How to Build a 2S1P 7.4V 21700 Li-ion Battery Pack (Beginner Guide with Parts & Wiring)

How to Build a 2S1P 7.4V 21700 Li-ion Battery Pack (Beginner Guide with Parts & Wiring)

⚠️ Safety First: Lithium-ion packs can short, vent, or catch fire if built incorrectly. Wear eye protection and gloves. Use a spot welder (not a soldering iron) on cell terminals. If a cell or BMS is damaged, do not use it.

This tutorial walks you through building a compact 2S1P 7.4V 21700 Li-ion battery pack with a 2S 20A (8.4V) BMS and an XT30 connector—great for Arduino projects, LED strips, small robotics, and portable DIY builds.

Bill of Materials (Parts)

Tools Needed

Pack Overview

  • Configuration: 2S1P (two cells in series)
  • Nominal Voltage: 7.4V (8.4V fully charged)
  • Capacity: ≈2700 mAh (depends on cell model)
  • BMS Leads (typical 2S): B−, B1, B+ to cells; P−/P+ to load/charger
  • Connector: XT30 (compact; suitable for low–mid current)

Wiring Notes (Read Before You Start)

Common 2S BMS pad functions:

  • B−: Pack negative (Cell 1 negative)
  • B1: Junction between Cell 1 (+) and Cell 2 (−)
  • B+: Pack positive (Cell 2 positive)
  • P− / P+: Output/charge to device/charger (P+ may be same as B+ on small boards)

Always follow the silk labels and wiring diagram printed on your exact BMS.

Step-by-Step Build

1) Inspect & Pre-Test Cells

  1. Measure open-circuit voltage of each cell with a multimeter (avoid any <3.0V or visibly damaged cells).
  2. Add fish-paper rings to each positive terminal. Wrap sharp edges or nickel ends with Kapton.

2) Arrange Cells in Series

  1. Place the two 21700 cells side-by-side with opposite orientation (one “+” up, the other “+” down).
  2. Use holders/spacers or Kapton to keep the cells aligned and insulated.

3) Spot-Weld Nickel

  1. Cut nickel for each end. On one end, bridge Cell 1 (+) to Cell 2 (−) to create the series link.
  2. Leave the remaining ends for Pack “−” (Cell 1 −) and Pack “+” (Cell 2 +).
  3. Use short, consistent welds; avoid overheating. Verify solid attachment with a gentle tug.

4) Wire the BMS (2S 20A)

  1. B− → Nickel at Cell 1 negative (Pack “−”).
  2. B1 → Nickel at the series bridge between Cell 1 (+) and Cell 2 (−).
  3. B+ → Nickel at Cell 2 positive (Pack “+”).
  4. P− / P+ → Output leads to the device/charger. If P+ is shared with B+, take positive from B+/P+ and negative from P−.

Route wires flat and secure with Kapton so they won’t pierce the outer heat-shrink.

5) Install the XT30 Connector

  1. Solder positive from P+ (or B+ if shared) to XT30 female +.
  2. Solder negative from P− to XT30 female −.
  3. Insulate joints with small heat-shrink and add strain-relief using Kapton.

6) Insulate & Wrap

  1. Place fish-paper pads on the ends and under the BMS as needed.
  2. Slide appropriately sized battery heat-shrink over the assembly and heat evenly.
  3. Leave the XT30 pigtail accessible.

7) Final Checks & First Charge

  1. Measure pack voltage at the XT30: ~7.2–7.6V depending on cell state.
  2. Verify polarity at the connector before connecting any load.
  3. Charge gently with a 2S Li-ion charger (8.4V); monitor temperature and confirm the BMS balances near full.

Troubleshooting

  • No output: Some BMS boards “wake up” after brief charging—connect an 8.4V charger for a few seconds.
  • Immediate cutoff: Load current may exceed BMS rating, or there’s a wiring error—check B−/B1/B+.
  • Cells unbalanced: Let the pack rest at near-full charge so the BMS can balance; re-verify the B1 lead.

Quick Links to Parts

Want more runtime and current? See the companion build: 2S2P 7.4V 21700 Battery Pack Guide.

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