Batteries: 18650 & 21700 Packs for RC Builds

Notes on building Li-ion battery packs from 18650 and 21700 cells: sizing the heat-shrink tube, recommended cells, and how series vs. parallel wiring affects voltage and capacity.

Heat-Shrink Tube Sizing

For a 4S1P pack using 21700 cells, the shrink tube should be 90 mm wide (21 mm × 4 cells = 84 mm + a small margin).

Tools You Need

To solder 18650 or 21700 cells you ideally need:

  • A spot welder (around €50 on Amazon)
  • Balance leads
  • Positive and negative main leads

Recommended Cell

For high-drain RC use, a solid choice is the Molicel INR21700-P45B (4500 mAh, 45 A continuous):

Molicel INR21700-P45B at NKON (EU)

Series vs. Parallel — Quick Refresher

Series (e.g. 2S = 1S + 1S)

Two cells of 3.7 V / 2000 mAh connected in series give:

  • Voltage adds up: 7.4 V (storage) / 8.4 V (fully charged)
  • Capacity stays the same: 2000 mAh

Note: 3.7 V is the nominal/storage voltage of a Li-ion or LiPo cell; fully charged it sits at 4.2 V.

Parallel (e.g. 1S2P)

Two cells of 3.7 V / 2000 mAh connected in parallel give:

  • Voltage stays the same: 3.7 V
  • Capacity adds up: 4000 mAh

References

Reference Images

Battery pack assembly reference
Series wiring diagram
Parallel wiring diagram
Battery pack reference photo

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