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)

Parts we stock for this build

Molicel P42A 21700 4200mAh 45A li-ion cell
Molicel P42A – 21700 4200mAh 45A
Recommended cell · order 2 for this build · 4200mAh pack capacity, well inside the 20A BMS
2S 8.4V 20A BMS protection board with balance for 18650 and 21700 packs
2S 8.4V 20A BMS Board with Balance
SKU BM101 · $4.99 — the exact BMS used in this build
XT30 female connector pigtail with pre-wired 18 AWG silicone leads
XT30 Female Connector Pigtail – Pre-Wired 18 AWG Leads
5-pack $8.99 · 10-pack $16.99 · 20-pack $33.99
XT30 male and female bare bullet connector pair, no wires
XT30 Connector Male + Female Pair – Bare, No Wires
5 pairs $6.49 · 10 pairs $11.99 · 20 pairs $21.99
21700 adhesive fish paper insulator rings for battery pack building
21700 Insulator Rings – Adhesive Fish Paper
200 pcs $14.99 · 400 pcs $26.99 · 600 pcs $36.99
Heat resistant polyimide Kapton tape rolls for battery pack building
0.15mm pure nickel strip roll 7mm width for battery spot welding
0.15 mm Pure Nickel Strip Roll – 7 mm Width
50 ft $19.99 — well suited to a low-current 2S1P pack

Tools Needed

Pack Overview

  • Configuration: 2S1P (two cells in series)
  • Nominal Voltage: 7.4V (8.4V fully charged)
  • Capacity: equal to one cell’s capacity (series does not add capacity)
  • 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.
8.4V 2A charger for 2S lithium-ion battery packs
8.4V 2A Charger – for 2S Lithium Packs
The correct charger for this pack — do not charge a 2S pack on a 1S or 3S supply

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.

Related Reading

Tools you may also need: spot welder, multimeter, and pack heat-shrink.

Back to blog

Leave a comment

Please note, comments need to be approved before they are published.