finished pack.
How Series Connections Affect Voltage
Series-connected cells add their nominal voltages. With a nominal 3.7V 18650 cell, the simplified voltage relationship is:
Pack voltage = number of series cells × cell nominal voltage
| Configuration | Cells in series | Approximate nominal voltage | Typical use direction |
|---|---|---|---|
| 1S | 1 | 3.7V | Single-cell portable devices |
| 2S | 2 | 7.4V | Small instruments and portable equipment |
| 3S | 3 | 11.1V | Handheld and compact equipment |
| 4S | 4 | 14.8V | Portable industrial devices and tools |
| 5S | 5 | 18.5V | Equipment needing a higher DC bus voltage |
| 6S | 6 | 22.2V | Higher-voltage portable systems |
Higher series counts raise voltage, so the protection electronics, charger and connected equipment must all be suitable for that series count. A six-series example is the ASOL 18650 6S1P 22.2V 3500mAh Cylindrical Li-ion Battery Pack.
How Parallel Connections Affect Capacity and Current Capability
Parallel-connected cells keep the same nominal voltage while their capacities add. The simplified relationship is:
Pack capacity = number of parallel cells × cell capacity
For example, three 2600mAh cells connected in parallel form a 3P group with approximately 7800mAh at the same nominal 3.7V. Parallel groups can also share the load, but the final current capability is limited by the actual cell rating, connections, protection circuit and thermal design.
For a practical single-series example, see the ASOL 18650 3P 3.7V 7800mAh Cylindrical Li-ion Battery Pack.
Series-Parallel Configurations: Combining Voltage and Capacity
Most multi-cell OEM packs use both series and parallel connections. The series count establishes the nominal voltage, while the parallel count establishes the capacity target and helps distribute current demand across cells.
For example, 3S2P means three cell groups in series with two cells in parallel in each group. It uses six cells in total. A real product example is the ASOL 18650 3S2P 11.1V 3600mAh Cylindrical Li-ion Battery Pack.
Likewise, 4S2P uses four series groups with two cells in each group, for eight cells in total. See the ASOL 18650 4S2P 14.6V 5000mAh Cylindrical Li-ion Battery Pack for an existing configuration reference.
Common 1S-6S 18650 Battery Pack Configurations
1S: 3.7V nominal
A 1S pack is a single cell or one parallel group at nominal 3.7V. It suits devices designed around a single lithium-ion cell voltage range, provided the protection, charging and load profile are appropriate.
2S: 7.4V nominal
A 2S pack provides approximately 7.4V nominal. Capacity can be increased with a parallel count such as 2S4P. One current example is the ASOL 18650 2S4P 7.4V 8000mAh Cylindrical Li-ion Battery Pack.
3S: 11.1V nominal
A 3S pack is often used when an application needs an approximately 12V-class lithium-ion supply. The final device and charger must be designed for the actual operating range of a 3S lithium-ion pack.
4S: 14.8V nominal
A 4S layout raises the nominal voltage to approximately 14.8V and is common where a higher-voltage DC input reduces current for a given power demand.
5S: 18.5V nominal
A 5S layout may be considered where the equipment’s electrical design requires an intermediate higher-voltage range. Check charger compatibility, switching components and protection limits early in the project.
6S: 22.2V nominal
A 6S layout provides approximately 22.2V nominal and is used in higher-voltage portable equipment. It requires a protection and charging solution designed specifically for a six-series lithium-ion assembly.
Quick Comparison: 1S-6S Nominal Voltage
| Series count | Nominal voltage with 3.7V cells | What changes | What does not change by series alone |
|---|---|---|---|
| 1S | 3.7V | Baseline cell voltage | Single-cell Ah capacity |
| 2S | 7.4V | Voltage doubles | Ah capacity |
| 3S | 11.1V | Voltage triples | Ah capacity |
| 4S | 14.8V | Higher DC bus voltage | Ah capacity |
| 5S | 18.5V | Higher DC bus voltage | Ah capacity |
| 6S | 22.2V | Higher DC bus voltage | Ah capacity |
To increase amp-hour capacity without changing nominal voltage, add parallel cells. For example, a 4S1P and 4S2P pack have the same nominal voltage, but the 4S2P configuration uses twice as many cells in parallel.
How to Choose an 18650 Battery Pack Configuration
1. Start with the device voltage requirement
Identify the permitted input-voltage range, not only a marketing voltage label. That determines the initial series-count range.
2. Calculate required energy and runtime
Estimate the load profile and required runtime. Energy in watt-hours is often more useful than capacity alone because it combines voltage and amp-hours.
3. Check continuous and peak current
Do not select cells by mAh alone. Confirm continuous load, peak load, start-up current and the allowed current for each parallel cell.
4. Select a cell suitable for the load profile
Capacity, discharge capability, internal resistance, available space and cell sourcing should be evaluated together. Higher capacity is not automatically the right choice for a high-current design.
5. Match the BMS or PCM to the architecture
The protection solution must match the cell chemistry, series count, expected current, charging method and required monitoring or balancing functions.
6. Review physical size and pack shape
Cell count and arrangement affect length, width, height, weight, wire routing and serviceability. Confirm the battery bay and mounting points before finalizing the electrical layout.
7. Define connector, cable and fuse requirements
Connector type, cable length, wire gauge, polarity control and any fuse or thermal protection need to match the actual load and installation conditions.
8. Consider the operating environment
Temperature, vibration, ingress exposure, charging environment and duty cycle can change the suitable cell, enclosure and protection approach.
9. Plan validation before volume production
Confirm the agreed specification, sample verification, functional testing and pack-level checks before a production release. Requirements depend on the product and destination market.
Standard 18650 Battery Packs and Custom Configuration Support
Existing configurations can provide a useful starting point when the voltage, capacity, pack size and interface match the application. If the application needs a different series count, parallel count, connector, wire length, enclosure or protection requirement, the configuration should be reviewed as a project-specific pack.
Explore ASOL cylindrical lithium-ion battery cells and packs to compare available formats and discuss a configuration around your device requirements.
Common 18650 Pack Configuration Mistakes
Using mAh without checking voltage or watt-hours
Capacity in mAh is not a complete measure of stored energy across different voltages. Compare watt-hours when evaluating packs with different series counts.
Choosing the parallel count without checking current
Parallel groups can share current, but the required load must still be within the selected cells’ limits and the limits of the interconnects and protection circuit.
Treating nominal voltage as the full operating range
Equipment, charger and protection settings must be designed for the actual operating range of the chosen lithium-ion series configuration.
Leaving mechanical design until the end
Insulation, cell spacing, wiring paths, connector placement and enclosure clearances are part of battery pack design from the beginning.
Assuming an existing pack is interchangeable
Even packs with the same S/P notation can differ in cells, current capability, protection, dimensions, wiring and interfaces. Confirm the full requirement before substitution.
What to Send for an 18650 Battery Pack Request
To review a configuration efficiently, provide the target voltage range, required runtime or energy, continuous and peak current, maximum dimensions, connector and cable details, charging method, application environment and expected quantity. Photos, drawings and the device power specification are helpful when available.
Frequently Asked Questions
Does series connection increase capacity?
No. Series connection increases nominal voltage. The amp-hour capacity remains at the level of one cell or one parallel group.
Does parallel connection increase voltage?
No. Parallel connection keeps the nominal voltage the same while adding capacity and helping distribute current demand.
How many cells are in a 4S2P pack?
A 4S2P pack has four series groups and two cells per parallel group, for eight cells in total.
What does 3S2P mean?
It means three groups connected in series, with two cells connected in parallel within each group. The layout uses six cells in total.
Can I replace a 3S pack with a 4S pack?
Not without confirming the device, charger, protection circuit and all connected components are designed for the higher voltage range. A 4S pack is not a drop-in replacement for a 3S pack.
Why does cell discharge capability matter?
It determines whether each cell and parallel group can support the application’s continuous and peak load without exceeding the intended operating limits.
What is the difference between an 18650 cell and an 18650 battery pack?
An 18650 cell is one cylindrical lithium-ion cell. A battery pack combines one or more cells with interconnects, insulation, protection electronics, wiring and a connector or enclosure for a specific application.
Can ASOL provide custom 1S-6S 18650 battery packs?
ASOL can review 1S-6S cylindrical pack requirements for B2B and OEM applications. Final suitability depends on the agreed electrical, mechanical, protection and validation requirements.
Find the Right 18650 Pack Configuration for Your Application
Use the voltage requirement to determine the starting series count, then size the parallel count around runtime, current and available space. For suitable existing models or project-specific support, review cylindrical lithium-ion battery cells and packs and share the details of your application.
