Getting Started: Battery Building
Getting Started: Build Your Battery Pack
Choose the right S/P configuration, BMS, nickel, and connectors for power tools, e-bikes/scooters, and DIY projects.
1) Understand S (Series) and P (Parallel)
• Series (S) sets pack voltage. More S → higher voltage.
• Parallel (P) sets capacity (Ah/Wh) and current capability.
| Common S | Nominal Voltage* | Typical Use |
|---|---|---|
| 2S | ~7.4V | RC, robotics, LED, portable electronics |
| 3S | ~10.8–11.1V | Small portable packs, RC, LED |
| 4S | ~14.4–14.8V | Light tools, robotics |
| 5S | ~18.0–18.5V | Many 18V power tools |
| 6S | ~21.6–22.2V | Higher-power tools, some scooters |
| 10S | ~36V | Common e-bikes / scooters |
| 13S | ~48V | E-bikes, higher-speed scooters |
*Nominal voltage depends on cell chemistry; check your device requirements.
Most 18V power tool packs use 5S2P — ten cells. Our guide on why every 18V brand uses that layout explains the reasoning.
2) Pick Your Cells
- Capacity (Ah) vs Current (A): Power tools favor high-drain cells; e-bikes balance capacity & current.
- Look at continuous discharge rating (A) and internal resistance (IR).
- 18650 and 21700 are not interchangeable — build in the format your pack was designed around. See 21700 vs 18650.
- Shop Cells — BAK, Molicel and Samsung.
3) Size Your Parallel Groups (P)
Estimate your required pack current and capacity:
Pack current capability ≈ cell continuous A × P
Pack capacity (Ah) ≈ cell Ah × P
Pack energy (Wh) ≈ pack nominal V × Ah
Example: If your tool can draw ~40A and your chosen cell is rated 20A continuous, pick at least 2P (20A×2 = 40A). Add margin for heat and longevity (e.g., 3P).
4) Choose a BMS
- Match BMS to your S count and pick a continuous current rating above your expected draw.
- Prefer balanced BMS for multi-S packs.
- Work through it with our BMS sizing guide.
- Shop BMS & Protection
5) Nickel, Busbars & Wiring
- Use pure nickel (not plated) sized for your current path. Thicker/wider for higher current — our sizing guide does the maths.
- Keep series paths robust; minimize long, thin traces that bottleneck current.
- Choose connectors rated above expected current (XT30/XT60/XT90, JST-XH for balance). Shop connectors & contacts, and see the connector guide.
- Shop nickel strips — rolls and pre-cut sets.
6) Housings & Fit
For a tool pack, the simplest route is a rebuild kit — it comes with an OEM-style housing, a matched BMS, nickel and hardware, so the fit is already solved. Cells are supplied separately.
7) Tools You’ll Need
- Spot welder (for nickel), soldering iron (for leads/connectors), heat gun (shrink), multimeter and an IR tester for cell matching. We don’t sell these — our build guides name the ones we use.
- Insulation and consumables we do stock: fish-paper rings and sheet, Kapton tape, nickel and copper strip.
- Never solder directly to a cell can — spot weld it.
Safety Essentials
- Wear PPE; prevent shorts with fish-paper rings & Kapton; verify polarity before every weld/solder.
- Use a BMS matched to S count and current; never bypass protection in finished packs.
- Charge with compatible chargers; follow local rules for shipping/transport of lithium.
