Do You Need Cell Spacers for 18650/21700 Packs? A Builder’s Guide
By RenewSpark • Updated: September 1, 2025
Cell spacers (plastic grids) keep cylindrical cells aligned, insulated, and rigid. This guide shows exactly when spacers are recommended—and when careful insulation (fish paper + heat shrink) can be enough for small DIY packs.
What are cell spacers?
Cell spacers are interlocking plastic grids sized for 18650 or 21700 cells. They set consistent gaps between cans, prevent metal-to-metal contact, and make a pack more rigid during spot welding and long-term use. You can grab 21700 cell holders or Molicel P42A cells for larger packs.
Why builders use spacers
- Safety & insulation: Prevents can-to-can rubbing and shorts from vibration or impacts.
- Structural strength: Keeps rows square so nickel welds don’t get stressed if the pack flexes.
- Thermal spacing: Leaves a small air gap that helps reduce local hot spots.
- Serviceability: Easier to handle, rework, and add harnessing or BMS leads neatly.
When spacers are recommended
- Medium–large packs (e.g., e-bike/scooter batteries, multi-row tool packs, 10+ cells total).
- High current builds (heavy tool use, frequent high-load bursts, or high ambient temps).
- Packs that will be handled a lot (swappable tool batteries, rental/field gear).
- Installations with vibration (power tools, off-road applications, equipment cases).
When spacers are optional
- Very small packs (e.g., 2S1P, 3S1P, 2S2P) used gently.
- Space-constrained prototypes where every millimeter counts.
- Low current / light duty applications with minimal handling and vibration.
If you skip spacers, be diligent with insulation: fish paper rings on positive ends, Kapton tape on series bridges, and heat shrink tubing with firm wrap. Add foam padding to stop cells from rubbing inside the enclosure.
Alternatives & supporting insulation
- Fish paper rings & sheets to isolate positives and cross-overs.
- Heat shrink & insulation films to bind and protect the pack.
- Thin foam pads (EVA/PE) to prevent rattle and abrasion inside the case.
- Cell spacer grids when you can spare the height and want pro rigidity.
Nickel strip note (thickness & layout)
Spacer choice is separate from nickel choice—but they interact. For high current builds, 0.20 mm nickel strips are stiffer and benefit from a rigid cell matrix (spacers) during welding. For lighter loads, many builders use 0.15 mm with shorter runs or doubled sections. Always insulate with fish paper sheets if you omit spacers.
Quick decision checklist
| Scenario | Use Spacers? | Notes |
|---|---|---|
| Power tool packs (multi-row, frequent swaps) | Yes | Rigidity + abrasion resistance under vibration/impacts. |
| E-bike / scooter packs | Yes | Thermal spacing + structure for longer series strings. |
| Small DIY packs (2S1P, 3S1P, 2S2P) | Optional | OK to skip with great fish paper + shrink + foam. |
| Prototypes with tight headroom | Optional | Consider 1-layer spacers or partial use in hot zones. |
FAQ
Do spacers replace fish paper?
No. Spacers position cells; fish paper provides dielectric insulation. Use both for pro results.
Can I use spacers only on the ends?
For small packs, end-only spacers with tight wrap can work. For larger or high-load packs, use spacers across the full matrix.
Do spacers affect cooling?
They add small air gaps that slightly improve airflow versus cells touching. Still design for heat: good nickel layout and no trapped hot spots.
Should I glue spacers?
Optional. Click-fit is common; a few dabs of high-temp adhesive can help during assembly. Don’t let glue creep onto cell cans.
18650 vs 21700—same spacer rules?
Yes. Larger 21700 packs benefit even more from rigidity because cells are heavier and loads often higher.
