Build plan: first test cell

Goal: build one replaceable cell module, measure what it really does, and decide whether to build the 10-cell pack, and with what changes.

Questions this cell must answer:

1. Usable capacity at 3.5 A (design target 70 Ah, estimate 40–70 Ah).
2. Charge voltage limit: does 1.50 V per cell recharge it, or does it need 1.80 V?
3. Internal resistance and burst current.
4. Whether cheaper MnO₂ is good enough.
5. Whether the printed PETG case, lug seals and vent hold up.
6. Self-discharge, and early signs of dendrites or corrosion.

Reference files:

• cell-module-blueprints.html: drawings, parts list, 15-step build sequence
• cell_module_case.scad: printable case
• parts_sources.md: where to buy
• note.txt: full design notes

Timeline overview

About 10–11 weeks in total. Most of it is cycling time, which runs unattended.

Phase 0: Safety and setup

Hazards and controls

Workspace:

• A bench for wet chemistry, separate from food areas.
• Labeled containers. Any pasta roller or pans used here are never used for food again.
• A plastic tray under the cell during all testing.

Decisions locked for this build (from the design notes):

• Electrolyte: 2 M ZnSO₄ + 0.1 M MnSO₄.
• 5 cathode plates and 6 zinc plates, 0.88 mm felt.
• Case printed in PETG.
• Materials route: mixed, meaning lab-grade MnO₂ for the full cell, with battery-grade MTI EMD compared in the coupon tests.

Phase 1: Shopping list

Materials (test cell plus coupons, with spare)

Tools and test equipment

Phase 2: Coupon tests (do these before committing to the full cell)

Coupons are small and quick. They answer the riskiest questions for a few dollars.

2A. Print and leak test

1. Print part = "fit_test" from cell_module_case.scad in PETG with the settings in the file header.
2. Check:

• The lip fits with a 0.2 mm gap.
• The M6 nut presses in firmly.
• The riser slot fits the titanium lug plus the EPDM sleeve.

1. Fill the box slice with water for 24 h, standing on paper towel. Pass: no damp spots.
2. If it fails, raise flow by 3% and the nozzle temperature by 5 °C, or coat the inside with epoxy, then reprint.

2B. Material soak test (2 weeks)

1. Put small pieces in jars of electrolyte at room temperature:

• PETG
• Cured epoxy
• EPDM
• Treated felt
• Cellophane
• Nickel-plated brass
• Titanium
• 316 stainless (if you're considering stainless mesh)

1. Weigh each piece and photograph it before soaking, after 1 week and after 2 weeks.

Pass: less than 1% mass change, no swelling, no discoloration of the electrolyte, and the brass plating still intact.

2C. Felt compression check

1. Clamp a 50 × 50 mm stack of 4 felt layers between flat plates.
2. Measure its thickness relaxed and at the target of 0.88 mm per layer.
3. Note how hard it is to squeeze. If 1/16" felt at 45% compression is very stiff, use a thinner felt or go to 4 cathode plates.

2D. Cathode pressing trial

1. Make 20 g of mix (75:25 MnO₂:carbon, using a 20% graphite + 5% Super P blend, plus 4% PTFE).
2. Roll a sheet and press it onto a mesh coupon to 2.90 mm.
3. Calculate porosity from mass and volume. Target: about 50%.
4. Check that the sheet doesn't crack when bent slightly and doesn't shed powder when tapped.

2E. Coupon cells: MnO₂ grade and charge voltage (the most important pre-test)

Coupon design

• One 30 × 30 mm cathode at full-cell thickness (2.90 mm, 50% porosity).
• Coupon mix: 4.87 g (3.65 g MnO₂ + 1.22 g carbon + 0.2 g PTFE), the same weight per area as the full cell.
• Faced on both sides by 30 × 30 mm zinc coupons through 0.88 mm felt.
• Clamped between two PP or PETG plates with nylon bolts, in a small jar of electrolyte. Printable parts and a press die for the coupons are in coupon_test_cell.scad.

Test current: 28 mA. This gives the same current per area as 3.5 A in the full cell (about 1.56 mA/cm²).

Expected capacity: about 0.56 Ah per coupon at the design's 50% depth of discharge, and 1.12 Ah at full theoretical capacity.

Test matrix (6 coupons)

Procedure for each coupon

1. Fill and rest 24 h. Record the resting voltage.
2. Run 10 cycles:

• Discharge at 28 mA to 0.90 V.
• Charge at 28 mA to the cutoff, then hold that voltage until the current falls to 3 mA.

1. Record per cycle:

• Discharge capacity (mAh).
• Charge capacity (mAh), so you can calculate coulombic efficiency.
• Average discharge voltage.

Run all 6 coupons in parallel. A cheap multi-channel analyzer, or a power supply switched by a microcontroller, makes this practical.

Decisions from the coupons

• Charge voltage: if 1.80 V gives clearly more capacity per cycle and coulombic efficiency stays at or above 95% with no heavy gassing, use 1.80 V for the full cell. Then update the BMS limits in note.txt (per-cell over-voltage 1.80–1.85 V, pack 18.0–18.5 V).
• MnO₂ grade: if lab grade reaches at least 80% of the MTI capacity, use lab grade for the pack. If not, budget for EMD.
• Cellophane: if C6 matches C2 in capacity, include cellophane in the full cell.

Scaling check: coupon capacity × 124 ≈ predicted full-cell capacity. That's the ratio of active area: (10 faces × 224 cm²) ÷ (2 faces × 9 cm²) = 2,240 ÷ 18. A 0.56 Ah coupon predicts about 70 Ah.

Phase 3: Build the full cell

Follow the 15-step build sequence on sheet 7 of cell-module-blueprints.html. Keep a traveler log (template below) and fill it in as you go.

Changes to that sequence from the coupon results

• Use the charge cutoff chosen in phase 2E.
• Add the cellophane envelope if C6 passed.
• Use the cathode mix and pressing method from phase 2D.

Critical checks during the build

Spare parts to make at the same time:

• 1 extra cathode plate and 2 extra zinc plates, in case a plate fails a check.
• A second printed box and lid.

Phase 4: Fill, formation and testing

The rig needs a power supply, an electronic load or analyzer, logging, a 30 A fuse, a thermocouple taped to the box face and a drip tray. Log voltage, current and temperature every 30 s.

4A. Wet-out (day 1–2)

1. Fill in 3–4 pours, 15 min apart.
2. Rest 24 h. Record the resting voltage every hour for the first 6 hours, then at 24 h.
3. Expect about 1.3–1.5 V, steady or slowly rising.

4B. Formation (days 2–6)

4C. Capacity test (cycles 5–7)

1. Discharge at 3.5 A all the way to 0.90 V. This is one-time only, to measure true capacity.
2. Recharge.
3. Then run one discharge at 7 A and one at 14 A to 0.90 V, to see how capacity falls with current.

4D. Resistance and pulse test (at about 50% charge)

1. Rest 1 h, then record the resting voltage.
2. Apply a 10 A pulse for 10 s, then a 20 A pulse for 10 s, with 5 min rest between.
3. Calculate R = ΔV ÷ ΔI at 0.1 s and at 10 s.
4. Then try 30 A for 5 s, stopping if the voltage falls below 0.90 V or the box warms by more than 5 °C.

4E. Cycle life (cycles 8–30)

1. Cycle at 3.5 A between 50% and 100% charge (35 Ah out, then back in).
2. Track capacity, coulombic efficiency and end-of-charge current.
3. Each cycle takes about 1 day, so this runs about 3 weeks.

4F. Self-discharge (after cycle 30)

1. Fully charge, then rest 7 days at room temperature.
2. Discharge at 3.5 A and compare the capacity with the previous cycle.

What to record every cycle

• Ah out and Ah in, and coulombic efficiency.
• Average voltage while discharging and while charging, and energy efficiency.
• Peak temperature.
• Any vent activity (listen for a puff, check for moisture at the vent).
• Visual check for leaks or bulging.

Phase 5: Go / no-go decision

Pass criteria for building the 10-cell pack

Teardown inspection (after the decision tests)

Discharge to 0.90 V, drain and rinse the cell, then cut open the lid weld and inspect:

Likely fixes depending on results

Budget (mixed materials route)

Traveler log template (copy one per build)

CELL ID: ZM-CELL-01-T1          Build start date: __________
Materials lot / supplier:
  MnO2 ________  Carbon ________  PTFE ________  Zinc ________
  ZnSO4 ________ MnSO4 ________   Felt ________  Ti mesh ______

Cathode plates (thickness mm / mass g incl. mesh):
  C1 ____/____  C2 ____/____  C3 ____/____  C4 ____/____  C5 ____/____
Zinc plates (thickness mm / mass g):
  Z1 ____/____  Z2 ____/____  Z3 ____/____  Z4 ____/____  Z5 ____/____  Z6 ____/____
Felt envelopes: relaxed ____ mm   compressed ____ mm   Triton batch ____
Cellophane used: Y / N
Clamped stack thickness: ______ mm
Isolation after welding (+ to -): ______ MOhm
Lid leak test 1.0 psi: PASS / FAIL   notes: __________
Electrolyte batch: ____ g ZnSO4·7H2O  ____ g MnSO4·H2O  to ____ mL   fill: ____ mL
Resting voltage: 1 h ____  6 h ____  24 h ____
Charge cutoff used: ______ V

Cycle log:
 #  | Ah out | Ah in | CE % | Vavg dis | Vavg chg | Tmax C | notes
----+--------+-------+------+----------+----------+--------+------
  1 |        |       |      |          |          |        |