Introduction

When people think about designing a custom battery pack, the first question is usually:

"Which battery cell should we use?"

While cell selection is an important step, it is only one piece of a much larger engineering process.

A reliable lithium battery pack requires careful consideration of electrical performance, mechanical integration, safety, thermal management, communication, certifications, and manufacturing. Even a high-quality cell can result in a poor battery if these factors are overlooked.

At Voltaplex, we've worked with customers across industries including UAVs, robotics, medical devices, industrial equipment, marine systems, and energy storage. One thing is consistent across every project:

A successful battery pack is the result of system engineering—not simply selecting the highest-capacity cell.

In this article, we'll walk through the key steps involved in designing a custom battery pack.

Step 1: Selecting the Right Cells

The battery cell is the heart of every battery pack.

However, choosing the highest capacity cell isn't always the right answer.

Engineers typically evaluate:

• Chemistry (Li-ion, LiFePO₄, LiPo, Sodium-ion, etc.)
• Nominal voltage
• Capacity (Ah)
• Continuous discharge current
• Peak discharge capability
• Maximum charging current
• Cycle life
• Operating temperature
• Availability and long-term supply
• Cost

For example:

• A UAV requires cells capable of very high discharge currents.
• A medical device prioritizes reliability and long service life.
• An energy storage system benefits from long cycle life rather than maximum energy density.

Choosing the wrong cell often leads to unnecessary redesigns later in the project.

Looking Beyond Initial Capacity

When comparing battery cells, many engineers focus on the initial capacity (Ah) or energy density (Wh/kg). However, an equally important metric is the total energy the battery can deliver over its lifetime.

Voltaplex LIFE cells are designed to maximize this lifetime energy output. As shown in the figure below, while many conventional lithium-ion cells experience rapid capacity degradation, Voltaplex LIFE cells maintain a much higher state of charge over hundreds of cycles. As a result, they can deliver up to three times more total energy over their service life compared to conventional alternatives. This translates into fewer battery replacements, lower maintenance costs, improved equipment availability, and a significantly lower total cost of ownership.

Figure 1. Capacity retention comparison between Voltaplex LIFE cells and conventional lithium-ion cells under 1.5C charge / C/2 discharge conditions.

Step 2: Determining the Correct Battery Configuration (Series/Parallel)

After selecting the cells, the next step is determining the battery configuration.

The two basic building blocks are:

• Series (S): increases voltage
• Parallel (P): increases capacity and current capability

For example:

• 7S12P
  • 7 cells in series
  • 12 cells in parallel

Changing the configuration affects:

• Battery voltage
• Energy (Wh)
• Maximum current
• Physical dimensions
• Weight
• Cooling requirements
• Cost

The ideal configuration depends on the application rather than simply maximizing capacity.

Figure 2. 7S12P Custom Battery Pack made by Voltaplex

Step 3: Choosing or Developing the BMS

The Battery Management System (BMS) is often considered the brain of the battery.

Its responsibilities include:

• Cell voltage monitoring
• Temperature monitoring
• Cell balancing
• Over-voltage protection
• Under-voltage protection
• Over-current protection
• Short-circuit protection
• CAN, SMBus, I²C, UART or RS485 communication
• State of Charge (SOC) estimation
• State of Health (SOH) estimation

One common question customers ask is:

"Can we use an off-the-shelf BMS?"

The answer depends on the project.

If the application requires:

• custom communication protocols
• special protection thresholds
• aviation requirements
• medical compliance
• faster charging
• unique mechanical constraints

then custom BMS development may be necessary.

Although custom development introduces Non-Recurring Engineering (NRE) costs, it often provides the best long-term solution.

Step 4: Mechanical Design

The battery must fit perfectly within the customer's product.

Mechanical design includes:

• Overall dimensions
• Mounting points
• Cell holders
• Structural support
• Enclosure design
• Waterproofing (IP rating)
• Shock resistance
• Serviceability

Many customers initially focus only on electrical performance.

In reality, mechanical constraints often become the biggest design challenge.

Step 5: Connector and Wiring Selection

Selecting the correct connector is just as important as selecting the cells.

Factors include:

• Maximum continuous current
• Peak current
• Charging current
• Connector size
• Locking mechanism
• Environmental conditions
• Ease of assembly

Common connector options include:

• XT30
• XT60
• XT90
• Anderson connectors
• JST connectors
• Molex connectors
• Custom aviation connectors

Similarly, wire gauge (AWG) must be selected based on the required current.

Undersized wiring can cause:

• Voltage drop
• Excessive heat
• Reduced efficiency
• Potential safety risks

Step 6: Thermal Management and Heating Pads

Temperature has a major impact on battery performance.

A good battery design must consider:

• Heat generated during discharge
• Heat generated during charging
• Ambient operating temperature
• Cell spacing
• Ventilation
• Cooling methods

Some applications operating in cold environments also require heating pads.

The heating system must be properly integrated with the BMS or the customer's control system to ensure the cells remain within their optimal operating temperature range.

Ignoring thermal management can significantly reduce battery life and compromise safety.

Step 7: Sample Validation

This is one of the most overlooked steps.

Many customers assume:

"If the manufacturer has tested the battery, we're ready for production."

Unfortunately, that's rarely the case.

A battery manufacturer can verify that the battery:

• meets the electrical specifications
• functions correctly
• passes internal quality tests

However, only the customer can verify how the battery performs inside the final product.

For example:

• communication with the host controller
• charger compatibility
• thermal behavior
• cable routing
• software integration
• enclosure fit
• vibration performance

Factory testing confirms that the battery works.

System validation confirms that it works in your application.

Skipping this stage often results in unexpected issues during production.

Step 8: Certifications

Certification requirements depend on the application and destination market.

Some of the most common certifications include:

UN38.3

UN38.3 is required for the transportation of lithium batteries.

It verifies that the battery can safely withstand:

• altitude simulation
• thermal cycling
• vibration
• shock
• external short circuit
• impact
• overcharge
• forced discharge

Without UN38.3, shipping lithium batteries can become difficult or impossible through many transportation channels.

IEC 62133

IEC 62133 applies primarily to portable rechargeable batteries.

It evaluates:

• electrical safety
• mechanical safety
• environmental performance

This certification is commonly requested for consumer electronics and medical equipment.

IEC 62619

IEC 62619 is designed for industrial lithium batteries.

Typical applications include:

• industrial equipment
• robotics
• AGVs
• energy storage systems
• telecommunications

It focuses on safety under demanding operating conditions.

CE

CE marking demonstrates that the product complies with applicable European regulations.

Depending on the battery system, this may include requirements related to:

• EMC
• RoHS
• Battery Regulation
• Product safety

Common Misconceptions

"The highest capacity cell is always the best."

Not necessarily.

The best cell depends on power, cycle life, charging speed, cost, safety, and long-term availability.

"A larger BMS is always better."

Oversizing the BMS increases cost and may complicate the design without adding value.

The BMS should be selected to match the application's actual requirements.

"Fast charging is always better."

Higher charging currents generate more heat and can reduce battery lifespan.

A balanced charging strategy often delivers the best long-term performance.

"Passing factory tests means the battery is production-ready."

Factory testing verifies battery performance.

Only system-level validation confirms compatibility with the final product.

"Certification can be done at the end."

Certification requirements should be considered from the beginning of the design process.

Late design changes can require additional testing, increasing both cost and development time.

Conclusion

Designing a custom battery pack is far more than selecting a lithium cell.

A successful battery requires the right combination of:

• Cell selection
• Battery configuration
• BMS development
• Mechanical design
• Connectors and wiring
• Thermal management
• System validation
• Battery certification

Each decision influences safety, performance, cost, and long-term reliability.

At Voltaplex, we work closely with customers from the concept stage through production, helping transform application requirements into reliable, manufacturable battery solutions tailored to each project.

Whether you're developing a UAV, medical device, industrial machine, or energy storage system, involving your lithium battery manufacturer early in the design process can save significant time, reduce development costs, and help ensure a successful product launch.