How to Build a 2S2P 7.4V 21700 Li-ion Battery Pack (High Capacity DIY Guide)

How to Build a 2S2P 7.4V 21700 Li-ion Battery Pack (High Capacity DIY Guide)

⚠️ Safety Warning: Lithium-ion batteries are dangerous if mishandled. Risk of fire, venting, or injury. Always wear gloves and safety glasses. Use a spot welder (not a soldering iron) for cell connections. Test every cell before use. If in doubt, do not proceed.

This tutorial shows how to build a 2S2P 7.4V 21700 Li-ion battery pack with a 30A BMS and XT60 connector. This configuration doubles both the runtime and discharge capability compared to a simple 2S1P pack, making it perfect for RC vehicles, drones, robotics, and high-drain portable electronics.

Bill of Materials (Parts)

Tools Needed

Pack Overview

  • Configuration: 2S2P (two series groups, each with two cells in parallel)
  • Nominal Voltage: 7.4V (8.4V full charge)
  • Capacity: ~5400mAh (2 × 2700mAh in parallel)
  • Max Current: Up to 30A continuous (BMS rated)
  • Connector: XT60 (ideal for high-drain applications)

Wiring Notes

For a 2S2P pack:

  • Parallel pairs must be matched (same cell type & capacity).
  • Weld each parallel pair first, then connect pairs in series.
  • BMS pads: B− → pack negative, BM → series junction, B+ → pack positive, P−/P+ → output leads.

Step-by-Step Build

1) Test & Pair Cells

  1. Measure all 4 cells with a multimeter; voltages should match closely.
  2. Group cells into 2 parallel pairs (P1 = Cell A + Cell B, P2 = Cell C + Cell D).
  3. Spot weld nickel to connect each pair in parallel (+ to +, − to −).

2) Create Series Connection

  1. Orient the two parallel groups in series (P1 + connects to P2 −).
  2. Spot weld a nickel bridge between them to complete the 2S2P layout.

3) Wire the BMS (2S 30A)

  1. B− → Connect to the negative side of the first parallel group (Pack −).
  2. BM → Connect to the middle junction between the two series groups (the bridge between the first group’s positive and the second group’s negative).
  3. B+ → Connect to the positive side of the second parallel group (Pack +).
  4. P− / P+ → Run to the XT60 connector (P+ to XT60 positive, P− to XT60 negative).

4) Install XT60 Connector

  1. Solder P+ (pack positive) to XT60 + terminal.
  2. Solder P− to XT60 − terminal.
  3. Insulate joints and strain relieve with Kapton tape.

5) Wrap & Insulate

  1. Apply fish-paper between cell groups and under BMS.
  2. Slide heat shrink wrap over the pack and shrink evenly.

6) Final Tests

  1. Check total pack voltage (≈7.2–7.6V initially).
  2. Confirm polarity at the XT60 connector.
  3. Charge with a 2S Li-ion charger (8.4V). Monitor closely on first charge.

Troubleshooting

  • Uneven voltage: Verify parallel groups were properly welded and cells matched.
  • No output: Some BMS modules require initial charging to activate.
  • Overheating: Check for shorts, loose welds, or current overload.

Quick Links to Parts

Looking for a smaller beginner build? Check out our 2S1P 7.4V 21700 pack guide.

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