Build Battery Packs

Cells

Welder

Battery Case

Cost

Balancer

balancer.jpeg

Assembly

DIY Charger

Cell Information for Packs

Parameter Why It Matters Target Variance
1. Discharge Capacity ($\text{mAh}$ / $\text{Wh}$) Sets the total energy reservoir. The weakest P-group determines when the entire pack's BMS cuts off. $\le 1\%$ variance across parallel groups
2. Internal Resistance ($m\Omega$) Governs voltage sag under load and heat generation. In parallel, low-IR cells do more work; in series, high-IR cells heat up faster. $\le 5\%$ variance across parallel groups
3. Self-Discharge Rate ($\Delta V / \Delta t$) Cells that bleed voltage while idle will constantly pull down their parallel group, overworking the BMS balance circuits. Eliminate outliers (reject self-discharge $> 0.01\text{V}$ / week)
4. Voltage Recovery Curve How fast voltage bounces back after a load is removed. Indicates internal chemical health and degradation. Reject cells that lag significantly behind the group recovery slope
5. Thermal Delta under Load ($\Delta T$) Temperature rise during a sustained high-current discharge. Reject cells that run $> 5^\circ\text{C}$ hotter than the batch average

Equipment for 21700 Cell Profiling

Metric / Parameter Primary Tool Recommendation Primary Function
1. Discharge Capacity ($\text{mAh}$ / $\text{Wh}$) SkyRC MC3000 $\rightarrow$ BLE Wireless $\rightarrow$ ESP32 Dev Board $\rightarrow$ TTL UART $\rightarrow$ MAX3232 $\rightarrow$ Console Server Automated C–D–C (Charge-Discharge-Charge) cycling with custom voltage cutoffs ($2.5\text{V}$–$2.8\text{V}$).
2. Internal Resistance ($m\Omega$) YAOREA YR1035+ $\rightarrow$ TTL to RS232 $\rightarrow$ console server $1\text{kHz}$ AC 4-wire Kelvin resistance measurement, bypassing contact resistance noise.
3. Self-Discharge ($\Delta V / \Delta t$) Fluke 87V High-resolution voltage tracking across a 7- to 14-day resting phase to catch micro-shorts.
4. Voltage Recovery Curve ZKETECH EBD-A20H $\rightarrow$ GX12-4pin Male Aviation Connector $\rightarrow$ ESP32 $\rightarrow$ TTL to RS232 $\rightarrow$ Console Server High-rate real-time logging of voltage rebound immediately after removing discharge load.
5. Thermal Delta ($\Delta T$) K-Type Glass Bead Thermocouple $\rightarrow$ K Type Thermocouple Module $\rightarrow$ MAX31855 $\rightarrow$ ESP32 $\rightarrow$ MAX3232 Direct surface temperature tracking during high-amp continuous discharge ($10\text{A}$–$20\text{A}$).
Component Example Part / Model Source Est. Cost Purpose / Function
10mΩ Resistance Standard Vishay / Dale WSL402610L00FEA or Ohmite FC4L110R010FER (10mΩ, 0.1%, 4-terminal) DigiKey / Mouser ~$4.00 20mΩ Range Calibration. Low AC inductance metal foil resistor for low-IR cell verification.
100mΩ Resistance Standard Vishay Y14880R10000B9R or Ohmite FC4L110R100FER (100mΩ, 0.1%, 4-terminal) DigiKey / Mouser ~$4.00 200mΩ Range Calibration. Calibration standard for higher-IR cells or module assemblies.
Breakout PCB / Terminal Board Custom 4-wire SMD test PCB or generic SOP/SOT adapter board OSH Park / Amazon ~$3.00 Holds the SMT sense resistors cleanly and exposes 4 isolated solder pads for Kelvin clips.
DC Voltage Reference Board AD584LH / AD584KH Precision Voltage Module eBay / AliExpress / Amazon ~$15.00 Voltage Calibration. Outputs 2.500V, 5.000V, 7.500V, and 10.000V with factory DMM calibration slip.
4-Wire Kelvin Test Clips / Leads High-quality gold-plated Kelvin clips Amazon / eBay ~$8.00 Dedicated test harness to ensure zero contact resistance during calibration passes.

Safety Equipment and PPE

Equipment Category Item Function & Primary Hazard Mitigation Essential Specifications
Personal Protection Full-Face Shield Face protection from molten nickel spatter during spot welding and high-current arc flashes. ANSI Z87.1 rated, worn over safety glasses
Class 00 Insulated Gloves Prevents shock and accidental short circuits across exposed high-voltage series strings. Rated for 500V AC / 750V DC working voltage
Multi-Gas/P100 Respirator Respiratory protection against toxic HF (hydrofluoric acid) vapor and organic solvents during cell venting. Organic Vapor / Acid Gas cartridge combination
Bench Infrastructure Flame-Retardant Silicone Mat Heat protection for the workbench during soldering and short-circuit containment. 500°C temperature rating
1000V VDE Insulated Hand Tools Prevents accidental cross-terminal shorts when tightening busbars or installing balance wires. IEC 60900 certified cutters, pliers, and wrenches
Kapton / Polyimide Tape Electrical insulation across cell shoulders, nickel crossovers, and ungrounded sensor probes. High dielectric strength, 260°C thermal limit
Thermal Emergency AVD / Lith-Ex Fire Extinguisher Direct cooling and thermal insulation to interrupt thermal runaway propagation. Aqueous Vermiculite Dispersion agent
High-Temp Battery Fire Blanket Containment cover for active thermal events, containing flaming debris and smothering surface fires. Heavyweight 100% woven high-silica fabric (1000°C+ rating) or 4–6 folded layers of heavy fiberglass
Heavy Aramid / Welding Gauntlets High-temperature hand protection to manually evacuate a venting/hot cell to an outdoor safe zone. Split cowhide or Kevlar/Nomex lining
Cell Storage Vented Steel Ammo Cans Physical containment for unstable, swollen, or quarantine cells. Metal construction with rubber lid gasket removed
Revision #59
Created 2026-09-03 14:42:16 UTC by luKe
Updated 2026-10-04 20:53:03 UTC by luKe