Lithium-Ion Battery Pack Design: Voltage, Capacity, BMS and Runtime

A lithium-ion battery pack is not defined by capacity alone. A suitable pack must deliver the required voltage, provide enough usable energy, support the device’s current demand, fit the available space and include the right protection functions.

For an OEM project, battery pack design also involves decisions about:

• Cell selection

• Series and parallel configuration

• Nominal voltage

• Capacity

• Peak and continuous current

• BMS or PCM

• Charging method

• Connector and cable

• Mechanical integration

• Operating temperature

• Expected runtime

• Testing and production requirements

A battery pack may have the correct capacity but the wrong voltage. It may provide sufficient energy but fail during peak current demand. It may fit the enclosure but lack the protection or communication functions required by the host device.

This guide explains the main principles of lithium-ion battery pack design and shows how OEM teams can connect electrical, mechanical and system requirements before moving into detailed development.

For commercial battery pack solutions, see ZERNE’s lithium battery packs. The product page is intended for product and solution enquiries, while this article focuses on the engineering decisions behind battery pack design.

Quick Answer: What Should Be Considered in Lithium-Ion Battery Pack Design?

A lithium-ion battery pack should be designed by matching the following factors:

1. Required operating voltage

2. Target capacity and usable energy

3. Continuous and peak current

4. Series and parallel cell configuration

5. BMS or PCM protection functions

6. Charging voltage and charging current

7. Available battery compartment

8. Connector and cable requirements

9. Operating and storage temperature

10. Required runtime

11. Testing and compliance needs

12. Expected production volume

The correct design is the one that satisfies the host device’s electrical and mechanical requirements while maintaining safe and repeatable production.

1. Start With the Host Device Requirements

Battery pack design should begin with the device, not with a battery model.

Before selecting cells, the OEM team should define:

• Device operating voltage

• Maximum input voltage

• Minimum operating voltage

• Continuous power demand

• Startup or peak power demand

• Expected operating time

• Charging method

• Available battery space

• Connector and cable position

• Operating temperature

• Product lifecycle

• Expected production volume

The battery should be evaluated as part of the complete power system. The charger, power-management circuit, motor, controller, display, communication module and other loads can all affect the battery specification.

For example, a device may consume only a small average current but require a high current pulse when a motor starts or a wireless module transmits. If only average power is considered, the battery may appear suitable but fail during actual operation.

For OEM requirements such as size, capacity, voltage, BMS and connector, ZERNE’s custom battery solutions provide a relevant commercial entry point.

2. Select the Required Battery Voltage

Voltage determines whether the battery can work with the device’s electrical system.

For many standard lithium-ion and lithium-polymer cells, the common nominal voltage is approximately 3.6 V or 3.7 V, while the full-charge voltage is commonly around 4.2 V. The exact values depend on the cell chemistry and specification.

When cells are connected in series, the voltage increases.

These values are common reference points for standard Li-ion or LiPo configurations. The actual battery design must follow the selected cell datasheet and the charging system requirements.

The device’s input voltage range should be checked carefully. A battery labeled 7.4 V does not remain at exactly 7.4 V throughout operation. Its voltage changes during charging and discharging.

Nominal Voltage and Operating Voltage

OEM engineers should distinguish between:

• Nominal voltage

• Full-charge voltage

• Typical operating voltage

• Cutoff voltage

• Device minimum input voltage

• Device maximum input voltage

A device that accepts only a narrow voltage range may require a DC-DC converter, voltage regulator or specific battery management strategy.

Practical Example: Choosing a Battery for a 7.4 V Device

If an OEM device requires a nominal battery voltage of approximately 7.4 V, a typical solution may use two 3.7 V cells in series, known as a 2S configuration.

However, the design team must still check:

• Whether the device accepts the full-charge voltage

• Whether the charger supports the 2S pack

• Whether cell balancing is required

• Whether the BMS supports two series cells

• Whether the device can operate at the minimum pack voltage

For a more focused comparison of series and parallel configurations, see Series vs. Parallel Battery Packs.

3. Determine the Required Capacity

Capacity describes how much electrical charge the battery can store and is commonly expressed in milliamp-hours or amp-hours.

However, capacity alone does not fully describe the available energy. Energy depends on both voltage and capacity.

The basic relationship is:

Energy in watt-hours = Voltage in volts × Capacity in amp-hours

For example:

• 3.7 V × 1 Ah = 3.7 Wh

• 7.4 V × 2 Ah = 14.8 Wh

• 14.8 V × 3 Ah = 44.4 Wh

When comparing different battery configurations, watt-hours are often more useful than milliamp-hours because voltage may be different.

Example With 3.7 V, 1000 mAh Cells

Assume each cell has:

• Nominal voltage: 3.7 V

• Capacity: 1000 mAh or 1 Ah

Different configurations produce different results:

Series connections increase voltage. Parallel connections increase capacity. A series-parallel configuration can increase both.

The detailed calculation of cell count, series connection and parallel connection should be handled in How to Calculate Lithium-Ion Battery Pack Capacity.

4. Match Continuous and Peak Current

A battery pack must provide enough current for both normal operation and temporary peak demand.

OEM buyers should identify:

• Average operating current

• Continuous current

• Peak current

• Peak duration

• Startup current

• Duty cycle

• Minimum battery voltage during peak load

The battery cell, BMS, wires, welding points and connector must all support the required current. A pack may contain enough stored energy but still fail if its discharge path cannot support the device’s current demand.

Continuous Current

Continuous current is the current the battery must deliver during normal operation. It affects cell selection, conductor size, BMS rating and thermal design.

Peak Current

Peak current may occur during:

• Motor startup

• Wireless transmission

• Heating

• Pump operation

• Lighting activation

• Rapid processor activity

• Temporary high-power output

The duration of the peak is important. A battery that supports a short pulse may not support the same current continuously.

Current and Power Relationship

When device power is known, current can be estimated using:

Current = Power ÷ Voltage

For example, a 20 W load supplied by a 7.4 V battery would require approximately:

20 W ÷ 7.4 V ≈ 2.7 A

The actual design should also consider conversion losses, voltage variation, peak demand and temperature.

5. Choose the Series and Parallel Configuration

The series-parallel configuration defines the electrical structure of the battery pack.

Series Connection

Cells connected in series increase voltage while the amp-hour capacity remains approximately equal to that of one cell.

A 2S1P pack made from 3.7 V, 1000 mAh cells provides approximately:

• 7.4 V nominal voltage

• 1000 mAh capacity

Parallel Connection

Cells connected in parallel increase capacity and current capability while the voltage remains approximately equal to that of one cell.

A 1S2P pack made from the same cells provides approximately:

• 3.7 V nominal voltage

• 2000 mAh capacity

Series-Parallel Connection

A 2S2P pack provides approximately:

• 7.4 V nominal voltage

• 2000 mAh capacity

The design must also consider:

• Cell matching

• Balancing

• Protection

• Welding or connection method

• Pack dimensions

• Heat generation

• BMS architecture

The principle is simple, but the final design should be checked against the actual device and production process. The article Series vs. Parallel Battery Packs provides a narrower comparison.

6. Select the Right Cell Format and Performance Grade

The physical and electrical characteristics of the cell affect the complete pack design.

Important cell parameters include:

• Cell format

• Dimensions

• Nominal capacity

• Discharge rate

• Internal resistance

• Cycle performance

• Operating temperature

• Charging conditions

• Weight

• Availability

• Production consistency

Pouch cells may support thin or shaped designs. Cylindrical cells may offer standardized dimensions and mechanical consistency. The best format depends on the host device and required performance.

Cell selection should not be based on capacity alone. A cell with a higher capacity may be unsuitable if it cannot provide the required current or fit the available space.

For a specific comparison of cylindrical cell formats, see 26650 vs. 18650 Battery: Size, Voltage and Capacity.

Cell Matching

For multi-cell packs, cells should be matched according to relevant parameters such as:

• Capacity

• Voltage

• Internal resistance

• Production batch

• Aging condition

Cell inconsistency can affect balancing, usable capacity, temperature and service life. A detailed discussion of this issue is available in Why Lithium-Ion Battery Packs Become Inconsistent and What to Do.

7. Decide Whether the Pack Needs a PCM or BMS

Protection electronics are an essential part of most rechargeable lithium battery packs.

PCM

A protection circuit module may be suitable for a simpler battery configuration. It can provide basic functions such as:

• Overcharge protection

• Over-discharge protection

• Overcurrent protection

• Short-circuit protection

BMS

A more advanced battery management system may be required when the pack includes multiple cells or needs additional control and communication functions.

A BMS may support:

• Cell voltage monitoring

• Cell balancing

• Current monitoring

• Temperature monitoring

• State-of-charge estimation

• Fault detection

• Communication with the host device

• Charging and discharging control

• Event or fault recording

When Is a BMS More Appropriate?

A BMS is usually more relevant when:

• The battery contains multiple cells in series

• Cell balancing is required

• The device needs battery status information

• The battery operates under high current

• The application requires detailed fault monitoring

• The host device communicates with the battery

• The project requires a smart battery function

The design should define the BMS requirements before the prototype is produced. Changing from a basic protection board to a smart BMS later may affect the battery dimensions, connector, firmware, testing and cost.

For additional background on how battery management affects charge status and battery behavior, see Why Your Lithium-Ion Battery's Last 1 Percent Holds Up Better Than You Think.

8. Estimate Battery Runtime

Runtime depends on the battery’s usable energy and the device’s actual power demand.

A basic estimate is:

Estimated runtime = Battery energy × usable efficiency ÷ device power

For a battery specified by voltage and capacity:

Battery energy = Nominal voltage × Capacity in amp-hours

Runtime Example

Assume:

• Battery pack: 7.4 V, 2000 mAh

• Nominal energy: 7.4 V × 2 Ah = 14.8 Wh

• Estimated usable system efficiency: 85%

• Device power: 5 W

Usable energy:

14.8 Wh × 0.85 = 12.58 Wh

Estimated runtime:

12.58 Wh ÷ 5 W ≈ 2.5 hours

This is an engineering estimate, not a guaranteed operating time.

Actual runtime may be affected by:

• Variable device load

• Voltage conversion efficiency

• Battery cutoff settings

• Temperature

• Battery age

• Discharge rate

• Cell resistance

• BMS protection limits

• Device standby consumption

For a complete runtime calculation method, see How to Calculate Runtime for a Lithium Battery Pack.

9. Check Mechanical Integration

Electrical performance is only one part of battery pack design. The battery must also fit safely into the host product.

The design should define:

• Maximum length

• Maximum width

• Maximum thickness

• Installation direction

• Mounting method

• Connector access

• Cable exit

• Compression limits

• Vibration exposure

• Heat sources

• Service and replacement requirements

A pouch battery should not be compressed by sharp edges or concentrated pressure. A cylindrical pack may require holders, spacers or structural protection. The battery compartment should account for manufacturing tolerances and the expected operating environment.

Detailed compartment design, clearance, heat and swelling allowances are covered separately in LiPo Battery Compartment Design: Clearance & Swelling.

In this article, the main design principle is:

The battery compartment should be designed together with the battery pack, not after the battery has already been selected.

10. Define the Connector and Cable Requirements

The connector and cable are part of the battery interface.

OEM teams should specify:

• Connector type

• Number of pins

• Current rating

• Voltage rating

• Polarity

• Locking method

• Cable length

• Wire gauge

• Wire-exit position

• Strain relief

• Space for installation

• Required signal or communication lines

The power connector may be different from the communication connector. A smart BMS may require additional signal pins for data communication, temperature sensing or identification.

Connector selection should also consider assembly, mating cycles, vibration and field maintenance.

Detailed connector selection is outside the scope of this design overview and is addressed in How to Choose a Connector for a Custom Lithium Battery Pack.

11. Consider Charging Requirements

The battery pack must be compatible with the charger and charging system.

OEM buyers should define:

• Charging voltage

• Charging current

• Charging time

• Charging method

• Charger output tolerance

• Temperature limits

• Charge termination method

• Balancing requirements

• Whether the battery is charged inside or outside the device

A 2S battery pack cannot be charged using a charger designed for a single cell. The charging system must match the pack configuration and the protection electronics.

The BMS or PCM should also be compatible with the charger’s operating conditions. Charging behavior can affect battery life, temperature and safety, so it should be tested with the actual device and charger.

12. Build a Complete Battery Design Specification

Before requesting a prototype, the OEM team should combine the electrical, mechanical and interface requirements into one controlled specification.

A useful specification should include:

A detailed requirement package reduces redesigns and makes supplier quotations easier to compare.

13. Worked OEM Example: Designing a 7.4 V Battery Pack

Assume an OEM device requires:

• Nominal voltage: approximately 7.4 V

• Target capacity: 2000 mAh

• Average power: 5 W

• Peak current: higher than normal operating current

• Compact rectangular installation space

• Rechargeable operation

• Battery status monitoring

A possible preliminary design direction could be:

• Two cells in series for a 2S configuration

• Two parallel cell paths for the required capacity

• A BMS with cell balancing

• Temperature monitoring

• A custom connector

• A pack designed around the host-device enclosure

• Runtime verification under the actual 5 W load

Using two 1000 mAh cells in parallel for each series group produces an approximate 7.4 V, 2000 mAh pack.

The nominal energy is:

7.4 V × 2 Ah = 14.8 Wh

Using an estimated 85% usable system efficiency, the expected usable energy is approximately:

14.8 Wh × 0.85 = 12.58 Wh

At a continuous 5 W load, the initial runtime estimate is approximately:

12.58 Wh ÷ 5 W = 2.5 hours

Before approval, the design team must still verify:

• Actual current demand

• Peak current response

• Charger compatibility

• BMS settings

• Thermal behavior

• Mechanical fit

• Connector reliability

• Runtime under real device conditions

This example illustrates the design logic. It should not replace an application-specific battery engineering review.

Common Lithium-Ion Battery Pack Design Mistakes

Selecting Capacity Before Checking Voltage

A battery with enough mAh may still be incompatible with the device if its voltage range is wrong.

Using Average Current Only

Peak demand can cause voltage drop, BMS cutoff or connector overheating even when average consumption appears acceptable.

Treating Nominal Voltage as Constant

The battery voltage changes during charging and discharging. The device must tolerate the complete operating range.

Choosing a BMS Too Late

BMS dimensions, wiring, communication and testing can affect the entire pack design.

Ignoring Mechanical Constraints

A battery that works electrically may not fit the enclosure or may experience harmful compression.

Assuming Runtime Equals Rated Capacity

Rated capacity does not equal usable energy under every load and temperature condition.

Comparing Packs Only by Capacity

Two packs with the same capacity may differ in cell quality, discharge capability, BMS functions, dimensions and testing.

Lithium-Ion Battery Pack Design Checklist

Before approving a design, confirm:

• Device voltage range is defined

• Nominal, minimum and maximum battery voltage are identified

• Capacity and usable energy are defined

• Continuous and peak current are known

• Series and parallel configuration is selected

• Cell model and performance grade are identified

• Cell matching requirements are defined

• PCM or BMS functions are specified

• Charging conditions are confirmed

• Battery dimensions are controlled

• Connector and cable requirements are documented

• Operating temperature is defined

• Runtime target is stated

• Host-device integration testing is planned

• Prototype approval criteria are defined

• Production and change-control requirements are understood

For broader OEM purchasing criteria, see How to Choose a Rechargeable Lithium-Ion Battery Pack for an OEM Device.

FAQ

What is the most important factor in lithium-ion battery pack design?

The most important factor is matching the battery to the host device’s complete electrical and mechanical requirements. Voltage, capacity, current, BMS, dimensions and charging must be evaluated together.

How do I calculate the energy of a lithium-ion battery pack?

Multiply nominal voltage by capacity in amp-hours. For example, a 7.4 V, 2 Ah battery pack has approximately 14.8 Wh of nominal energy.

Does connecting cells in series increase capacity?

No. Series connections primarily increase voltage. Parallel connections increase capacity and current capability. A series-parallel configuration can increase both voltage and capacity.

Does every lithium battery pack need a BMS?

The required protection system depends on the pack configuration and application. A simple single-cell pack may use a PCM, while a multi-cell pack may require a BMS with balancing and monitoring functions.

How long will a lithium battery pack run a device?

Runtime depends on battery energy, device power, conversion efficiency, cutoff settings, temperature and load variation. A basic estimate is battery watt-hours divided by device power, adjusted for usable efficiency.

What is the difference between a custom battery pack and a standard battery?

A custom battery pack is designed around the OEM product’s voltage, capacity, current, dimensions, connector, BMS and application requirements. A standard pack usually uses a predefined configuration.

Should voltage or capacity be selected first?

The device’s voltage range should normally be established first. Capacity can then be selected according to runtime, available space, current demand and weight limitations.

Conclusion

Lithium-ion battery pack design is a system-level process. A reliable design must connect:

• Voltage

• Capacity

• Energy

• Current demand

• Series and parallel configuration

• Cell selection

• PCM or BMS

• Charging

• Runtime

• Mechanical integration

• Connector requirements

• Testing and production needs

A 3.7 V cell can be configured into different battery packs depending on the required voltage and capacity. However, the electrical configuration is only one part of the design. The battery must also operate correctly with the charger, host device, protection system and mechanical enclosure.

If you are developing a custom lithium-ion battery pack, share the application, target voltage, capacity, current demand, dimensions, connector requirements, runtime target and expected production volume with the ZERNE technical team. ZERNE supports custom lithium battery pack development for OEM and ODM projects, from initial requirement review through sample development and production planning.