Views: 0 Author: ZERNE Battery Technical Content Team Publish Time: 2026-08-26 Origin: Site
Calculating lithium-ion battery pack capacity requires more than adding the capacity printed on each cell.
The final capacity depends on how the cells are connected:
Series connections increase voltage.
Parallel connections increase amp-hour capacity.
Series-parallel configurations increase both voltage and capacity.
For example, four 3.7 V, 1000 mAh cells can be arranged in different ways:
4S1P: approximately 14.8 V, 1000 mAh
2S2P: approximately 7.4 V, 2000 mAh
1S4P: approximately 3.7 V, 4000 mAh
The same number of cells can therefore produce very different battery pack specifications.
This guide explains how to calculate lithium-ion battery pack capacity in mAh, Ah and Wh. It also clarifies the difference between rated capacity and usable capacity so OEM engineers can prepare a more accurate battery specification.
For the broader relationship between voltage, capacity, BMS and runtime, see Lithium-Ion Battery Pack Design: Voltage, Capacity, BMS and Runtime.
For matched cells with the same capacity:
Pack capacity (Ah) = Capacity of one cell (Ah) × Number of cells in parallel
Pack capacity (mAh) = Capacity of one cell (mAh) × Number of parallel cell groups
Pack voltage (V) = Nominal cell voltage (V) × Number of cells in series
Pack energy (Wh) = Pack voltage (V) × Pack capacity (Ah)
The number of cells connected in series is represented by S. The number of cells connected in parallel is represented by P.
For example:
2S1P means two cells in series and one parallel path.
1S2P means one cell group in series and two cells in parallel.
3S2P means three series groups with two parallel cells in each group.
Before calculating a battery pack, it is important to distinguish between three basic values.
Capacity describes how much electrical charge a battery can store. It is commonly expressed in:
Milliamp-hours, or mAh
Amp-hours, or Ah
The conversion is:
1 Ah = 1000 mAh
A 3000 mAh cell is equivalent to a 3 Ah cell.
Voltage describes the electrical potential of the battery. It is commonly expressed in volts.
For many standard lithium-ion and lithium-polymer cells, the nominal voltage is approximately 3.6 V or 3.7 V. The exact value depends on the cell chemistry and manufacturer specification.
Energy combines voltage and capacity. It is expressed in watt-hours.
Energy (Wh) = Voltage (V) × Capacity (Ah)
A higher mAh rating does not always mean a higher total energy if the battery voltage is different.
For example:
3.7 V × 2 Ah = 7.4 Wh
7.4 V × 2 Ah = 14.8 Wh
The two packs have the same amp-hour capacity but different energy because their voltages are different.
When cells are connected in series, the positive terminal of one cell connects to the negative terminal of the next cell.
Series connections increase voltage.
For matched cells connected in series:
Voltage increases
Ah capacity remains approximately the same
Total energy increases because the voltage increases
Assume each cell has:
Nominal voltage: 3.7 V
Capacity: 1000 mAh
A 2S1P pack contains two cells in series.
Pack voltage:
3.7 V × 2 = 7.4 V
Pack capacity:
1000 mAh
Pack energy:
7.4 V × 1 Ah = 7.4 Wh
Therefore, a 2S1P battery pack is approximately:
7.4 V, 1000 mAh, 7.4 Wh
Configuration | Number of Cells | Nominal Voltage | Capacity |
|---|---|---|---|
1S1P | 1 | 3.7 V | 1000 mAh |
2S1P | 2 | 7.4 V | 1000 mAh |
3S1P | 3 | 11.1 V | 1000 mAh |
4S1P | 4 | 14.8 V | 1000 mAh |
These are nominal values based on 3.7 V cells. The full-charge voltage is higher and must be considered when selecting the charger and protection system.
When cells are connected in parallel, their positive terminals are connected together and their negative terminals are connected together.
Parallel connections increase capacity.
For matched cells connected in parallel:
Voltage remains approximately the same
Ah capacity increases
Current capability may increase
Total energy increases because capacity increases
Assume each cell has:
Nominal voltage: 3.7 V
Capacity: 1000 mAh
A 1S2P pack contains two cells in parallel.
Pack voltage:
3.7 V
Pack capacity:
1000 mAh × 2 = 2000 mAh
Pack energy:
3.7 V × 2 Ah = 7.4 Wh
Therefore, a 1S2P battery pack is approximately:
3.7 V, 2000 mAh, 7.4 Wh
Configuration | Number of Cells | Nominal Voltage | Capacity |
|---|---|---|---|
1S1P | 1 | 3.7 V | 1000 mAh |
1S2P | 2 | 3.7 V | 2000 mAh |
1S3P | 3 | 3.7 V | 3000 mAh |
1S4P | 4 | 3.7 V | 4000 mAh |
Parallel capacity calculations assume that the cells have the same model, capacity, condition and electrical characteristics.
A series-parallel battery pack combines both connection methods.
The general formulas are:
Pack voltage = Cell voltage × S
Pack capacity = Cell capacity × P
Total cell count = S × P
Where:
S = number of cells in series
P = number of cells in parallel
Assume each cell has:
Nominal voltage: 3.7 V
Capacity: 1000 mAh
For a 2S2P pack:
Pack voltage:
3.7 V × 2 = 7.4 V
Pack capacity:
1000 mAh × 2 = 2000 mAh
Total number of cells:
2 × 2 = 4 cells
Pack energy:
7.4 V × 2 Ah = 14.8 Wh
The result is:
7.4 V, 2000 mAh, 14.8 Wh
Using the same 3.7 V, 1000 mAh cells:
Pack voltage:
3.7 V × 3 = 11.1 V
Pack capacity:
1000 mAh × 2 = 2000 mAh
Total number of cells:
3 × 2 = 6 cells
Pack energy:
11.1 V × 2 Ah = 22.2 Wh
The result is:
11.1 V, 2000 mAh, 22.2 Wh
For a broader explanation of how these configurations affect device compatibility, see Series vs. Parallel Battery Packs.
If the target capacity and individual cell capacity are known, the required number of parallel cells can be estimated with:
Parallel count = Target pack capacity ÷ Cell capacity
Assume:
Target nominal voltage: 7.4 V
Target capacity: 3000 mAh
Cell nominal voltage: 3.7 V
Cell capacity: 1500 mAh
Step 1: Determine the series count.
7.4 V ÷ 3.7 V = 2S
Step 2: Determine the parallel count.
3000 mAh ÷ 1500 mAh = 2P
The preliminary configuration is:
2S2P
Total number of cells:
2 × 2 = 4 cells
Assume:
Target nominal voltage: 11.1 V
Target capacity: 5000 mAh
Cell nominal voltage: 3.7 V
Cell capacity: 2500 mAh
Series count:
11.1 V ÷ 3.7 V = 3S
Parallel count:
5000 mAh ÷ 2500 mAh = 2P
The preliminary configuration is:
3S2P
Total number of cells:
3 × 2 = 6 cells
This calculation provides an initial configuration. The final design still needs to verify current demand, available space, cell discharge capability, protection requirements and production feasibility.
Amp-hour capacity is useful for comparing batteries with the same voltage. Watt-hours are more useful when comparing packs with different voltage levels.
The formula is:
Pack energy (Wh) = Nominal pack voltage (V) × Pack capacity (Ah)
Battery Pack | Capacity | Energy |
|---|---|---|
3.7 V, 2000 mAh | 2 Ah | 7.4 Wh |
7.4 V, 2000 mAh | 2 Ah | 14.8 Wh |
11.1 V, 2000 mAh | 2 Ah | 22.2 Wh |
14.8 V, 2000 mAh | 2 Ah | 29.6 Wh |
The mAh value is the same in this example, but the total energy changes with voltage.
This is why OEM teams should not compare battery packs using mAh alone. When the voltage differs, compare watt-hours as well.
The capacity printed on a battery is normally a rated or nominal capacity measured under specified test conditions.
The actual usable capacity may be lower because of:
Discharge current
Cutoff voltage
Temperature
Cell aging
Internal resistance
BMS protection limits
Device power conversion
Required service-life reserve
A simple conceptual formula is:
Usable capacity ≈ Rated capacity × Applicable derating factor
The derating factor is not a universal number. It must be established according to the cell datasheet, discharge conditions and application requirements.
For example, a 2000 mAh pack may not deliver exactly 2000 mAh in every application if:
The load is high;
The device stops before the cell reaches its minimum voltage;
The temperature is low;
The pack has aged;
The BMS disconnects the load for protection.
The pack should therefore be specified with both:
Rated capacity
Required usable capacity under defined conditions
The basic capacity formula assumes that all cells are matched.
Cells used in the same pack should be compatible in terms of:
Capacity
Voltage
Internal resistance
Cell model
Production batch
Aging condition
Charge state
If cells have significantly different characteristics, the weakest cell may reach its voltage limit earlier than the others. This can reduce the usable capacity of the complete pack and increase balancing requirements.
For a deeper discussion of cell inconsistency, see Why Lithium-Ion Battery Packs Become Inconsistent and What to Do.
In an OEM specification, it is better to define the required cell model and matching requirements rather than only stating the target battery capacity.
A higher-capacity cell may reduce the number of parallel cells needed, but it may also be physically larger or have different discharge characteristics.
OEM engineers should evaluate:
Cell dimensions
Pack layout
Available space
Target capacity
Current demand
Weight
Thermal conditions
Cell availability
For example, a 3000 mAh cell may allow a smaller parallel count than a 1500 mAh cell. However, if the 3000 mAh cell is too large or cannot meet the required current, it may not be the best choice.
The capacity calculation should therefore be combined with mechanical and electrical feasibility review.
Assume an OEM device requires:
Nominal voltage: approximately 7.4 V
Target capacity: 4000 mAh
Cell nominal voltage: 3.7 V
Cell capacity: 2000 mAh
Step 1: Calculate the series count.
7.4 V ÷ 3.7 V = 2S
Step 2: Calculate the parallel count.
4000 mAh ÷ 2000 mAh = 2P
Step 3: Calculate total cell count.
2S × 2P = 4 cells
Step 4: Calculate nominal energy.
7.4 V × 4 Ah = 29.6 Wh
The preliminary pack specification is:
2S2P, 7.4 V nominal, 4000 mAh, approximately 29.6 Wh
Before this configuration is approved, the OEM and battery manufacturer should verify:
Whether the cells can supply the required current;
Whether the pack fits the available space;
Whether a BMS with balancing is required;
Whether the charger is compatible;
Whether the connector and wiring support the load;
Whether the target capacity is measured under the required test conditions.
A 2S1P pack made from two 1000 mAh cells is not a 2000 mAh pack. It is approximately 7.4 V and 1000 mAh.
If the pack uses multiple parallel paths, the capacity must be multiplied by the number of parallel cells.
A 2000 mAh pack at 3.7 V and a 2000 mAh pack at 7.4 V do not contain the same amount of energy.
Nominal energy is normally calculated using nominal voltage, not the maximum charging voltage.
Actual usable capacity depends on current, temperature, cutoff voltage, aging and protection settings.
Cells with different capacities, internal resistance or aging conditions should not be combined casually in the same pack.
Capacity calculation alone does not determine runtime. Runtime also depends on device power and system efficiency.
For the separate runtime calculation method, see How to Calculate Runtime for a Lithium Battery Pack.
Before requesting a battery pack quotation, confirm:
Target nominal voltage
Minimum and maximum device voltage
Target capacity in mAh or Ah
Required usable energy in Wh
Cell nominal voltage
Cell rated capacity
Required series count
Required parallel count
Total cell count
Continuous current
Peak current
Operating temperature
Available battery space
Charging method
BMS or protection requirements
Capacity test conditions
Expected runtime
Prototype and production quantity
The capacity figure should always be connected to a defined test condition. A quotation stating only “4000 mAh battery pack” is incomplete unless the supplier also identifies the voltage, cell model, configuration and measurement conditions.
Multiply the capacity of one cell by the number of cells connected in parallel. For example, four 1000 mAh cells in a 2S2P configuration provide approximately 2000 mAh capacity.
No. A series connection increases voltage. The amp-hour capacity normally remains approximately equal to the capacity of one parallel group.
Yes. A parallel connection increases the amp-hour capacity according to the number of parallel cells.
If each cell is rated at 3.7 V and 1000 mAh, a 2S2P pack is approximately 7.4 V and 2000 mAh, with four total cells.
Multiply nominal pack voltage by capacity in amp-hours. A 7.4 V, 2 Ah pack contains approximately 14.8 Wh of nominal energy.
No. The 7.4 V, 2000 mAh pack contains approximately twice the nominal energy of the 3.7 V, 2000 mAh pack.
The result can be affected by discharge current, temperature, cutoff voltage, cell aging, internal resistance, BMS settings and measurement conditions.
The total cell count is:
Total cells = Series count × Parallel count
The final count must also satisfy voltage, capacity, current, size, weight and protection requirements.
Calculating lithium-ion battery pack capacity starts with understanding the difference between series and parallel connections.
The core formulas are:
Pack voltage = Cell voltage × Series count
Pack capacity = Cell capacity × Parallel count
Total cells = Series count × Parallel count
Pack energy = Pack voltage × Pack capacity in Ah
For example, using 3.7 V, 1000 mAh cells:
1S1P = 3.7 V, 1000 mAh
2S1P = 7.4 V, 1000 mAh
1S2P = 3.7 V, 2000 mAh
2S2P = 7.4 V, 2000 mAh
These calculations provide the starting point for a battery pack design. The final battery must still be checked for current demand, cell matching, usable capacity, BMS requirements, mechanical fit, charging compatibility and production feasibility.
