Development Process of a Custom Lithium-Ion Battery Pack

By Greg Weber, BSEE, Vice President OEM Sales, Apex Mobile Power

Developing Lithium-Ion (Li-ion) battery packs is a multi-stage process that combines electrochemistry, mechanical design, electronics, safety engineering, and rigorous testing. In my role as a sales engineer, I realize that understanding the intricacies that go into developing a custom battery pack can benefit our customer and improve efficiency. Here is a detailed breakdown of the end-to-end development process, from concept to production:

 1. Requirement Definition

Before any design work begins, clear performance and use-case requirements are set. We like to do this in our first meetings. It’s important to get as much detail as possible both technically and commercially about the application to ensure synergy. The following questions will help us begin understanding your needs:

a. What is the application? (medical device, autonomous mobile robot, land mobile radio, energy storage system, power tool, drone)

b. What are the electrical specs? Voltage, capacity, power, charge/discharge rate, energy density should all be determined.

c. What are the mechanical constraints? Size, weight, shape, cooling method if needed.

d. What are the environmental factors to be considered? Temperature range, humidity, vibration, safety standards are all key determinations in battery pack design.

e. What regulatory and safety compliance tests needed for your market and should be conducted? UN38.3, UL 2054, IEC 62133, CE, etc.

f. What is the total application volume? It’s important to determine whether full automation is needed and the number of cavities for the plastic enclosure tooling.

g. Where will the battery packs ship to? Tariffs may be a consideration or specific regulatory testing needed.

h. Do you have a specific target pricing or target market? Helps in determining cell manufacturer to meet targets. 

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2. Cell Selection and Evaluation

After the initial review with the customer a discussion with engineering takes place to start the process of choosing the right Li-ion cell. This is critical for performance, safety, and value.

We start with a Chemistry selection of the Lithium-Ion technology. We may choose LFP (LiFePO₄) for a safer, longer life, but lower energy density battery pack.  NMC (LiNiMnCoO₂) may be chosen for a more balanced energy/power battery pack or LCO (LiCoO₂) for portable electronics and polymer designs.

Form factors are also a major consideration for size and weight as well as capacity.

Cylindrical cell 18650 and 21700 are the most common. But there are others as well, smaller like the 14500 or larger like the 26700 and46700. Then there are the rectangular prismatic, or Li-Polymer pouch cells.

We also need to determine the proper cell characteristics for capacity, impedance, thermal behavior, cycle life, etc.

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3. Electrical Design

Here is where we need to do some exciting math to get you the voltage needed and capacity for your device’s runtime and power demands. Most applications may need more than one cell to get the voltage and capacity for the application. Lithium-Ion cells are generally around 3.7 volts and LiFe is at 3.2V so to get a voltage of 12V or higher you would need 4 cells.

Therefore, battery packs consist of series and parallel cell configurations.

• Series     (S) increases voltage. For Li-Ion 1S equals 3.7V, 2S equals 7.4V, 3S equals 11.1V etc.
• Parallel     (P) increases capacity/current. For a pack with a cell rated at 2.0Ah if you were to use 3P is would mean a 6.0Ah battery pack not accounting for electrical losses.

Example of pack nomenclature: a “10S3P” pack = 10 cells in series, 3 in parallel.

Considerations that are keys to success are cell balancing (sometimes needed, but not always), current path design (busbars, connectors), fusing or protection components, voltage, current, and thermal monitoring.

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4. Battery Management System (BMS) Design

The BMS is the brains of the Lithium-Ion battery pack. It can be just a simple protection circuit or a full-on smart circuit with communication of 100s of data points and cell balancing.

The BMS ensures safety, reliability, and longevity of the Lithium-Ion battery pack.

Functions of the BMS include:

- Cell voltage and temperature monitoring

- Cell balancing (active or passive and not commonly needed in smaller packs)

- Key Li-Ion Protection Points: overcharge, over discharge, over current, short circuit, thermal runaway

- State-of-Charge (SOC) and State-of-Health (SOH)estimation

- Communication (SMBus, CANBus, UART, I2C, etc.)

Other critical design areas include hardware design for sensor circuits, microcontroller, isolation, and redundancy. Software design for algorithms supporting estimation, diagnostics, and fault detections is also needed.

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5. Mechanical and Thermal Design of the Battery Pack

The physical structure ensures stability, cooling, and protection of the battery pack. The designers should be made aware of any certifications that are needed.

a. Mechanical architecture:

These include cell holders, compression mechanisms, vibration resistance.

Enclosure design for ingress protection (IP rating). Waterproof and dustproof designs. Also key in Intrinsically Safe battery packs design.

b. Thermal management:

Space between cells, if enough is available, as well as passive cooling components (heat spreaders, aluminum plates). Active cooling (liquid or air cooling for larger packs where space permits. Simulation of thermal behavior under load conditions.

c. Safety design:

Features such as pressure vents, flame arrestors, fuses, insulation, thermal runaway mitigation. Even the orientation of the cell ventis critical.

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