Inside Custom Battery Pack Assembly — Cell Selection, BMS and Build Process

Not every device has a ready-made replacement battery available. The manufacturer went bust, the model was discontinued, the original battery costs more than the device itself, or you need a capacity that doesn't exist in the standard range. In such cases, a bespoke battery pack is the only solution.

Custom battery packs need correct cells, a suitable BMS, safe physical layout, and proper joining. We work mainly with lithium-ion 18650 and 21700 cells from Sony/Murata, Samsung, Molicel, LG, and Panasonic.

When You Need a Custom Build

Device Has Been Discontinued

The manufacturer closed the product line, went bankrupt, or simply stopped producing spare parts. The battery is no longer available anywhere — neither original nor compatible. This is common with:

• Professional power tools from older series
• Medical equipment (portable monitors, nebulisers, infusion pumps)
• Early robot vacuums (original iRobot Roomba 500/600/700 series, Samsung NaviBot)
• Industrial equipment (portable barcode scanners, test instruments)
• Emergency lighting and specialist equipment

Original Battery Is Disproportionately Expensive

Some manufacturers price replacement batteries at half the cost of a new device. Yet inside are the same standard 18650 or 21700 cells that cost €3–6 each.

You Want Increased Capacity

The standard battery is 3.0 or 5.0 Ah. By fitting higher-capacity cells, you can gain 20–40% additional runtime without replacing the tool.

How We Select Cells

Three Key Parameters

1. Capacity (mAh). How much energy the cell stores.
2. Maximum continuous discharge current (A). How much current the cell can deliver continuously.
3. Internal resistance (mΩ). Lower means less heat and fewer losses.

Our Cell Selection

• Sony/Murata VTC5A (18650, 2,600 mAh, 25 A) — for high-drain power tool applications
• Sony/Murata VTC6 (18650, 3,000 mAh, 15 A) — versatile all-rounder
• Samsung 30Q (18650, 3,000 mAh, 15 A) — excellent capacity-to-current ratio
• Molicel P42A (21700, 4,200 mAh, 45 A) — one of the finest cells in the world for demanding applications
• Samsung 40T (21700, 4,000 mAh, 35 A) — powerful and capacious
• Samsung 50E (21700, 5,000 mAh, 10 A) — maximum capacity for low-drain devices

We never use cells of unknown origin, "reclaimed" cells from disassembled packs, or unverified cells from AliExpress.

Assembly Process

Spot Welding

We join cells using spot (resistance) welding — the only correct method. A brief current pulse (thousandths of a second) welds a nickel strip to the cell terminal without heating it.

Why not soldering? A soldering iron requires sustained contact (5–10 seconds) at 300–400°C. This overheats the cell, damages the internal separator, and reduces lifespan by 20–30%.

BMS (Battery Management System)

Every pack is fitted with a BMS matched to its configuration: correct series cell count, maximum discharge current, balancing current, and temperature protection.

Testing the Finished Pack

After assembly, each pack undergoes: individual cell voltage check, full charge, discharge under load, protection verification, and balance check.

Devices We Build Batteries For

• Power tools: Makita, Bosch, DeWalt, Milwaukee, Hilti, Metabo
• Robot vacuums: iRobot Roomba, Roborock, Xiaomi, Dreame, Ecovacs
• Medical equipment
• Industrial equipment
• Emergency lighting
• Hobby projects and prototypes

Ordering Process

1. Bring the device or old battery. We'll inspect it, measure parameters, and determine the configuration.
2. We discuss requirements. What capacity? What maximum current? Any size constraints?
3. We select cells and BMS. Quote price and timeframe.
4. Assembly and testing. Spot welding, BMS installation, full test cycle.
5. Collection. You receive the finished pack with usage and storage recommendations.

Old battery diagnostics — free of charge. Typical turnaround — 2–5 working days.