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Build Battery Packs
Cells
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
Cost
- Me: $497+$774+$390=$1660
- Milwaukee (with worse cells): $279*8=$2232
Assembly
DIY Charger
The 5 Key Data Points to Gather
Parameter
Why It Matters
Target Variance
1. Discharge Capacity ($\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
