build steps

Battery packs built

Three 4S16P banks: 192 cells spot-welded, busbars in, BMS wired, balanced and capacity-checked. At 14 kg the pack is half the vehicle's mass and the number every other number is derived from.

1. 1check

   Cells received, counted and accepted

   Before anything is welded, prove the cells are healthy and that you have enough.

   Check

   1. Count: 192. Three banks of 4S16P. Order spares — a damaged cell mid-build stops everything.
   2. Visual on every can: no dents, no torn wrap, no corrosion at either terminal. A dented cell is scrap, not a discount.
   3. Resting voltage on every cell. Factory storage charge is ~3.6 V. Log them all.

   Done when

   192 cells present, undamaged, and every one between 3.4 V and 3.8 V.

   Watch out for

   ⚠️ Any cell below 2.5 V is dead — do not charge it. A deeply discharged lithium cell can plate copper internally and short later, in a sealed tube, at depth. Set it aside permanently. ⚠️ Big spread in resting voltage (>100 mV) suggests mixed batches. That matters for the next step.

   
2. 2do

   Sort cells into matched 16-cell groups

   Cells in parallel must be matched. Mismatched cells fight each other — the strong ones dump current into the weak ones the moment they're joined, forever.

   Steps

   1. Charge every cell to the same voltage (a common storage level, ~3.7 V) and let them rest 24 h.
   2. Re-measure. Cells that sag more than their neighbours over that rest have higher self-discharge — segregate them.
   3. Measure internal resistance if your charger supports it. Group by IR, not just voltage.
   4. Sort into 12 groups of 16, keeping each group as tightly matched as you can. Spread any outliers across groups rather than concentrating them.
   5. Label every group. Masking tape and a marker. You will not remember later.

   Done when

   Twelve labelled groups of 16, each internally matched to within ~20 mV and similar IR.

   Watch out for

   ⚠️ This is the step everyone skips and regrets. Parallel mismatch is invisible on day one and shows up as a bank that won't balance, months later, sealed inside a tube.

3. 3do

   Print and fit-check the pack hardware

   Steps

   1. Print per bank: 4 cell plates, 4 collars, 1 top plate. Ribs (rib_tip_d = 103.8) on the top and bottom plates only — ribs on every part means four sets of interference fighting a 5 kg pack down a 400 mm tube.
   2. Dry-stack one full bank with real cells: bottom plate, 4 layers, collars, top plate.
   3. Measure the assembled height. Expect ~309 mm against 361.8 mm of usable tube.
   4. Slide it the full length of a tube — then pull it back out.

   Done when

   A full dry stack goes in and comes out with firm but manageable force, and you know the real spare length.

   Watch out for

   ⚠️ Insertion force with 5 kg behind it is the number that matters, not how one ring felt by hand. If it's a fight, drop rib_tip_d to 103.5 and reprint before committing to twelve plates. ⚠️ A pack you can't extract from a blind 400 mm tube is a genuinely bad situation. Err loose.

4. 4do

   Set up the spot welder and prove it on scrap

   Never learn to weld on the real pack. This step is entirely about scrap cells.

   What is actually happening

   Spot welding is resistance welding. Both probes press on the nickel a few millimetres apart. A huge current pulse goes down one probe, through the nickel, dives into the cell can, and comes back up the other probe. The resistance where nickel meets can is where heat concentrates — that is what fuses them.

   ~~~ probe A probe B | | v v -----#------------------#----- nickel strip \ / \________________/ current dips through the joint =====================#======= cell can <- fusion happens HERE ~~~

   Every technique rule below follows from that path.

   1. Probe spacing - the single most important setting

   5-8 mm apart.

   ~~~ TOO FAR (>10mm) CORRECT (5-8mm) A B A B | | | | -#------------#- -#----#- \__________/ \__/ current shortcuts current dives THROUGH the nickel INTO the can -> two burn marks, -> a real weld no weld ~~~

   If your welder has a fixed head, this is already set. If the probes are handheld, make a jig - a block of wood with two holes at 6 mm - rather than eyeballing it each time.

   2. Pressure

   Press until the probes stop moving, THEN trigger.

   Light pressure leaves an air gap and the pulse arcs across it. You get a bright flash, spatter, and a black pit - not a weld, and the pit is damage to the can.

   3. Energy

   Start at the low end of the dial and work up until the pull test passes.

   WARNING: Pure nickel needs MORE energy than nickel-plated steel, and nearly every tutorial online uses plated steel. Nickel is more conductive so less heat develops for the same pulse. Your welder's numbers will not match what you see in videos. Scrap testing is the only calibration that counts.

   4. The pull test - how you actually know

   Weld a tab to a scrap cell, grab it with pliers, and peel it back hard.

   ~~~ GOOD BAD nickel TEARS tab PEELS OFF CLEAN small discs stay welded only discolouration left to the can on the can

   ___ ___ ____________ | \/ | <- torn | | <- intact # # . . ^^^^^^^^^^ ^^^^^^^^^^ discs remain nothing remains ~~~

   WARNING: The dangerous result is the middle one - a tab that resists a little then releases. That is a marginal weld, and it looks perfect from above.

   Done when

   Ten consecutive tears, no peels, at one setting. Write the setting down. Do not touch the dial again.

   Watch out for

   WARNING: Never let the probes bridge a cell's two terminals - that is a dead short through your welder. WARNING: Eye protection. Bad welds throw sparks. WARNING: Do not dwell. A long pulse heats the cell rather than the joint. If a cell gets warm, stop. WARNING: If a test cell hisses or swells, take it outside immediately and leave it there.

5. 5do

   Build and weld the first 16-cell layer

   One layer done carefully is the template for the other eleven.

   Tab geometry

   Cut nickel tabs 8 mm wide x ~30 mm long from 0.15 mm strip. That length is deliberate:

   ~~~ |<-- 10mm -->|<- 6mm ->|<---- 14mm ---->| [ welded to ][ through ][ folds over to ] [ the cell ][ plate ][ the copper ] ~~~

   The extra length is what lets you solder away from the cell later. Do not cut them short.

   Steps

   1. Seat 16 cells in a bottom plate, all the same orientation, positives up.

   1. WARNING: Verify polarity across the whole layer before striking a single arc. One reversed cell shorts the other fifteen through itself. Walk the layer with a meter - every cell should read the same sign.

   1. Weld a tab to each cell, four welds per tab:

   ~~~ tab ___________ | o o | <- pair 1 | | | o o | <- pair 2 |___________| cell ~~~

   Two pairs, spaced apart. Four, not two - one marginal weld out of four costs nothing; one out of two is a joint that fails under load inside a sealed tube.

   1. Inspect every weld as you go (see below). This is the last time you will see them.

   1. Measure the layer. It should read one cell's voltage and now behave as a single 85 Ah cell.

   Inspecting welds on the real pack

   Pull-testing destroys the joint, so on real cells you use:

   • Visual - two clean dimples with a bright fused ring. No black spatter, no pits.
   • The tug - lift the tab edge with a fingernail or thin blade. A good weld does not move at all.
   • Redundancy - four welds means one bad one does not matter.
   • The load test later (step 80) finds anything that slipped through, as a hot spot.

   Done when

   A 16-cell layer at one stable voltage, every tab tug-tested, every weld visually clean.

   Watch out for

   WARNING: 16 cells in parallel is 85 Ah. A short across that is thousands of amps - it welds tools instantly and starts a fire faster than you can react. WARNING: Insulated tools only. No watch, no rings. Non-conductive work surface. One hand behind your back when probing. WARNING: Kapton over exposed metal as you go. Never leave a live layer open on the bench.

6. 6do

   Copper collectors and series links

   The nickel carries 3 A per cell. The collector carries all 48 A. That is why it is copper.

   Why copper, in one line

   ~~~ 48 A bank current / 16 cells = 3 A per cell -> nickel tab: fine (0.15mm x 8mm handles ~8A)

   the collector sums all 16 = 48 A -> needs ~6 mm2 of copper. Nickel would need 4 layers. ~~~

   The stack at each interface

   ~~~ solder joint (10mm clear of the cell) | v copper ====================================== collector / nickel tab _/ <-- HEMOSTAT CLAMPS HERE | (sinks heat before it printed | reaches the cell) plate ----+---------------------------------- (insulator) | welds x4 ==#== cell top ########## ~~~

   The printed plate does two jobs: it insulates the can rim (bare copper touching both the button and the rim is a dead short across that cell) and it physically separates the solder joint from the cell.

   Steps

   1. Cut collectors from 0.5 mm copper sheet - a disc roughly matching the plate. Tin snips, then file the edge smooth. Five per bank: bottom (pack -), three interfaces, top (pack +).

   1. Fit the printed plate over the welded layer. Tabs come up through the 12 mm windows.

   1. Fold each tab over onto the collector.

   1. WARNING: Clamp a hemostat or alligator clip on each tab between the solder joint and the cell. This is not optional - it sinks the heat before it travels down the nickel. Heat is what damages cells, and soldering puts in far more than welding does.

   1. Solder tab to copper. Nickel needs an active flux - it does not wet like copper. Use enough iron (80-100 W) to heat fast; slow heating gives the heat time to conduct downward, which is exactly what you are avoiding.

   1. The collector between two layers IS the series link - it joins layer N's positives to layer N+1's negatives.

   ~~~ top plate ------------------ collector ================== pack + layer 4 [][][][][][][][][] collector ================== series link layer 3 [][][][][][][][][] collector ================== series link layer 2 [][][][][][][][][] collector ================== series link layer 1 [][][][][][][][][] collector ================== pack - bottom plate ------------------ ~~~

   1. Measure ~14.8 V across the finished bank (4 x ~3.7 V).

   Testing the solder joints

   • Visual - a shiny concave fillet wetting both the nickel and the copper. Dull and blobby means it did not wet; reflux and redo.
   • Tug - the tab should not move.
   • Load test (step 80) - a resistive joint reveals itself as a warm spot under current.

   Done when

   A 4S16P bank at ~14.8 V, every joint soldered and tugged, every series link in copper.

   Watch out for

   WARNING: You cannot spot-weld to bare copper - it conducts the weld energy away before anything fuses. Weld to the cell, solder to the copper. WARNING: Verify every weld BEFORE the collector goes on. Once copper covers them you cannot see or reach them. WARNING: Voltage climbs as you stack. Insulate each series joint as you make it, not at the end.

7. 7do

   Fit the BMS and balance leads

   Steps

   1. Route balance leads through the interstitial channels between cell trios — ~3.3 mm of clear space runs the length of the bundle, better protected than the annular gap and it doesn't fight the tube.
   2. Bring them out through the wire notch aligned across all the plates.
   3. ⚠️ Connect balance leads in order, negative-most first. Out of order puts full pack voltage across a single BMS input and destroys it.
   4. Mount the BMS. See the open backlog item — the Daly must be 4S - a 7S board sees three dead cells and will not turn on. 75x48mm is the right form factor and does not fit the ~53 mm of spare tube in any orientation.
   5. Verify the BMS reads all four cell groups within ~20 mV.

   Done when

   BMS reporting all four groups, balanced, with leads routed and strain-relieved.

   Watch out for

   ⚠️ A balance lead that chafes through against a cell can is a dead short inside a sealed tube. Strain-relieve everything and check nothing is trapped where a plate meets the bundle.

   Fit the bank fuse before it goes in the tube

   Each bank gets its own fuse at the pack B+, inside the battery tube, with the shortest lead you can manage. 80-100A - it is short-circuit protection, not overcurrent, and a fuse near 48A would open during normal full thrust.

   WARNING: Never put this fuse in the electronics tube. The run from pack terminal, through the tube, through a penetrator, across to the electronics tube would be completely unprotected - a chafe anywhere along it is a dead short with 85 Ah behind it inside a sealed hull.

   Use a bolt-down MIDI/AMI fuse, not a blade holder - a blade holder is ~50 x 25 x 20mm and the tube has only ~53mm spare, 15mm of which the BMS takes.

   Full plan for every fuse on the vehicle: docs/FUSING.md

8. 8check

   Bank tested — capacity, balance, and a real load

   Prove it before it goes in a tube you have to unseal to fix.

   Check

   1. Full charge to 16.8 V. Watch balance throughout; groups should track within ~30 mV.
   2. Capacity discharge at a modest rate. Expect ~85 Ah; anything under ~78 suggests a weak group.
   3. Load test at realistic current, watching for hot spots — a joint that warms under load is a bad weld.
   4. Rest 24 h and re-measure. Any group that drifts has a self-discharge problem.
   5. Log everything. This is the baseline you'll compare against for the pack's life.

   Done when

   Measured capacity within 10% of nominal, balanced after a rest, and no joint warmer than its neighbours under load.

   Watch out for

   ⚠️ Do this outside or in a fire-safe space, not on the bench beside everything else you've built. ⚠️ A bank that won't balance now will never balance. Find out here, not at 300 m.

9. 9do

   Repeat for banks 2 and 3

   Same process, twice more.

   Watch out for

   ⚠️ Don't let familiarity erode the checks. Bank 3 is where polarity errors happen, because by then it feels routine. ⚠️ Keep the three banks matched to each other as well — they'll be ORed onto a common bus, and a bank that's meaningfully weaker will be back-fed by the other two.

   Done when

   Three banks built, tested, logged, and labelled — ~3.7 kWh total, ready for their tubes.