Build Battery Packs
Cells
- DIY500AMP.COM
- Order #1584 2025-09-25
- Ampace JP40 "LT Wrap" 21700 4000mah 70A Battery - Tabless Cell × 120
- Subtotal $474.00 Shipping $22.99 Total $496.99 USD
- Order #3924 2026-09-09
- Ampace JP40 "LT Wrap" 21700 4000mah 70A Battery - Tabless Cell × 1 $5.99
- FEB 21700 Battery Cell – 6500mAh 13A Ultra High Capacity Energy Cell × 30 $224.70
- Subtotal $230.69 Shipping $10.80 Total $241.49
Welder
- Heltec HT-SW02H
- Manual
- Order #114-4340207-9060214 2025-10-30
- Spot Welder & Milliohm Meter
- 42 KW 7000A Capacitor Energy Storage Pulse Welding Machine
- Subtotal $729.90 Tax: $43.79 Total: $773.69
Battery Case
- 15 cell m18 kit (8 batteries)
- $389.20
Cost
- Me: $497+$774+$390=$1660
- Milwaukee (with worse cells): $279*8=$2232
Extra
- Balancer: 2A Active Balancer 2S–24S with Built-in Bluetooth RS485 and Temp Sensor for LiFePO4 Li-ion LTO 20V–100V Smart Equalizer Module (JK-DZ11-B2A24S)
- HD26 connectors: shipping
- male CN-DSUBHD26PN-000
- female CN-DSUBHD26SK-000
- 20 wire ribbon 28AWG
Assembly
DIY Charger
Cell Information for Packs
| Parameter | Why It Matters | Target Variance |
|---|---|---|
| 1. Discharge Capacity ($\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 $\rightarrow$ RS-232 Wire $\rightarrow$ ZPE 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 ($10\text{A}$–$20\text{A}$). |