Test Procedures

In addition to teardowns, we run 4 electrical tests on our batteries. Each test measures battery characteristics that matter for real-world performance.

The tests:

1. Internal Resistance Testing
   The electrical resistance of a battery reflects its build quality. Higher resistance is caused by low-quality 18650 cells or poor electrical construction, and leads to overheating.
2. Total Capacity Testing
   High-capacity is often the primary advertising point of power tool batteries. In reality, it's only one of a few important electrical characteristics of a pac
3. High-Power Cycling Testing
   We cycle the batteries at high-power (600-800W) to reflect their actual usage in high-load scenarios like circular saw use. We monitor for overheating, which causes tools to "quit out".
4. Run To Failure Test
   We run all batteries at increasingly high current until something fails: cells vent, solder melts, plastic ignites. Batteries that survive the longest are entered into our Hall of Fame scatterplot.

Internal Resistance Test Procedure:

Pack heating losses scale quadratically with current and overheating quickly becomes a problem for poorly constructed power tool batteries. Higher resistance is caused by low-quality 18650 cells and/or poor electrical construction.

Power tool batteries can exceed 100A of current

We can measure R_internal by comparing the voltage drop at two different operating currents. We use I_small = 1A, I_big = 15A:

We take measurements at 17.5V < V_pack < 18.5V because it's the most stable regime for internal resistance. We also spend ~30 mins heating up the packs (by running them at 15A) to reach consistent internal temperatures. Internal resistance is a function of pack voltage and temperature

Total Capacity Test Procedure:

Battery capacity, in watt-hours, is purely a function of the 18650 cells used. Almost all battery vendors report capacity in Ah.

We discharge at low current relative to the battery's Ah capacity, around ~0.2 - 0.5C, to minimize energy lost to heat

$$V_{\text{load}}=V_{\text{oc}}-V_{\text{int}}$$

$$P_{\text{delivered}}=P_{\text{discharged}}-P_{\text{internal}}$$

$$P_{\text{delivered}}=V_{\text{oc}}I-I^2{R}_{\text{int}}$$

$$t_{\text{discharge}}=\frac{Q_{\text{bat}}}{I}$$

$$E_{\text{delivered}}=P_{\text{delivered}}\cdot \frac{Q_{\text{bat}}}{I}$$

$$E_{\text{delivered}}=(V_{\text{oc}}-I{R}_{\text{int}})Q_{\text{bat}}$$

The lower the discharge current, the higher the E_delivered; we discharge at 2A.

High Power Cycling Test Procedure: