18650 lithium battery pack customization tutorial

If you’ve ever used devices like electric scooters, mobile power supplies, or emergency lighting, you’ll know the value of a well-built 18650 lithium-ion battery pack. I’ve found that learning to assemble one yourself can be rewarding, especially for high-current or rechargeable devices.

Screening the Cells

The first step is to screen the 18650 cells for voltage, internal resistance, and capacity. This process, known as capacity separation, ensures the stability and performance of the final pack. After preparing the materials, I assemble the cells, then check the voltage and internal resistance. Usually, I allow a voltage difference of 5 millivolts and an internal resistance difference of 3 milliohms when grouping cells.

Only by carefully selecting cells like this can the pack perform reliably. A core separation device is used to achieve this precision.

Assembling Cells in Series and Parallel

When assembling the pack, I always install a bracket to hold the cells. This improves heat dissipation and keeps the structure independent, reducing the risks from vibration. A well-supported battery is both safer and more durable.

Welding the Battery Pack

For spot welding, nickel strips are used. There are two types: pure nickel and nickel-plated steel.

• Pure nickel has low internal resistance, high current capacity, and rust resistance, but it’s more expensive.
• Nickel-plated steel is cheaper but has higher resistance, lower current capacity, and rusts more easily.

Choosing Nickel Strip Thickness

The thickness of the nickel strip matters:

• 0.15 mm – typical for standard products
• 0.1 mm – for low current
• 0.2 mm – for high current

Avoid strips that are too thin or too thick. When spot welding, make sure the welding power is correct: too low can fail to bond the cell; too high can cause welding bursts. After welding, each joint should pass a 7 kg tensile test.

Protecting the Battery Pack

For 48V ternary lithium packs, the protective cover must be welded according to instructions, starting from B-, B0, B1, up to B13.

BMS Protection Board

The Battery Management System (BMS) is crucial. After welding its leads, I insulate them with heat shrink tubing to prevent shorts. The BMS monitors:

• State of charge
• Overvoltage and undervoltage protection
• Balanced charging
• Charging management
• Temperature monitoring

Insulating and Packaging

Next, I insulate and shape the battery pack, then secure wires and pack the unit. Applying PVC film protects against water and dust, ensuring durability.

Installing in the Chassis

When placing the pack in the chassis, I connect the exposed wires to the shell connector for charging, discharging, fusing, and switching.

• Charger port wires: thinner, lower current
• Discharge port wires: thicker, high current

Wire sizes and welding are based on the structural diagram of the battery pack.

Final Battery Test

Before shipping or use, I run a final test, including:

• Charge/discharge cycle
• Capacity
• Internal resistance
• Open circuit voltage
• Overcurrent, overcharge, overdischarge
• Short circuit test

Equipment used includes charge/discharge aging boxes, complete test machines, and chargers.

When assembling an 18650 lithium battery pack, following these steps carefully is essential to ensure safety, reliability, and high performance.

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