18650 / 21700 Cell Holder and Module Design for 3D Printing

The problem

Cell holders keep cylindrical cells spaced, aligned and protected from vibration, but printed holders often fit too tightly or too loosely because printer tolerance was never measured. They are also printed in materials that soften near cell temperatures. A properly designed and tested holder makes small packs safer and repeatable.

Objectives

• Build a parametric holder that switches between 18650 and 21700 cells with configurations.
• Design a tolerance test coupon and set pocket clearance from measurement.
• Add busbar windows, interlocking edges and bolt bosses for module assembly.
• Choose a print material with a suitable temperature and flammability rating.
• Assemble a module and produce print-ready files and drawings.

What you need

Key concepts

Printer tolerance

The difference between modelled and printed size. Holes usually print smaller than modelled, so pocket clearance must be found by test.

Cell spacing

The gap between cells, set by holder walls. It affects module size, heat dissipation and the risk of heat passing from one cell to another.

Heat deflection temperature

The temperature at which a polymer deforms under a standard load. Holder material should have a value well above the cell's maximum operating temperature.

UL 94 rating

A flammability classification for plastics, such as V-0. Flame-retardant grades are preferred for battery parts.

Step-by-step guide

1. 1

   Collect cell and module requirements

   Record 18650 and 21700 nominal diameter, height and mass from the chosen datasheets, and measure five real cells of each with a caliper to see the variation. Define an example module, such as 4S3P for a small e-bike or tool, and a target spacing between cells of about 1–2 mm. Note maximum cell operating temperature from the datasheet. Note any sleeve or wrap thickness that adds to diameter.

   Checkpoint: A table of datasheet and measured cell sizes, the module configuration and spacing target.

2. 2

   Design a tolerance coupon

   Model a small plate with a row of pockets from cell diameter plus 0.0 mm to plus 0.6 mm in 0.1 mm steps, each labelled with embossed text. Add a few wall thicknesses between pockets to see which print cleanly. Export as STL or 3MF and print in the intended material and settings. Measure pocket diameters and test-fit cells. Print the coupon twice to see repeatability.

   Checkpoint: A measured table showing the pocket clearance that gives a firm but easy fit for each cell size.

3. 3

   Build the parametric holder

   Model a holder with Global Variables or a Design Table for cell diameter, clearance, spacing, rows and columns. Create one pocket with a sketch circle and a cut, then use a Linear Pattern driven by rows and columns. Add a lip at the pocket base so cells sit at a fixed depth. Make configurations for 18650 and 21700 using the same table.

   Checkpoint: Switching configuration resizes the pockets and the whole holder without rebuild errors.

4. 4

   Add busbar windows and assembly features

   Cut windows over each cell end that leave room for the busbar weld or spring contact, keeping enough rim to hold the cell. Add dovetail or peg-and-hole interlocking edges so holders join side by side. Add bosses for through-bolts that clamp top and bottom holders together. Check minimum wall thickness against the coupon results. Add a small chamfer at each pocket entry so cells slide in without catching on a printed edge.

   Checkpoint: No wall is thinner than the thinnest wall that printed cleanly on the coupon.

5. 5

   Choose the material

   Compare candidate print materials using their datasheets: heat deflection temperature, UL 94 rating and impact strength. PLA softens at low temperatures and is not suitable near cells; PETG, ABS, ASA or PC-based and flame-retardant grades are better candidates, depending on your printer. Record the choice and the reason, and assign a matching material in SOLIDWORKS for mass. Note also whether the material creeps under clamp load over time.

   Checkpoint: A material table with temperature and flammability data and a justified choice above the cell operating temperature.

6. 6

   Assemble the module

   Create an assembly with top and bottom holders, cells as parts with datasheet mass, and busbar strips. Use a pattern of cells driven by the holder pocket pattern so cell count follows the configuration. Alternate cell polarity for series connection. Run Interference Detection and fix any clashes between busbars and holder features. Assign datasheet cell mass and holder material so later mass checks are meaningful.

   Checkpoint: Interference Detection passes for both 18650 and 21700 configurations.

7. 7

   Check spacing and mass

   Measure the minimum gap between cells in the assembly and compare it with the target. Use Mass Properties for module mass and compare holder mass with cell mass. Check that clamping bolt holes are clear of busbars. If the module is too large, reduce wall thickness only as far as the coupon results allow. Report holder mass as a share of module mass, since it affects pack energy density.

   Checkpoint: Cell spacing matches the target and module mass is reported for both configurations.

8. 8

   Print and test

   Print one top and one bottom holder for each cell size, insert cells and clamp. Check fit, alignment of busbar windows and whether the holder flexes when the module is lifted by one corner. Measure printed pocket sizes and compare with the model and the coupon. Adjust clearance in the Design Table and reprint if needed. Record print orientation and settings so results are repeatable.

   Checkpoint: Cells fit firmly without force, and measured pocket sizes match the coupon results within about 0.1 mm.

9. 9

   Drawings and report

   Produce a holder drawing with key dimensions, a note of the clearance found by test, print orientation and material, and a module assembly drawing with bill of materials. Export final STL or 3MF files. The report includes coupon results, material choice, spacing and mass, test photos and lessons on printer tolerance. Note that holders for real packs should be tested for temperature and vibration.

   Checkpoint: Print-ready files, drawings and a report with measured fit results.

Testing & validation

• Pocket clearance set by measured coupon results.
• Configurations rebuild for both cell sizes.
• Interference Detection passes for both modules.
• Material temperature rating above the cell operating limit.
• Printed holders fit cells as predicted.

Expected results

A parametric, printer-calibrated cell holder for 18650 and 21700 cells, an assembled module with spacing and mass checks, a justified material choice and print-ready files with drawings.

Take it further

• Add cooling channels between cells and test temperature rise under a known load.
• Design a holder for injection moulding with draft and uniform walls.
• Run a Frequency study of the module to check holder stiffness.

Viva and review questions

Learn it at DIYguru

These DIYguru courses and programmes teach what this project needs.