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=$2232
Assembly
DIY Charger
- 5 Key Data Points to Gather
| Parameter |
Why It Matters |
Target Variance |
| 1. Discharge Capacity ($\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 |
Equipment for 21700 Cell Profiling
| Metric / Parameter |
Primary Tool Recommendation |
Secondary / Alternative Tool |
Primary Function
1. Discharge Capacity ($\text{mAh}$ / $\text{Wh}$)
SkyRC MC3000
ZKETECH EBD-A20H
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+
RC3563 Milliohm Meter
$1\text{kHz}$ AC 4-wire Kelvin resistance measurement, bypassing contact resistance noise.
3. Self-Discharge ($\Delta V / \Delta t$)
Fluke 87V (or precision DMM)
Benchtop 5.5 / 6.5 digit DMM
High-resolution voltage tracking across a 7- to 14-day resting phase to catch micro-shorts.
4. Voltage Recovery Curve
ZKETECH EBD-A20H
West Mountain Radio CBA V
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
FLIR Thermal Camera
Direct surface temperature tracking during high-amp continuous discharge ($10\text{A}$–$20\text{A}$).
Software & Drivers for Linux Setup
- Serial Communication Interface: Integrated CH340 / PL2303 USB-to-UART bridge mapped natively to
/dev/ttyUSB0.
- Open-Source Controller: Community Python scripts (
pyserial) or ebc-battery-tester (Rust/WebUSB).
- Legacy Windows GUI: Official ZKETECH EB Tester software running under Wine with a
com1 symlink (~/.wine/dosdevices/com1 -> /dev/ttyUSB0).
