Series vs. Parallel Battery Packs: Which Configuration Fits an OEM Device?

The way lithium-ion cells are connected determines the basic electrical behavior of a battery pack.

• Series connections increase voltage.

• Parallel connections increase capacity and current capability.

• Series-parallel configurations increase voltage and capacity at the same time.

For an OEM device, the correct battery pack configuration must match:

• Device voltage range

• Required capacity

• Continuous and peak current

• Runtime target

• Charger design

• BMS or PCM requirements

• Available installation space

• Weight and thermal limits

• Production and testing requirements

Choosing between series and parallel is therefore not only an electrical calculation. It is a system-level design decision.

Quick Answer: Series vs. Parallel Battery Packs

For example, using 3.7 V, 2500 mAh cells:

• 2S1P = 7.4 V, 2500 mAh

• 1S2P = 3.7 V, 5000 mAh

• 2S2P = 7.4 V, 5000 mAh

The right configuration depends on the target voltage, capacity, current and mechanical design.

1. What Does “S” and “P” Mean?

Battery pack configuration is commonly written using the letters S and P.

S Means Series

The number before S indicates the number of cells or cell groups connected in series.

Examples:

• 1S: one cell group in series

• 2S: two cell groups in series

• 3S: three cell groups in series

• 4S: four cell groups in series

Series connections increase voltage.

P Means Parallel

The number before P indicates the number of cells connected in parallel within each series group.

Examples:

• 1P: one cell in each parallel group

• 2P: two cells in parallel

• 3P: three cells in parallel

• 4P: four cells in parallel

Parallel connections increase capacity and current capability.

Total Cell Count

The total number of cells is:

Total cells = S × P

For example:

• 2S1P = 2 cells

• 1S2P = 2 cells

• 2S2P = 4 cells

• 3S2P = 6 cells

2. How Series Battery Packs Work

Cells connected in series are connected end to end. The positive terminal of one cell connects to the negative terminal of the next cell.

For matched cells in series:

Pack voltage = Cell voltage × Number of cells in series

The amp-hour capacity remains approximately equal to the capacity of one cell or one parallel group.

Example: 2S1P Pack

Assume each cell has:

• Nominal voltage: 3.7 V

• Capacity: 2500 mAh

A 2S1P pack provides:

• Voltage: 3.7 V × 2 = 7.4 V

• Capacity: 2500 mAh

• Energy: 7.4 V × 2.5 Ah = 18.5 Wh

The result is:

7.4 V, 2500 mAh, 18.5 Wh

Example: 3S1P Pack

Using the same cells:

• Voltage: 3.7 V × 3 = 11.1 V

• Capacity: 2500 mAh

• Energy: 11.1 V × 2.5 Ah = 27.75 Wh

The result is:

11.1 V, 2500 mAh, 27.75 Wh

A series configuration is suitable when the device or power system requires a higher nominal voltage.

3. How Parallel Battery Packs Work

Cells connected in parallel have their positive terminals connected together and their negative terminals connected together.

For matched cells in parallel:

Pack capacity = Cell capacity × Number of cells in parallel

The voltage remains approximately equal to the voltage of one cell or one series group.

Example: 1S2P Pack

Assume each cell has:

• Nominal voltage: 3.7 V

• Capacity: 2500 mAh

A 1S2P pack provides:

• Voltage: 3.7 V

• Capacity: 2500 mAh × 2 = 5000 mAh

• Energy: 3.7 V × 5 Ah = 18.5 Wh

The result is:

3.7 V, 5000 mAh, 18.5 Wh

A parallel configuration is suitable when the device operates at the same voltage as one cell or series group but needs more capacity, longer runtime or higher current capability.

4. Series vs. Parallel: What Changes?

The detailed capacity formulas are covered in How to Calculate Lithium-Ion Battery Pack Capacity.

5. What Is a Series-Parallel Battery Pack?

Most custom OEM battery packs use a combination of series and parallel connections.

A series-parallel pack uses:

• Series connections to achieve the required voltage;

• Parallel connections to achieve the required capacity and current capability.

Example: 2S2P Pack

Using four 3.7 V, 2500 mAh cells:

• Series count: 2

• Parallel count: 2

• Total cells: 4

• Nominal voltage: 7.4 V

• Capacity: 5000 mAh

• Energy: 37 Wh

Calculation:

3.7 V × 2 = 7.4 V

2500 mAh × 2 = 5000 mAh

7.4 V × 5 Ah = 37 Wh

The result is:

7.4 V, 5000 mAh, 37 Wh

A 2S2P configuration may be appropriate for an OEM device that needs a 7.4 V platform and more energy than a 2S1P pack can provide.

6. Practical Comparison Using the Same Two Cells

Using two identical 3.7 V, 2500 mAh cells, the cells can be connected in different ways.

Both configurations contain approximately the same nominal energy because they use the same two cells. However, they are not interchangeable.

The device must be designed for the correct:

• Input voltage;

• Charging voltage;

• Current;

• BMS;

• Connector;

• Mechanical layout.

This is an important point for OEM design: two battery packs can have similar watt-hours but still be unsuitable for the same product.

7. When Should an OEM Device Use a Series Configuration?

A series configuration is generally considered when the device requires a higher voltage than one cell can provide.

Typical reasons include:

• The motor or power system requires a higher voltage;

• Higher voltage can reduce current for a given power level;

• The device already uses a 2S, 3S or 4S electrical architecture;

• A higher voltage input is needed for a converter or motor controller;

• The available system components are designed for a defined voltage platform.

For a constant-power load:

Current = Power ÷ Voltage

At the same power, a higher battery voltage can reduce the current drawn from the pack. This may affect conductor sizing, connector selection and power-management design.

However, higher voltage does not automatically mean better performance. The device electronics, charger, insulation, BMS and motor controller must all support the selected voltage.

For practical voltage-level comparisons, see 7.4V vs. 11.1V vs. 14.8V Battery Packs.

8. When Should an OEM Device Use a Parallel Configuration?

A parallel configuration is generally considered when the device needs more capacity or higher current at the same nominal voltage.

Typical reasons include:

• Longer required runtime;

• Higher continuous current;

• Short high-current demand;

• More energy within the same voltage platform;

• Use of a cell with limited individual capacity;

• Need to distribute current across multiple cells.

Parallel cells can share current, but the design must still account for:

• Cell matching;

• Current distribution;

• Connection resistance;

• Cell temperature;

• Welding quality;

• BMS or protection design;

• Mechanical support.

Parallel cells should not be treated as automatically identical. Differences in internal resistance or state of charge can affect current sharing and long-term pack consistency.

9. Cell Matching Requirements

Series and parallel battery packs both require compatible cells.

Cells used in the same pack should be matched by relevant characteristics such as:

• Model;

• Nominal capacity;

• Voltage;

• Internal resistance;

• Production batch;

• Aging condition;

• Temperature history.

Why Matching Is Important in Series Packs

In a series pack, the weakest cell may reach its upper or lower voltage limit before the other cells. This can reduce usable pack capacity and cause premature protection cutoff.

Why Matching Is Important in Parallel Packs

In a parallel group, cells with different voltage or internal resistance may not share current evenly. One cell may carry more load or receive more charging current than another.

For more information about pack inconsistency and cell differences, see Why Lithium-Ion Battery Packs Become Inconsistent and What to Do.

10. BMS Requirements for Series and Parallel Packs

The battery management system must match the pack configuration.

Series Packs

A pack with multiple cells in series generally requires monitoring of individual cell groups.

The BMS may need to provide:

• Cell voltage monitoring;

• Cell balancing;

• Overcharge protection;

• Over-discharge protection;

• Overcurrent protection;

• Short-circuit protection;

• Temperature monitoring.

As the series count increases, the BMS must be designed for the corresponding number of cells.

Parallel Packs

Parallel cells may share the same voltage measurement point, but the BMS or protection system still needs to account for:

• Total current;

• Cell matching;

• Temperature;

• Charging behavior;

• Connection resistance;

• Fault conditions.

Series-Parallel Packs

A series-parallel pack typically requires the BMS to monitor each series group while managing the total pack current.

The BMS should be selected during the battery design stage, not added after the cell configuration is finalized.

11. Charging Requirements

Series and parallel configurations require different charging considerations.

Series Charging

The charger must support the total series voltage.

For example:

• 1S pack: charger designed for one cell;

• 2S pack: charger designed for a 2S battery;

• 3S pack: charger designed for a 3S battery;

• 4S pack: charger designed for a 4S battery.

A charger designed for a single cell should not be used directly with a multi-series pack.

Parallel Charging

A parallel pack still uses the voltage of one cell or one series group, but the charging system may need to supply more current or accept a longer charging time.

Series-Parallel Charging

The charging system must match both:

• Total series voltage;

• Pack capacity and charging-current requirements.

The charger, battery and BMS should be tested as one system.

12. Match the Configuration to the OEM Device

The correct configuration depends on the device’s actual requirements.

This is only a preliminary selection guide. The final configuration should also be checked against current demand, dimensions, charging, temperature and production requirements.

13. OEM Example: Choosing a Configuration for a 7.4 V Device

Assume an OEM product requires:

• Nominal voltage: approximately 7.4 V;

• Target capacity: 5000 mAh;

• Continuous current: moderate;

• Compact rectangular battery compartment;

• Rechargeable operation;

• Cell-level monitoring.

A preliminary configuration could be:

2S2P using 3.7 V, 2500 mAh cells

Calculation:

• Voltage: 3.7 V × 2 = 7.4 V;

• Capacity: 2500 mAh × 2 = 5000 mAh;

• Total cells: 2 × 2 = 4 cells;

• Nominal energy: 7.4 V × 5 Ah = 37 Wh.

The design team must then confirm:

• Whether the 2S voltage range is compatible with the device;

• Whether a 2S BMS with balancing is required;

• Whether the cells can support the current;

• Whether the charger is designed for 2S;

• Whether the pack fits the enclosure;

• Whether the connector supports the required current;

• Whether the battery passes the planned testing.

ZERNE’s 7.4V lithium battery pack page can be used as a product-level reference for 2S battery pack configurations, while custom designs should be evaluated against the actual OEM device.

14. Configuration Example for a Higher-Voltage Device

Assume another OEM product requires:

• Nominal voltage: approximately 11.1 V;

• Capacity: 5000 mAh;

• Stable output during continuous operation.

A preliminary configuration could be:

3S2P using 3.7 V, 2500 mAh cells

Calculation:

• Voltage: 3.7 V × 3 = 11.1 V;

• Capacity: 2500 mAh × 2 = 5000 mAh;

• Total cells: 3 × 2 = 6 cells;

• Nominal energy: 11.1 V × 5 Ah = 55.5 Wh.

An 11.1 V pack requires a charger and BMS designed for a 3S configuration.

For an existing product reference, see ZERNE’s 11.1V lithium battery pack page.

15. Mechanical Design Considerations

Series and parallel connections also affect physical pack layout.

The designer should consider:

• Cell orientation;

• Cell spacing;

• Welding points;

• Busbars or conductors;

• Insulation;

• BMS location;

• Cable routing;

• Connector position;

• Housing dimensions;

• Heat dissipation;

• Mounting points;

• Vibration and impact.

A configuration that is electrically correct may still be unsuitable if it cannot fit the available enclosure.

For cylindrical cell packs, the cell arrangement may affect pack width, length and height. For pouch-cell packs, series and parallel tab locations may influence the internal layout and cable exit.

Detailed battery-compartment clearances, compression and swelling allowances should be handled separately in LiPo Battery Compartment Design: Clearance & Swelling.

16. Common Series and Parallel Design Mistakes

Mistake 1: Assuming Series Increases mAh

Series connections increase voltage, not normally amp-hour capacity.

Mistake 2: Assuming Parallel Increases Voltage

Parallel cells retain approximately the same voltage as one cell or one series group.

Mistake 3: Using a Charger for the Wrong Series Count

A charger for a 1S pack is not suitable for a 2S, 3S or 4S pack.

Mistake 4: Connecting Unmatched Cells

Different capacity, resistance or aging conditions can reduce pack consistency and usable performance.

Mistake 5: Selecting the BMS After the Pack Layout

The series count, current demand and communication requirements should be known before the BMS is selected.

Mistake 6: Comparing Only mAh

A 3.7 V, 5000 mAh pack and a 7.4 V, 5000 mAh pack do not contain the same amount of energy.

Mistake 7: Ignoring Peak Current

The configuration must support the device’s peak demand, not only its average current.

17. Series and Parallel Battery Pack Checklist

Before approving the battery configuration, confirm:

• Device nominal voltage;

• Device minimum and maximum voltage;

• Target capacity;

• Continuous current;

• Peak current;

• Required series count;

• Required parallel count;

• Cell model and capacity;

• Total cell count;

• Cell matching requirements;

• BMS or PCM structure;

• Charger compatibility;

• Connector and cable rating;

• Mechanical fit;

• Temperature conditions;

• Runtime target;

• Testing requirements;

• Expected production volume.

For broader battery design criteria, see Lithium-Ion Battery Pack Design: Voltage, Capacity, BMS and Runtime.

FAQ

What is the difference between a series and parallel battery pack?

A series battery pack increases voltage, while a parallel battery pack increases capacity and current capability. A series-parallel pack combines both effects.

Does connecting batteries in series increase capacity?

Normally, no. Connecting equal-capacity cells in series increases voltage while the amp-hour capacity remains approximately the same.

Does connecting batteries in parallel increase voltage?

Normally, no. Connecting equal-voltage cells in parallel increases capacity while maintaining approximately the same voltage.

What does 2S2P mean?

2S2P means two cell groups connected in series, with two cells connected in parallel in each group. It contains four cells in total.

What is the capacity of a 2S2P pack using 2500 mAh cells?

The pack capacity is approximately 5000 mAh. If the cells have a nominal voltage of 3.7 V, the pack voltage is approximately 7.4 V.

Do series battery packs need a BMS?

Multi-series lithium battery packs generally require appropriate protection and balancing. The exact BMS structure depends on the cell chemistry, series count, current, charger and application.

Are series-parallel battery packs better than series-only or parallel-only packs?

Not automatically. A series-parallel configuration is useful when the device requires both higher voltage and higher capacity, but the final choice depends on the application and physical constraints.

Can different battery cells be connected in parallel?

Cells should not be connected together casually if they differ in model, capacity, voltage, internal resistance or condition. A qualified battery design should use compatible and properly matched cells.

Conclusion

The basic rule for battery pack configuration is:

Series increases voltage.

Parallel increases capacity and current capability.

Series-parallel increases both voltage and capacity.

For an OEM device, the correct configuration must also match:

• Voltage range;

• Capacity and energy;

• Current demand;

• Charger;

• BMS;

• Cell matching;

• Mechanical space;

• Connector;

• Temperature;

• Testing requirements.

For example:

• 2S1P provides higher voltage;

• 1S2P provides higher capacity at the same cell voltage;

• 2S2P provides both higher voltage and higher capacity.

If you are developing a custom battery pack, share the device voltage, capacity target, current demand, dimensions, charging method and expected quantity with the ZERNE technical team. ZERNE provides custom lithium battery pack solutions for OEM and ODM projects and can help evaluate the appropriate series, parallel or series-parallel configuration.