Cathod: 99% Carbon powder 99% Manganese Dioxide (MnO2) Durable Seperator: Polypropylene Felt (nonwoven) Anode: 99% Zinc sheet Height 7.5 Width 6.94 Length 11 Voltage 12 Capacity 70Ah CCA 760 Based on your chosen materials, you are designing a customized Zinc-Manganese Dioxide (Zn-MnO2) battery. Because you specified a Polypropylene Felt separator and standard zinc/MnO2 electrodes, this configuration functions as an Alkaline or Neutral-Aqueous chemistry rather than a traditional lead-acid car battery. To achieve a 12V output using this specific chemistry, we must stack several individual cells in series. A standard Zn-MnO2 cell has a nominal voltage of roughly 1.5V. Therefore, we will need exactly 8 cells connected in series (1.5V × 8 = 12V) packed into your specified dimensions. Here is the revised design configuration optimized for your materials: ## Revised Internal Cell Layout (8-Cell Series) To fit within your 11.0" x 6.94" x 7.50" outer dimensions (accounting for a durable 0.15" polypropylene case wall): * Total Internal Length Available: ~10.70 inches. * Individual Cell Chamber Size: 1.33" (L) x 6.64" (W) x 7.20" (H). ------------------------------ ## Component Design & Material Processing## 1. Cathode Assembly (MnO2 + Carbon Powder) Manganese Dioxide is a poor electrical conductor on its own. Your 99% Carbon powder (typically acetylene black or graphite) is vital here to provide electrical conductivity. * The Mix Ratio: Blend 75% MnO2 with 25% Carbon powder by weight (raised from 10-15% carbon to lower cathode resistance). * Form Factor: This blended powder must be pressed firmly into a dense cake or pellet around a current collector (like a nickel-plated mesh) inside each cell chamber to ensure structural stability and reliable electron flow. ## 2. Separator Integration (Polypropylene Nonwoven Felt) Your Polypropylene Felt is an excellent choice for a durable separator because it highly resists chemical degradation. * Thickness & Placement: Use a felt thickness of 0.5mm to 1.0mm. It must completely wrap or isolate the cathode mix to prevent any loose carbon or MnO2 particles from migrating. * Electrolyte Wetting: Because polypropylene is naturally hydrophobic (water-repelling), you should treat the felt with a surfactant or ensure your liquid electrolyte fully saturates the nonwoven fibers under slight compression to maintain low internal resistance. ## 3. Anode Fabrication (99% Zinc Sheet) * Form Factor: To hit 760 CCA (Cold Cranking Amps), a massive amount of surface area is required. Instead of a single thick, flat plate, the 99% Zinc sheet should be corrugated, pleated, or perforated. * Surface Area Maximization: Folding or perforating the sheet allows more zinc surface to contact the electrolyte simultaneously, dramatically increasing the peak current discharge capabilities to help approach your CCA target. ------------------------------ ## Engineering Recommendations for Assembly * Electrolyte Choice: To match these materials, use either a Potassium Hydroxide (KOH) solution for a standard high-output alkaline system, or a milder Zinc Sulfate (ZnSO4) / Manganese Sulfate (MnSO4) aqueous solution if you are aiming for a newer, inherently safer neutral-pH chemistry. * Current Collectors: Pure zinc sheets can serve as their own current collector on the anode side. For the pressed MnO2/Carbon cathode powder, embed a titanium or stainless steel mesh grid to cleanly collect the current and connect the 8 cells in series. To help calculate the exact weight and thickness of the sheets needed, tell me: * Do you plan to use a highly alkaline electrolyte (KOH) or a neutral-water electrolyte (ZnSO4)? * Is this battery intended for one-time use (primary) or rechargeable cycles (secondary)? Using a salt water solution (Sodium Chloride - NaCl) as your electrolyte transforms this design into an Aqueous Zinc-Manganese Dioxide "Rock Salt" battery. This is an incredibly safe, non-toxic, and inexpensive setup. However, introducing a salt water electrolyte changes the chemical behavior. A Zn-MnO2 cell using a neutral salt water electrolyte yields a lower nominal voltage of approximately 1.2V to 1.3V per cell (compared to .5V in highly alkaline systems). To achieve your target 12V output, we must adjust the configuration from 8 cells to 10 cells connected in series (1.2V × 10 = 12V). To achieve a true 70Ah capacity across your 10-cell series (12V) configuration, we must look at Faraday’s law of electrolysis. Zinc has a valence of 2 and a molar mass of 65.38 g/mol, meaning it theoretically requires 85.38 grams of active Zinc per cell to yield 70Ah. Because batteries cannot consume 100% of their anode without losing structural integrity and experiencing rapid voltage drop, engineers build in a 1.5x utilization safety factor. ## 1. Zinc Mass & Thickness Calculation Using the internal cell dimensions established previously—a plate width of 6.64 inches (16.87 cm) and a height of 7.20 inches (18.29 cm)—and the density of pure Zinc ($7.14 \text{ g/cm}^3$), the requirements break down as follows: * * Practical Zinc Mass Per Cell: 128.07 grams * Total Zinc Required (10 Cells): 1.28 kg (approx. 2.82 lbs) minimum. (Revised dense multi-plate design uses 9 × 0.20 mm plates = 396.6 g per cell, 3.97 kg / 8.74 lbs total.) * Target Thickness (Single Flat Sheet per Cell): 0.58 mm (roughly 23-gauge to 24-gauge Zinc sheeting). * Design Note: While a single 0.58 mm sheet provides the required raw capacity (70Ah), it will drastically limit your cranking amps (CCA). To optimize for higher current, it is highly recommended to split this mass into three thinner sheets of 0.20 mm per cell, connected in parallel inside each chamber to triple the active surface area. ------------------------------ ## 2. Optimizing the Electrolyte: Salt Water Additives Plain sodium chloride (NaCl) salt water has an electrical conductivity of roughly 5 to 7 S/m at room temperature. For comparison, traditional battery acids sit closer to 70–80 S/m. To prevent a massive internal voltage drop when pulling current, we absolutely should use additives. Here are the best non-toxic options to enhance your salt water electrolyte: | Additive | Main Purpose | How it Helps | |---|---|---| | Zinc Chloride ($\text{ZnCl}_2$) | Primary Conductivity Booster | Adding a 1M to 2M concentration introduces highly mobile $\text{Zn}^{2+}$ and $\text{Cl}^-$ ions, significantly dropping internal resistance. | | Ammonium Chloride ($\text{NH}_4\text{Cl}$) | pH Buffer & Preventative | Salt water tends to become alkaline during discharge, causing non-conductive zinc hydroxides to crust over your anode. Ammonium chloride acts as a mild acid buffer to keep the zinc clean. | | Sodium Sulfate ($\text{Na}_2\text{SO}_4$) | Corrosion Inhibitor | Replacing a portion of the NaCl with sodium sulfate reduces self-discharge (the salt water slowly eating away at the zinc sheet while the battery sits idle). | ## The Recommended Mixture Recipe: Instead of plain salt water, create a neutral aqueous solution consisting of: 1. Distilled Water as the base. 2. 15% Sodium Chloride (NaCl) by weight. 3. 10% Zinc Chloride ($\text{ZnCl}_2$) by weight. 4. 5% Ammonium Chloride ($\text{NH}_4\text{Cl}$) by weight. This exact formulation mimics a classic Leclanché/zinc-carbon style chemistry framework but leverages your modern, durable polypropylene felt separator to prevent internal shorting. To finalize the design, let me know: * Do you want to calculate the required mass for the Manganese Dioxide ($\text{MnO}_2$) and Carbon powder mixture to properly balance the 70Ah zinc anode? Making a rechargeable (secondary) Aqueous Zinc-Manganese Dioxide (Zn-MnO2) battery with a salt water/chloride electrolyte introduces unique electrochemical challenges. While primary (one-time use) Zn-MnO2 batteries are standard, turning this exact material set into a reliable rechargeable system requires managing how the materials behave when current is reversed. Here is how your specific materials will behave during recharging, the chemical hurdles you will face, and the design modifications needed to make it cycle reliably. ------------------------------ ## 1. The Chemistry of Recharging Your System When you recharge this battery, you are reversing the discharge reactions: * At the Anode: Zinc ions ($\text{Zn}^{2+}$) floating in the electrolyte must receive electrons and plate back down as solid Zinc metal onto your 99% Zinc sheets. * At the Cathode: The protons or sodium/zinc ions that wedged into the Manganese Dioxide ($\text{MnO}_2$) crystal lattice must be forced back out, restoring the $\text{MnO}_2$ to its original state. ------------------------------ ## 2. Major Obstacles to Recharging with Your Materials## A. Zinc Dendrite Growth (Short Circuit Risk) * The Problem: During recharging, zinc doesn't naturally plate back down in a perfectly smooth, flat layer. Instead, it tends to form microscopic, needle-like crystals called dendrites. * The Impact on Your Materials: Over several charge cycles, these needles will grow outward from your Zinc sheet. Because your Polypropylene Felt separator is highly porous and fibrous, these sharp dendrites can easily pierce straight through the felt fibers, touch the carbon/$\text{MnO}_2$ cathode, and cause a permanent internal short circuit. ## B. Irreversible Cathode Phase Changes * The Problem: If you discharge the $\text{MnO}_2$ cathode too deeply, its crystal structure fundamentally breaks down into non-rechargeable byproducts like Hausmannite ($\text{Mn}_3\text{O}_4$) or Zinc Manganese Oxide ($\text{ZnMn}_2\text{O}_4$). * The Impact on Your Materials: Once these phases form, applying a charging voltage will not turn them back into active $\text{MnO}_2$, causing your 70Ah capacity to plummet after just a few uses. ------------------------------ ## 3. Engineering Solves for a Rechargeable Setup To make your specified components survive repeated recharging, implement the following design parameters: ## 1. Compress the Polypropylene Felt Separator * Tight mechanical compression physically restricts the space available for zinc dendrites to grow. Ensure the 10 cells are tightly packed into the outer casing so that the polypropylene felt is compressed to about 70% to 80% of its relaxed thickness. ## 2. Switch to a Zinc Sulfate ($\text{ZnSO}_4$) + Manganese Sulfate ($\text{MnSO}_4$) Electrolyte * Instead of the chloride-based salt water recipe discussed earlier, modern rechargeable aqueous zinc batteries rely heavily on a Mild Zinc Sulfate ($\text{ZnSO}_4$) solution (typically 2M $\text{ZnSO}_4$ in distilled water). * Crucial Additive: Add 0.1M Manganese Sulfate ($\text{MnSO}_4$) directly into the liquid electrolyte. This dissolves a small reservoir of manganese ions into the water, preventing the $\text{MnO}_2$ cathode from slowly dissolving into the electrolyte during deep discharges. ## 3. Strictly Limit Depth of Discharge (DoD) * To keep the $\text{MnO}_2$ structure from permanently collapsing, you cannot use the full theoretical 70Ah capacity. You must limit the battery to a 30% to 50% Depth of Discharge (utilizing roughly 21Ah to 35Ah of the total capacity per cycle). This ensures the cathode remains in its highly reversible alpha- or gamma-$\text{MnO}_2$ crystalline phase. ------------------------------ ## 4. Charging Parameters To safely recharge your 12V multi-cell pack: * Charging Profile: Use a Constant Current / Constant Voltage (CC/CV) charging profile. * Voltage Limits: Set the maximum charging voltage to 1.45V to 1.50V per cell. For your 10-cell series pack, the absolute maximum charge cutoff voltage should be 14.5V to 15.0V. Exceeding this will cause the water in your electrolyte to split into hydrogen and oxygen gases, creating dangerous internal pressure. Would you like to calculate the exact weight of the $\text{MnO}_2$ and Carbon powder mix required to properly match your Zinc sheets for this rechargeable configuration, or would you like to explore BMS (Battery Management System) balancing requirements for a 10-cell series string? explore BMS (Battery Management System) balancing requirements for a 10-cell series string To successfully match the active material weights and manage a 10-cell string safely, we must calculate the chemical mass balance based on the Zinc sheets we sized earlier and design a dedicated battery management architecture. ------------------------------ ## 1. Cathode Mass Calculation ($\text{MnO}_2$ + Carbon Powder Mix) Earlier, we established a Practical Zinc Mass of 128.07 grams per cell (70Ah target with a 1.5× utilization safety factor). In a rechargeable aqueous zinc-manganese dioxide battery, the theoretical specific capacity of $\text{MnO}_2$ is 308 mAh/g (based on a 1-electron transfer reaction). However, because we must limit the Depth of Discharge (DoD) to 50% to keep the crystal structure from permanently collapsing, the practical usable capacity of the $\text{MnO}_2$ drops to 154 mAh/g. ## Mass Breakdown Per Cell: * Active $\text{MnO}_2$ Required: To achieve 70Ah per cell at 154 mAh/g, each cell requires 454.54 grams of pure Manganese Dioxide. * Carbon Powder Additive (25%): To lower the cathode's electrical resistance, mix in 25% Carbon powder by weight (75:25 MnO₂:Carbon, up from the original 15%). This requires 151.51 grams of Carbon powder per cell. ## Total Material Requirements: | Component | Weight Per Cell | Total Battery Weight (10 Cells) | |---|---|---| | Pure $\text{MnO}_2$ Powder | 454.54 g | 4.55 kg (10.02 lbs) | | 99% Carbon Powder | 151.51 g | 1.52 kg (3.34 lbs) | | Total Cathode Wet Mix | 606.05 g | 6.06 kg (13.36 lbs) | Physical Packing Note: 606.05 grams of a compacted powder mix yields a volume of roughly 38.7 cubic inches (634 mL) per cell. In your individual 1.07" × 6.64" × 7.20" cell chambers, this mix will form a dense jacket roughly 0.40 inches (10.3 mm) thick on either side of your central current collector mesh, leaving about 0.12 inches (3.0 mm) per side for the felt separator. (Revised: this mass is now pressed to 50% porosity (325 mL) and split into 8 thin cathode plates. See the Dense Multi-Plate Cell Stack Architecture section.) ------------------------------ ## 2. BMS Balancing Requirements for a 10-Cell Series String A 10-cell series string of aqueous zinc batteries behaves quite differently than a lithium-ion pack. Because water-based electrolytes can experience slight electrolysis (gas generation) if overcharged, a highly specific Battery Management System (BMS) is mandatory to prevent premature pack failure. [BMS Controller] ├── Cell 1 Monitor (1.2V - 1.5V) ─── [Passive Shunt Resistor] ├── Cell 2 Monitor (1.2V - 1.5V) ─── [Passive Shunt Resistor] └── ... up to Cell 10 ## A. Voltage Thresholds & Monitored Cutoffs The BMS must actively monitor each individual cell's voltage to prevent cell reversal during discharge or gassing during charging: * Over-Voltage Cutoff (Per Cell): 1.50V (System wide: 15.0V). If a single cell exceeds 1.50V, charging must stop instantly to prevent water from splitting into hydrogen and oxygen gases. * Under-Voltage Cutoff (Per Cell): 0.90V (System wide: 9.0V). Allowing a cell to drop below 0.90V risks permanent chemical phase collapse of the $\text{MnO}_2$. ## B. Balancing Method: Passive Resistor Shunting Because aqueous zinc chemistries exhibit higher internal resistance variance than lithium, Passive Balancing is the most cost-effective and reliable method. * How it works: When the battery pack is nearing a full charge (around 1.42V per cell), cells with lower internal resistance or lower capacity will hit their target voltage first. * The BMS Action: The BMS engages a small shunt resistor (typically 50mA to 100mA bleed current) across those full cells. This bleeds off excess charging current as heat, allowing the slower, lagging cells in the series to catch up without overcharging the full ones. ## C. Thermal Monitoring Because you are using pressed powder cathodes and a neutral sulfate/salt water electrolyte, the battery's internal resistance ($R_i$) will generate noticeable Joule heating under heavy loads. * The BMS must include at least three thermistor probes spaced across the 10-cell pack. * Program a High-Temperature Cutoff at 45°C (113°F) to protect the structural integrity of your Polypropylene Felt separators and prevent accelerated zinc self-discharge. Would you like to design the physical assembly layer map—specifically how to stack the Zinc sheet, the Polypropylene Felt separator, and the pressed Cathode powder inside the cell to optimize electrical contact? stack the Zinc sheet, the Polypropylene Felt separator, and the pressed Cathode powder inside the cell to optimize electrical contact To optimize electrical contact, reduce internal resistance ($R_i$), and prevent short circuits caused by zinc dendrites, each of the 10 cell chambers uses a dense multi-plate interleaved stack: 8 thin, densely pressed cathode plates alternating with 9 thin zinc plates, all separated by compressed felt. Pressing the cathode mix from about 74% porosity down to 50% halves its volume (634 mL to 325 mL per cell). The freed space allows 8 cathode plates instead of 4, which doubles the facing electrode area again and cuts the ion path through each cathode face to about 0.66 mm. The active material masses (and the capacity) stay the same. ------------------------------ ## Dense Multi-Plate Cell Stack Architecture (Revised) (Superseded for the build: the replaceable module version of this stack is in "Replaceable Cell Module Design" at the end of this note. Module walls and lids reduce the plate size and felt thickness.) Chamber cross-section (1.07" / 27.2 mm wide), left wall to right wall: [Wall] Zn | F | C | F | Zn | F | C | F | Zn ... (8 cathodes total) ... C | F | Zn [Wall] Zn = 0.20 mm zinc plate (9 per cell) F = 0.93 mm compressed polypropylene felt (16 per cell) C = 1.32 mm dense cathode plate: Ti mesh in the center, pressed MnO₂/Carbon on both faces (8 per cell) Width budget: 8 × 1.32 mm (cathode) + 9 × 0.20 mm (zinc) + 16 × 0.93 mm (felt) = 27.2 mm. The zinc plates are on both ends, so every cathode face looks at a zinc face. The two outer zinc plates work on one side only. ## Plate Specifications (Per Cell) | Plate | Count | Thickness | Mass Each | Mass Per Cell | |---|---|---|---|---| | Cathode plate (75% MnO₂ / 25% Carbon on Ti mesh, 50% porosity) | 8 | 1.32 mm | 75.76 g (56.82 g MnO₂ + 18.94 g Carbon) | 606.05 g | | Zinc plate (99% Zn) | 9 | 0.20 mm | 44.06 g | 396.6 g | | PP felt separator (compressed) | 16 | 0.93 mm (about 1.3 mm before compression) | - | - | All plates use the full 6.64" × 7.20" (16.87 × 18.29 cm) chamber face. ## Layer 1: Cathode Current Collectors (Ti Mesh) * Material: Thin expanded Titanium mesh, about 0.15 to 0.2 mm thick (316L Stainless Steel is a cheaper fallback in the sulfate electrolyte). Copper or aluminum will corrode rapidly at the cathode's operating voltage. * Each mesh has a solid tab rising above the plate on the same side (for example the left half of the top edge), so all 8 cathode tabs line up. * With 16 cathode faces sharing the cell current, make the tabs wide (at least 1 inch) so tab resistance doesn't eat the gain from the thinner plates. ## Layer 2: Dense Pressed Cathode Plates * Material: 75.76 g per plate of 75% MnO₂ / 25% Carbon. Add a binder of about 3% to 5% PTFE (as a water dispersion), on top of the 75:25 mix, so the thin plates hold together. Loose powder will crumble at 1.32 mm. The binder adds only about 4% to the cathode volume. * Densifying: Press each plate to 1.32 mm total thickness (about 0.56 mm of mix per face over a 0.2 mm mesh), which brings the porosity to about 50%. * Pressing method: Pressing a full 6.64" × 7.20" plate flat to this density needs tens of tonnes of force. Use one of these instead: * Roll it: Make the mix into a dough-like sheet with the PTFE, run it through a roll press (calender) to the per-face thickness, then roll it onto both sides of the mesh. This is how commercial thin electrodes are made, and the force is concentrated on a narrow line. * Press in tiles: Press smaller tiles in a hydraulic shop press and lay several side by side on each mesh. * Check the density: Each finished plate should weigh about 75.8 g (dry mix, plus the mesh and binder) at 1.32 mm thick. A thicker plate is less dense. ## Layer 3: Polypropylene Felt Separators * Material: About 1.3 mm PP felt treated with Triton X-100 (see the surfactant section), which compresses to 0.93 mm in the stack (about 28%). * Placement: Fold each felt sheet into a U-shaped envelope around each cathode plate, covering both faces and the bottom edge. Leave about 3 mm of felt overhang past the plate edges on the sides and top so zinc can't grow around the edges. ## Layer 4: Zinc Plates * Material: 0.20 mm 99% Zinc sheet, 9 plates per cell. * Each zinc plate has a tab on the opposite side of the top edge from the cathode tabs (for example the right half), so the two tab groups never touch. * Total zinc is 396.6 g per cell (3.97 kg / 8.74 lbs for the pack). Only about 22% of it is used in a full 70 Ah discharge, which is good for cycle life but adds weight. Using 0.15 mm zinc would save about 1 kg per pack and give each felt layer about 0.03 mm more room, but the plates are flimsier. ------------------------------ ## Assembly Sequence for the 10-Cell Pack 1. Prep the Cases: Construct or mold your 10-chamber polypropylene case with solid internal partition walls dividing each cell. 2. Build the Cathode Plates: Make the PTFE-bound cathode sheets and roll or press them onto each Ti mesh (8 per cell, 80 total). Weigh and measure every plate. Slip each finished plate into its felt envelope. 3. Stack Each Cell Outside the Case: Starting and ending with a zinc plate, alternate Zn, enveloped cathode, Zn, and so on (9 zinc, 8 cathode). Squeeze the stack to 27.2 mm and hold it with a temporary clamp. 4. Gang the Tabs: Weld the 8 cathode tabs to a titanium strap, and solder or weld the 9 zinc tabs to a zinc or brass strap. Keep both straps above the electrolyte line. 5. Insert the Stacks: Slide each clamped stack into its chamber and release the clamp so the felt stays compressed against the walls. 6. The Inter-Cell Connections (Series Link): Connect the cathode strap of Cell 1 over the partition wall to the zinc strap of Cell 2 with a bolted or welded stainless/titanium bridge. Repeat this down the line for all 10 cells. (A lead-tin cast-on strap won't bond to titanium.) 7. BMS Integration: Tap a thin voltage-sensing wire directly onto each inter-cell bridge so the Battery Management System can monitor the exact voltage of each individual cell. 8. Flood and Seal: Fill the chambers with your modified Zinc Sulfate (ZnSO₄) electrolyte until the liquid just covers the top of the felt (below the straps), then seal the top lid with pressure relief valves installed. ## Expected Effect on Internal Resistance (Estimate) | Per Cell | Single Thick Cathode | 4-Plate Stack | 8-Plate Dense Stack | |---|---|---|---| | Cathode porosity | 74% | 74% | 50% | | Cathode/zinc facing area | 617 cm² | 2,468 cm² | 4,937 cm² | | Ion path through cathode (per face) | 10.3 mm | 2.57 mm | 0.66 mm | | Felt thickness per gap | 3.0 mm | 0.70 mm | 0.93 mm | | Separator ionic resistance | about 15 mΩ | about 0.9 mΩ | about 0.6 mΩ | | Cathode ionic resistance | about 17 mΩ | about 1.1 mΩ | about 0.25 mΩ | These are rough estimates (2M ZnSO₄ at about 5 S/m). Contacts, tabs, straps, and the electrode reactions add more on top of these figures, and with this stack they become the largest part. Expect around 1.5 to 2.5 mΩ per cell (15 to 25 mΩ per pack), good for very roughly 200 to 300 A bursts at room temperature if the tabs and straps are heavy. That is still short of 760 CCA, especially in the cold. Would you like assistance in drafting a test plan for the first few charge/discharge cycles to safely break in the MnO₂ cathode, or do you need help designing the venting layout to handle potential gas buildup? Here is the Formation Protocol (Break-In Test Plan) and the Venting System Layout designed to safely initialize your custom 12V 70Ah Aqueous Zn-MnO₂ battery. ------------------------------ ## Part 1: Cathode Formation & Break-In Test Plan Freshly pressed $\text{MnO}_2$/Carbon powder cathodes require a low-current "formation" period. This initial wetting and slow cycling allows the aqueous electrolyte to deeply penetrate the micro-pores of the carbon matrix, sets up a stable solid-electrolyte interface (SEI) on the Zinc sheet, and prevents structural fracturing of the cathode jacket from rapid expansion. ## Phase 1: The Rest Period (Wetting) * Action: After filling the battery with the electrolyte, let the battery sit entirely idle for 24 hours. * Purpose: Allows the hydrophobic polypropylene felt and dense carbon powder mix to fully absorb the liquid solution before any electrical current passes through. * Success Metric: Measure the Open Circuit Voltage (OCV) of the pack. It should naturally stabilize between 12.5V and 13.0V. ## Phase 2: Cycle 1 & 2 (Low-Current Conditioning) To protect the crystal lattice of the cathode, these initial cycles are limited to a very shallow 20% Depth of Discharge (DoD). * Discharge: Apply a constant current of 1.4 Amps (0.02C). Stop discharging as soon as 14 Ah has been drawn from the battery, or if any single cell drops below 1.05V. * Charge: Charge at a constant current of 1.4 Amps. Stop charging when the pack voltage hits 14.3V (1.43V per cell) and allow it to rest for 1 hour. ## Phase 3: Cycle 3 & 4 (Target Capacity Stepping) Step up the conditioning to your maximum operating envelope of 50% DoD. * Discharge: Apply a constant current of 3.5 Amps (0.05C). Stop discharging when 35 Ah has been drawn, or if any single cell drops below your hard cutoff of 0.90V. * Charge: Charge using a Constant Current / Constant Voltage (CC/CV) profile. * CC Stage: Charge at 3.5 Amps until the pack hits 14.5V. * CV Stage: Hold the voltage tightly at 14.5V until the current naturally tapers down to 350mA. Stop charging immediately. ------------------------------ ## Part 2: Venting Layout for Gas Buildup Management During recharging, minor inefficiencies or slight overvoltage spikes will cause the water in your electrolyte to experience electrolysis. This splits the water into Hydrogen ($\text{H}_2$) and Oxygen ($\text{O}_2$) gases. Because $\text{H}_2$ is highly flammable, pressure management is critical. A Valve-Regulated (VRLA) Manifold Venting Design built directly into the battery lid is the safest choice for this 10-cell footprint. [Cell 1] [Cell 2] [Cell 3] ... [Cell 10] │ │ │ │ ◄── (Individual Gas Splash Baffles) └─────────┴─────────┴─────────────┴───► [Common Internal Manifold Channel] │ [Rubber Bunsen/EPDM Valve] │ [Flame Arrestor Filter] │ └──► (Atmosphere Vent) ## 1. Individual Cell Splash Baffles * Directly above each of the 10 cell chambers, mold an angled, zig-zag splash baffle directly into the underside of the top lid. * This ensures that any violent bubbles or liquid electrolyte splashing up during charging hits the baffle and drips right back down into the chamber, while allowing free gas to escape upward. ## 2. Common Internal Manifold Channel * Above the splash baffles, run a single sealed horizontal channel (manifold) molded across the length of the 11.0" top cover. * This channel collects the gas escaping from all 10 individual cells into one central chamber, eliminating the need to install 10 separate physical valves. ## 3. Pressure Relief Valve (The Vent Core) * At the exit terminal of the manifold channel, install a one-way EPDM Rubber Bunsen / Umbrella Valve. * Pressure Calibrations: The valve must remain hermetically sealed during normal operation to prevent atmospheric oxygen from entering and drying out or self-discharging the zinc sheets. It should be calibrated to pop open only when internal pressure hits 1.5 to 2.5 psi (0.10 to 0.17 bar) to cleanly dump excess pressure. ## 4. Flame Arrestor Integration * Directly on the exterior side of the EPDM rubber valve, press-fit a porous Sintered Polypropylene or Ceramic Flame Arrestor disk. * Because hydrogen gas is vented, this porous disk acts as a thermal barrier. If an external spark occurs near the battery, the disk safely cools the flame and prevents an external ignition from traveling backwards through the valve into the hydrogen-rich interior of the battery. To finalize the design before physical assembly, let me know: * Would you like a recommendation on a specific surfactant or wetting agent to treat the polypropylene felt separator? * Do you need help calculating the exact volume of liquid electrolyte (in milliliters) required to fill the internal space of the 10 chambers? ## 1. Surfactant / Wetting Agent Recommendation Raw nonwoven polypropylene (PP) felt is inherently hydrophobic, exhibiting a water contact angle greater than 90°. Without chemical modification, an aqueous electrolyte will sit on top of the felt rather than soaking through, skyrocketing the battery's internal resistance. [1] For a custom aqueous zinc battery build, you have two primary options: ## Option A: Pre-treated Industrial Membrane (Highly Recommended) If you are sourcing materials commercially, skip raw felt and use an industry-standard hydrophilic surface-modified microporous polypropylene such as Celgard 3501 or Celgard A273. These are monolayer PP membranes pre-grafted with a proprietary hydrophilic surfactant layer. They completely eliminate dry spots and allow spontaneous, immediate wet-out in water-based zinc salt solutions. ## Option B: Coating Raw Polypropylene Felt From Scratch If you are modifying standard raw PP felt yourself, treat it with Triton X-100 (Polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether). * * Why it works: Triton X-100 is a non-ionic surfactant. Unlike ionic soaps, non-ionic surfactants will not breakdown, react with, or alter the chemical charge distribution of your $\text{Zn}^{2+}$ or $\text{SO}_4^{2-}$ electrolyte ions. * Application Protocol: Prepare a mild 0.1 wt% to 0.5 wt% solution of Triton X-100 in Isopropyl Alcohol (IPA). Submerge your PP felt sheets into this mixture for 5 minutes, remove them, and allow the IPA to fully evaporate. The non-polar segments of the surfactant chain bind permanently to the PP felt backing, leaving a microscopic layer of hydrophilic, water-loving tails exposed to lock down the aqueous electrolyte. * ------------------------------ ## 2. Electrolyte Volume Calculation To find the precise liquid volume needed, we must account for the geometric space of the cell chamber and subtract the true physical displacement of the solid active materials. The remaining volume represents the internal "void spaces" (porosity) of the components that must be entirely flooded with liquid. [2] ## A. Single Cell Material Space Analysis * * Individual Chamber Volumetric Footprint: 1.07" (L) × 6.64" (W) × 7.20" (H) = $838.27 \text{ cm}^3$ (or mL) * The Anode Displacement: The 396.6 grams of pure solid Zinc sheet per cell (9 × 0.20 mm plates, Density: $7.14 \text{ g/cm}^3$) displaces $55.54 \text{ mL}$ of volume. * The Cathode Void Volumetric Space: The dry mass (454.54g $\text{MnO}_2$ + 151.51g Carbon) is pressed to 50% porosity and split across 8 cathode plates (10.54 mm total thickness), occupying an overall spatial volume of $325.06 \text{ mL}$. The absolute atomic volume of those powders combined takes up $162.53 \text{ mL}$, leaving $162.53 \text{ mL}$ of pore space for liquid electrolyte within the cathode plates. * The Separator Void Volumetric Space: Pushed into the remaining chamber gaps, the wet-treated PP felt compresses slightly. Assuming a standard industrial nonwoven felt porosity of 75%, the 16 felt layers (0.93 mm each, 458.0 mL total) will hold $343.53 \text{ mL}$ of liquid. * Top Headspace Pool: Allowing a baseline 0.25-inch clear safety reservoir above the core elements inside the chamber adds $29.11 \text{ mL}$ of open liquid pool area. * (Superseded for the build: the replaceable module design needs about 359 mL per cell / 3.59 L per pack. See "Replaceable Cell Module Design" at the end of this note.) ## B. Final Volumetric Matrix Breakdown | Parameter Area | Single Cell Volume | Total Pack Volume (10-Cell String) | |---|---|---| | Cathode Plate Pores | 162.53 mL | 1,625.30 mL | | PP Felt Matrix Absorption | 343.53 mL | 3,435.30 mL | | Upper Headspace Pool | 29.11 mL | 291.10 mL | | Total Liquid Electrolyte Needed | 535.17 mL | 5,351.70 mL ($\approx$ 5.35 Liters; the 6.41 L recipe below leaves about 1 L spare for top-ups) | Assembly Advice: When filling the battery, pour roughly 535 mL of electrolyte per cell slowly across multiple small stages. Let each stage sit for 15 minutes to allow capillary action to fully push the solution deep into the cathode pores before topping it off to the final headspace margin line. Would you like assistance in formulating the exact mass balance checklist (in grams) to mix all 6.41 liters of the optimized Zinc Sulfate / Manganese Sulfate electrolyte recipe? formulating the exact mass balance checklist (in grams) to mix all 6.41 liters of the optimized Zinc Sulfate / Manganese Sulfate electrolyte recipe To mix exactly 6.41 liters of the optimized rechargeable electrolyte solution, we will prepare a high-performance 2M Zinc Sulfate ($\text{ZnSO}_4$) + 0.1M Manganese Sulfate ($\text{MnSO}_4$) aqueous chemistry. When preparing this solution, it is highly recommended to use heptahydrate and monohydrate crystalline salts, as they dissolve much more readily in water than completely anhydrous powders. The calculations below account for the water weight bound inside those crystal lattices. ------------------------------ ## The Electrolyte Mass Balance Checklist To ensure absolute precision during your build, measure out the following exact masses using a digital scale: | Material Component | Concentration | Mass in Grams (g) | Mass in Ounces (oz) | |---|---|---|---| | Distilled Water ($\text{H}_2\text{O}$) | Base Solvent | 5,100.0 g | 179.90 oz | | Zinc Sulfate Heptahydrate ($\text{ZnSO}_4 \cdot 7\text{H}_2\text{O}$) | 2.0 M (Active Ion) | 3,685.9 g | 129.96 oz | | Manganese Sulfate Monohydrate ($\text{MnSO}_4 \cdot \text{H}_2\text{O}$) | 0.1 M (Cathode Stabilizer) | 108.3 g | 3.82 oz | | Total Combined Mass | Final Prepared Weight | 8,894.2 g | 313.68 oz | Note: The total volume will equal exactly 6.41 Liters because the high concentration of dissolved heavy metal salts increases the final solution's density to approximately 1.39 g/mL. ------------------------------ ## Safe Mixing & Preparation Protocol 1. Equip Safety Gear: Put on nitrile gloves, a dust mask, and protective safety glasses. While these salts are vastly safer than traditional sulfuric lead-acid battery fluid, they are severe eye irritants and can cause skin irritation in high concentrations. 2. Measure the Water First: Pour exactly 5,100 grams (or 5.10 Liters) of pure distilled water into a heavy-duty plastic mixing carboy or glass beaker. Do not use tap water, as trace minerals like chlorine or iron will permanently poison the zinc plates. 3. Dissolve the Manganese Sulfate: Add the 108.3 grams of Manganese Sulfate Monohydrate to the water first. Stir thoroughly until the crystals are entirely dissolved and the liquid turns a very faint, clear pinkish hue. 4. Gradually Blend the Zinc Sulfate: Slowly add the 3,685.9 grams of Zinc Sulfate Heptahydrate in 3 or 4 separate batches, stirring continuously. * What to expect: Dissolving this volume of zinc salt is an endothermic reaction, meaning the liquid will physically turn noticeably cold as you mix it. Keep stirring until the solution becomes completely clear and free of any undissolved white powder at the bottom. 5. Rest and Check: Let the fully mixed liquid rest for 30 minutes to return to room temperature before pouring it evenly across your 10 battery cells. ------------------------------ ## Replaceable Cell Module Design Each of the 10 cells is a separate sealed module that slides into the outer case. A failed or worn cell can be unbolted, lifted out, and replaced without opening the other 9 cells or draining the pack. ## 1. Outer Case Layout (11.0" × 6.94" × 7.50") * Outer case: 0.15" (3.81 mm) polypropylene walls and floor, with no internal partition walls. The inside is 271.8 mm long × 168.6 mm wide. * Module slots: 10 slots at 26.88 mm pitch, plus a 3.0 mm end compression plate (10 × 26.88 + 3.0 = 271.8 mm). * Guide grooves: Molded into the two long side walls (front and back) for each slot, 1.5 mm deep. * Polarity keying: Each module has one wide guide rail (on its + end) and one narrow rail. Odd-numbered slots have the wide groove at the front, and even slots have it at the back. This forces modules to alternate orientation so the series links are short and straight, and a module can't be installed backwards. * End compression plate: A 3.0 mm stiff plate (glass-filled PP or stainless) at one end, pushed by 2 nylon-tipped screws through the end wall. Tightening it clamps the 10 modules together, so neighbors support each other's thin walls against the felt compression force. * Height (assumes the 7.50" height includes the top cover but not the main terminal posts): 3.8 mm floor + 175 mm module + 11.7 mm for link bars and the snap-on top cover = 190.5 mm (7.50"). ## 2. Cell Module Box | Part | Specification | |---|---| | External size | 26.88 mm × 167.6 mm × 175 mm (plus guide rails) | | Side walls | 1.2 mm PP (injection molded, or hot-plate welded PP sheet) | | Bottom | 2.0 mm PP with 3 mm tall support ribs inside (plates rest on the ribs, and sludge collects between them) | | Lid | 3.0 mm PP, hot-plate welded to the box after the stack is installed (permanently sealed) | | Inside size | 24.48 mm × 165.2 mm × 170 mm | | Lift tabs | Molded finger lip on each end of the lid for pulling the module out | Each module lid carries: * Two bolt-hole terminal lugs (no posts). See section 5. * One vent valve: Its own EPDM umbrella valve (1.5 to 2.5 psi) with a splash baffle molded under the lid. This replaces the shared internal manifold, since the modules are now sealed separately. * One fill port: A threaded plug for filling and topping up electrolyte. ## 3. Internal Stack (Per Module) The stack uses 5 cathode plates instead of 8 so the felt can be thicker (0.88 mm instead of 0.51 mm), which gives more protection against zinc dendrites. The active material masses (and the 70 Ah target) are unchanged. [Wall] Zn | F | C | F | Zn | F | C | F | Zn | F | C | F | Zn | F | C | F | Zn | F | C | F | Zn [Wall] Inside height budget (170 mm): 3 mm bottom ribs + 140 mm plates + 3 mm felt overhang + 5 mm electrolyte above the felt + 19 mm for tabs, straps, and lug feedthroughs. | Plate | Count | Size | Thickness | Mass Per Cell | |---|---|---|---|---| | Cathode plate (75% MnO₂ / 25% Carbon + 3-5% PTFE, 50% porosity, Ti mesh) | 5 | 160 × 140 mm | 2.90 mm | 606.05 g mix (121.21 g each) | | Zinc plate (99% Zn) | 6 | 160 × 140 mm | 0.20 mm | 191.9 g (32.0 g each) | | PP felt separator (envelope around each cathode) | 10 layers | 165 × 146 mm (flat pattern 165 × 290) | 0.88 mm (about 1.2 mm before compression, about 27%) | - | Width budget: 5 × 2.90 mm + 6 × 0.20 mm + 10 × 0.88 mm = 24.48 mm. * About 44% of the zinc is used in a full 70 Ah discharge. * Trade-off: Fewer, thicker cathode plates and thicker felt raise the estimated ionic resistance to about 1.2 mΩ (separator) + 1.2 mΩ (cathode) = 2.4 mΩ per cell, up from about 0.9 mΩ with 8 plates. Expect roughly 100 to 200 A bursts at room temperature instead of 200 to 300 A. * Middle option: 6 cathode plates (2.42 mm) with 0.71 mm felt gives about 1.7 mΩ per cell. ## 4. Electrolyte Per Module | Item | Per Module | Pack (10 Modules) | |---|---|---| | Filled space (up to 5 mm above the felt) | 610.7 mL | 6,107 mL | | Minus solids (cathode powder, zinc, felt fibers, mesh) | 251.5 mL | 2,515 mL | | Liquid electrolyte needed | about 359 mL | about 3.59 L | Batch for 3.8 L (about 5% extra): 2,185.5 g Zinc Sulfate Heptahydrate + 64.2 g Manganese Sulfate Monohydrate, dissolved in distilled water and then topped up to exactly 3.8 L. Use the same mixing steps as the 6.41 L recipe above. This is a lean amount of electrolyte: a full 70 Ah discharge uses up roughly 60% of its dissolved zinc as zinc hydroxide sulfate forms on the cathode, so don't discharge past the 50% DoD limit. ## 5. Terminal Lugs, Series Links, and Top Cover * Terminal lugs (2 per module): Flat bars that come up through a sealed slot in the lid, bend over, and lie flat on a molded boss on the lid. Each has a 6.5 mm bolt hole over a captive M6 stainless hex nut pressed into the boss. * + lug: Titanium, 4 × 25 mm, welded inside to the titanium cathode strap (titanium to titanium welds cleanly, while titanium to stainless or copper welds are brittle). Titanium conducts poorly, so keep the lug short. About 0.13 mΩ for a 30 mm long lug. * − lug: Nickel-plated brass, 3 × 20 mm, soldered or welded inside to the zinc strap. * Lid seal: Where each lug passes through, use a molded well filled with epoxy around the lug, plus an EPDM gasket under the boss. * Mark + and − on the lid next to each lug. * Link bars: Short nickel-plated copper bars, 3 × 15 mm, with two bolt holes. Each bar spans from the + lug of one module to the − lug of the next, with an M6 stainless bolt and spring washer into each captive nut. They sit outside the sealed modules, so copper is safe here. * BMS sense wires: Ring lugs under each link bar bolt (one per inter-module link, plus the two pack ends). * Main terminals: The pack + and − posts are on the outer top cover, connected to the end modules' lugs with heavy bars. * Top cover: A snap-on or screwed PP cover over the link bars and module lids, with gas channels collecting from all 10 module vents into one outlet with the sintered flame arrestor. * Hold-down: The top cover presses down on the module lids so the modules can't lift when the pack is moved. ## 6. Module Weight (Estimate) About 1.5 kg per module: 606 g cathode mix + about 25 g binder + 192 g zinc + about 40 g Ti mesh + about 48 g felt + about 500 g electrolyte + about 110 g box, lid, lugs, and straps. ## 7. Replacing a Module 1. Disconnect the load and charger, then remove the main terminal links. Use insulated tools. 2. Remove the top cover. 3. Unbolt the two link bars on the module being replaced, and remove its BMS sense lugs. 4. Back off the end compression plate screws. 5. Lift the module straight up by its lift tabs. 6. Prepare the replacement: It must already be filled, sealed, and broken in (run the formation cycles on the single module at the same currents scaled to 1 cell: 1.4 A and 3.5 A, with per-cell voltage limits). 7. Match the charge: Charge or discharge the new module until its resting voltage is within 0.02 V of the other modules' average. 8. Insert it (the keyed rails only allow the correct orientation), retighten the end plate screws, and bolt the link bars and sense lugs back on at the same torque as the others. 9. Refit the cover, reconnect, and let the BMS balance the pack over the next charge. A new module next to aged modules will have more capacity than they do. The pack is still limited by its weakest module, so if several modules have faded, replace them together. Multi-Cell Blocks: Series and Parallel Layouts ============================================== The printed case (cell_module_case.scad, Rev D) now takes two parameters: cells_series (S) sealed compartments in one block, wired in series cells_parallel (P) plate stacks sharing each compartment Parallel cells are at the same voltage, so they share ONE compartment and ONE electrolyte. A 2P compartment holds two standard plate kits side by side (10 cathodes, 12 zinc), with 718 mL of electrolyte, one vent and one pair of lugs. All cathode tabs go to the Ti riser and all zinc tabs to the brass riser. Series cells are at different voltages, so each needs its own sealed compartment. Partitions (2.4 mm) are printed solid with the box and are never notched or drilled. The epoxy in the lid lip gaps seals each partition top. Every compartment keeps its own vent, fill port and two lugs through the lid. Alternate compartments are turned 180 degrees, so one cell's - lug sits beside the next cell's + lug. A raised link pad joins the two bosses, and a short Ni-brass link bar bolts across it. The links stay outside the sealed space, so every cell can still be measured by the BMS. Why the links are on top of the lid: the headspace above the plates is only a few mm, so a connector through or over a partition would sit at or below the electrolyte line. On top of the lid it stays dry. The block thickness is always S x P x 26.88 mm, so any block fits the same slotted outer case. Rails repeat at every slot, and spare thickness goes into the side walls (1.2 mm for 1P, 2.4 mm for 1S2P, 3.6 mm for 2S2P). Layout Thickness Nominal Electrolyte/comp. PETG (box+lid, approx.) 1S1P 26.9 mm 1.3 V 359 mL 140 g 2S1P 53.8 mm 2.6 V 359 mL 280 g 1S2P 53.8 mm 1.3 V 718 mL 275 g 2S2P 107.5 mm 2.6 V 718 mL 550 g Pack options with 20 cells (40-70 Ah per cell estimate, unverified): 10S2P ~13 V, 80-140 Ah, 1.0-1.8 kWh: 5 x 2S2P blocks (recommended) 2 x 10S strings in parallel: same totals; fuse each string; 20 BMS taps 20S ~26 V, 40-70 Ah: charger ~30 V (1.50 V/cell) or ~36 V (1.80 V/cell) Trade-off of shared parallel compartments: a dendrite short in one stack also drains the plates beside it, so the whole compartment is lost. The BMS must catch it from the rise in self-discharge, because a slow soft short never blows a fuse. STLs: stl/cell_case_{1s1p,2s1p,1s2p,2s2p}_{box,lid}.stl Life Expectancy in Solar Storage (estimate, unverified) ======================================================= End of life = 80% of starting capacity. About 1 cycle per day in solar use (cloudy days give shallow cycles). Based on published Zn-MnO2 work, not on tests of this design. The coupon tests will replace these numbers. Scenario Cycles to 80% Daily use First build, still learning 100-200 4-8 months Good build at 50% depth of discharge 300-500 1-1.5 years 30% depth, tuned charge voltage, maintained 800-1,500 2-4 years LiFePO4, for comparison 3,000-6,000 8-15 years Published results of thousands of cycles come from thin electrodes, tiny loadings and fast rates, which hide the slow damage. Thick plates at slow solar rates age faster. What wears the cell out (roughly most to least important): 1. Zinc reshaping and dendrites. Each cycle uses ~45% of the zinc's theoretical capacity (at 70 Ah). Plating back is uneven: edges thin, spikes grow, then a short or capacity loss. Likely life limiter. 2. Mn dissolving from the cathode. MnSO4 slows it but doesn't stop it. 3. Zinc hydroxide sulfate build-up clogging cathode pores (rising resistance). 4. Hydrogen gassing and water loss from slow zinc corrosion. Dries the electrolyte and drifts pH, which caps calendar life at a few years even with light use. 5. Case: PETG creep under stack pressure and heat, ageing epoxy seals. Probably 5+ years, so not the first thing to fail. What helps most: - Shallower depth of discharge: size the pack so a normal day uses 30% or less. Doubling the pack can more than double the cycles. - Keep it at 15-25 C. Ageing roughly doubles for every 10 C rise. - Set the charge voltage correctly (the 1.50 vs 1.80 V coupon test). - Charge slowly. Solar is gentle; avoid fast generator top-ups. - Top up with distilled water through the fill port every few months. - Rebuild instead of replacing: swap the zinc plates and felt (~$10-20 per module) and reuse cathodes that still test well. With rebuilds the pack could last many years, even if each set of zinc plates lasts only one. Estimating life from the coupon tests: cycles to 80% = 0.20 / (fade per cycle) 0.1%/cycle -> ~200 cycles (~7 months) 0.05%/cycle -> ~400 cycles (~1 year) 0.02%/cycle -> ~1,000 cycles (~3 years) Ten cycles is too few to measure small fade. Run at least one coupon for 50-100 cycles. One cycle takes about 1.5 days at 28 mA, so 50 cycles take ~2.5 months and 100 cycles ~5 months. Verdict: lifetime cost per kWh can't beat LiFePO4 unless rebuilds prove cheap and easy. Strengths: non-flammable, cheap non-toxic materials, repairable. Good for learning and backup; as main solar storage, expect regular rebuilds. Coupon Test Cells (what a "coupon" is) ====================================== A coupon is a miniature test cell: a small square cut from the full design, with the same materials and the same plate thickness. It tests the chemistry cheaply and quickly before building a full-size cell. ("Coupon" is the usual engineering word for a small test sample.) Full procedure: build_plan_test_cell.md, phase 2. One coupon: Cathode 1 x 30 x 30 mm, 2.90 mm thick (same as a full plate), 4.87 g mix = 3.65 g MnO2 + 1.22 g carbon + 0.2 g PTFE, pressed onto Ti mesh with a Ti tab (+). Zinc 2 x 30 x 30 mm, 0.20 mm, one each side of the cathode; tabs joined as the - terminal. Separator the same treated PP felt each side (+ cellophane on C6). Housing printed from coupon_test_cell.scad: PETG cup (54 x 28 x 62 mm inside), clamp plates that set the full-cell stack gap, lid with 3 tab slots and a vent. The press die in the same file forms the cathode at the right density. Electrolyte 2 M ZnSO4 + 0.1 M MnSO4, a few tens of mL. Why coupons first: - Cheap: 3.65 g MnO2 per coupon vs ~455 g per full cell. - Fast: six coupons in an afternoon vs a week for a full cell. - Six variables side by side (the test matrix): C1 lab-grade MnO2, 1.50 V C2 lab-grade, 1.80 V C3 MTI battery-grade, 1.50 V C4 MTI battery-grade, 1.80 V C5 pottery grade, 1.80 V C6 lab-grade + cellophane, 1.80 V - Low risk: a failure costs a few grams of material. Scaling to the full cell: Active area ratio = (10 faces x 224 cm2) / (2 faces x 9 cm2) = 124. Full-cell capacity ~ coupon capacity x 124 (0.56 Ah coupon -> ~70 Ah). Test current 28 mA = 1.56 mA/cm2, the same density as 3.5 A in a full cell. Test rig: six channels run all coupons at once (coupon-test-rig-wiring.html, coupon_rig/coupon_rig.ino, coupon_rig/coupon_logger.py). One cycle takes ~1.5 days, so the 10-cycle test takes ~2 weeks. The results decide the charge voltage (1.50 vs 1.80 V), the MnO2 grade and whether to use cellophane, before the first full-size cell is built.