How to Build a 4S2P 32700 LiFePO4 Battery Pack with a 3D Printed Frame

Eight 32700 LiFePO4 cells, wired 4S2P, gives you 12Ah at 12.8V nominal. Around 154Wh in a package you can carry with one hand. This guide walks through building one inside a set of 3D printed parts I designed, from printing through to the finished shrink wrapped pack.

You do not need to model anything. Download the STL files, print them, and follow along.

Before you start

Building lithium packs is genuinely dangerous. Cells can short, vent or catch fire if they are damaged, wired incorrectly, charged improperly or protected inadequately. The frame in this guide is printed plastic and nothing more. It provides no electrical protection, it is not fire rated, and it does not replace a properly specified BMS.

If you have never welded cells before, practice on scrap or on cells you do not mind ruining before you build something you intend to rely on.

What you are building

• 8 x 32700 LiFePO4 cells, 6000mAh each
• Wired 4S2P: four groups in series, two cells in parallel per group
• 12Ah capacity, 12.8V nominal, about 154Wh
• Full charge around 14.6V, empty around 10V
• A 4S BMS mounted on a printed top plate
• The whole assembly shrink wrapped in PVC

What you need

Electrical

• 8 x 32700 LiFePO4 cells, ideally from the same batch
• A 4S LiFePO4 BMS ( I used THIS one)
• Nickel strip, 8mm wide, 0.15mm thick
• Silicone wire for the output leads, sized for your current
• Fish paper or similar insulating rings
• PVC shrink wrap tube, wide enough to go around the assembled pack
• Your chosen output connector

Hardware, all M3 self tapping into the plastic

Buy spares. Self tapping into printed plastic strips a thread occasionally.

Tools

• Spot welder capable of 0.15mm nickel
• Multimeter
• Soldering iron for the balance leads
• Scissors or shears for the nickel
• A corner rounding punch if you want tidy strip ends
• Heat gun or hair dryer for the shrink wrap

Step 1: Print the parts

Print in ABS, ASA or PETG. PLA is fine for a test fit but do not use it for the finished pack, because a battery is exactly the sort of thing that ends up in a hot car.

• Layer height 0.2mm
• 3 to 4 perimeters
• 30 percent infill or more
• No supports needed
• Brim or enclosure recommended for ABS and ASA

Print one spacer first and check it before committing to the whole set. The spacers are long and thin, so they are the main warp risk, and they are also the part where hole alignment matters most.

Step 2: Test fit everything dry

Before any welding, assemble the frame empty.

Drop a cell into a spacer and check it seats on the retaining lip without falling through and without needing force. Stand the four posts in the corner pockets of the bottom spacer, put the top spacer on, and drive the eight corner screws. Check the panels line up with the edge holes.

Fix any fit problems now. Once cells are welded in, everything gets much harder to change.

Step 3: Prepare the nickel strips

Every connection in this pack spans one cell pitch, whether it is a parallel link across a column or a series link between columns. That means every strip is the same length: about 48mm.

Count what you need. Four parallel links, one per column, and three series links between columns. Seven strips total, plus a couple of spares.

Cut them with shears. If you want them looking tidy, round the ends with a corner rounding punch. This is cosmetic but it also removes sharp corners that can snag or cut through insulation later.

Step 4: Insulate the positive ends

On a cylindrical cell the positive terminal is a small raised button in the centre, and the rim around it is connected to the negative can. An 8mm strip is wider than that button, so without insulation it can bridge onto the rim and dead short the cell.

Cut a fish paper ring for each positive end: hole big enough to expose the button, outer diameter big enough to cover the rim. Lay it over the positive end before welding. The negative end is the whole can and needs nothing.

Identify which end of your cells is positive before you do anything else.

Step 5: Load the cells

Load them into the bottom spacer in a 2 x 4 grid, alternating orientation column by column. Both cells in a column face the same way, and each column faces opposite to its neighbour. That is what lets a single strip join a column in parallel and the next strip carry the series link to the next column.

Check every cell with a multimeter before you weld. They should all read close to each other. If one is well below the others, do not use it.

Then check your orientation twice. Getting a column backwards means unwelding it later.

Step 6: Weld

Lay a strip into the groove, and weld. Two spots per cell minimum. Four if you want lower resistance under load.

Keep both weld spots roughly in line, since the current wants a straight path between the probes.

Peel test first. Weld a scrap of nickel to a spare cell and try to pull it off. A good weld tears the nickel and leaves the welded dots behind. If the strip lifts off cleanly, turn the power up and try again. Do this before you touch the real pack.

Work through the four parallel links first, then the three series links.

When you are done, check voltage across the whole string. You should read roughly four times a single cell, so around 13V for cells at rest.

Step 7: Solder the leads and balance taps

You need seven wires off the pack.

Two heavy leads: pack negative from one end of the string, pack positive from the other. Use silicone wire sized for your expected current.

Five balance taps: pack negative, the three junctions between series groups, and pack positive. These are thin wires and they go to the BMS balance connector.

Solder them on now, while you can still reach everything, and leave enough length to reach the top plate comfortably. Then cap the pack with the second spacer, flipped so its retaining lip and grooves face outward.

Step 8: Assemble the frame

Stand the four posts in the corner pockets of the bottom spacer. Put the top spacer on so the posts seat in its pockets too.

Drive four M3x16 up from underneath the bottom spacer into the posts. The screw heads recess so the pack still sits flat.

Both spacers have a wire exit notch. Because you flip the top one, the notches end up on opposite faces by default. If you would rather have all your wiring on one side, spin the top spacer 180 degrees flat before you assemble it. Every hole still lines up and only the notch moves. One side is tidier.

Route your wires up through the notches to the top.

Step 9: Mount and wire the BMS

Screw the BMS to the top plate with four M3x8. There are four dedicated holes for it, separate from the general hole grid. Use the grid for zip ties and cable management.

Wiring is simpler than it looks:

• Pack positive goes straight out to your output positive. It never touches the BMS.
• Pack negative goes to the BMS B- pad.
• The BMS P- pad goes to your output negative.
• The balance harness plugs into the BMS balance connector.

That switched negative path is the whole point of the BMS. It opens that path to protect the cells.

Plug the balance connector in last, and check the tap order against your BMS documentation before you do. Getting balance leads out of order can damage the board.

Now sit the top plate on the four standoffs and drive four M3x20 down through the plate, standoff and top spacer into the posts.

Step 10: Output cable and strain relief

Run the output leads out through the hole in the cable exit end panel, and fit your connector on the cable ends rather than on the pack.

Add strain relief inside, so a pull on the cable cannot reach your solder joints. The simplest version that works: tie a knot in the cable inside the panel, or loop it and zip tie the loop to the top plate grid. The anchor has to sit between the tug and the joints.

Step 11: Fit the panels

Screw on the side and end panels with M3x10. The pads face inward and seat into the pockets in the spacer edges, so the outside stays flat.

Before you close it up, look over the whole thing. Nothing pinched, no bare wire touching nickel, no strands stray, the BMS not resting against anything conductive. A strip of fish paper over the busbars under the top plate is cheap insurance.

Step 12: Shrink wrap

Slide the assembly into PVC tube and shrink it with a heat gun on low or a hair dryer.

Run the tube so the open ends are the top and bottom, wrapping around the side panels. Doing it the other way means shrinking film down over the BMS and the connector, which gets lumpy.

Keep the heat moving. Do not park it in one spot, both because concentrated heat can soften a thin printed wall and because PVC scorches. Do it in ventilated air.

Cut a clean slit where the cable exits and reinforce that spot with tape or heat shrink over the cable so the film edge does not saw into the insulation.

Step 13: First charge and checks

Before charging, measure output voltage and confirm polarity at the connector. Measure it twice.

Charge with a LiFePO4 specific charger. Do not use a lead acid or a generic lithium charger, because the voltages are different. A 4S LiFePO4 pack charges to around 14.6V.

Charge the first time somewhere you can watch it, on a non flammable surface, and stay in the room. Check that the pack stays cool. Warm is normal, hot is not.

After the first full charge, check the pack sits where you expect and that the BMS has balanced the groups.

A note on capacity monitoring

LiFePO4 has a very flat discharge curve. From roughly 90 percent down to 20 percent the voltage barely moves, so a voltmeter tells you "full", "getting low" or "empty" and very little in between.

Files

The STL files are on Printables, free to download. Print the quantities in the table above.

If you build one, I would like to see it.

Disclaimer

This is a hobby project I built for myself and am sharing as is. The frame is a mechanical part only. It provides no electrical protection, it is not fire rated, it is not certified to any standard, and it is not a substitute for a properly specified BMS and sensible charging practice.

There is no warranty here of any kind. If you print these files, modify them or build something from them, you are responsible for confirming the design suits your cells, your BMS and your intended use, and you do so entirely at your own risk. I accept no liability for any damage, injury or loss arising from their use or misuse.