Battery Basics
Datasheets tell you what a cell should do. Testing tells you what it actually does. Here's how to measure real capacity, discharge curves, and internal resistance — and the exact method behind CellBench's lab data.
To test a lithium cell, fully charge it, then discharge it at a known constant current down to a safe cutoff voltage (typically 2.5V) while logging voltage and time. From that you get its real capacity, its discharge curve, and — with a current step — its internal resistance. Repeat at several currents to see how it behaves under real loads.
Four numbers a datasheet can't give you for your specific cell and your specific load.
The real amp-hours a cell delivers from full to its cutoff voltage at a given current — almost always less than the datasheet headline, and it drops as current rises.
Voltage plotted against capacity as the cell drains. The curve's shape tells you usable runtime and how the cell behaves under your actual load, not a lab-ideal one.
How far voltage drops the instant load is applied. High sag means less usable power for motors and high-drain devices — invisible on a spec sheet.
The cell's opposition to current flow. Lower is better; it drives sag, heat, and efficiency. Rises with age, so it's also a health indicator for used cells.
The same constant-current discharge procedure, whatever equipment you use.
Charge to 4.2V (CC/CV) and let it rest ~30 minutes so the surface charge settles and you start every test from the same true full state.
Use heavy gauge wire for the current path and separate thin sense leads measured right at the cell terminals. This removes lead and contact resistance from your voltage reading — essential for accurate sag and internal-resistance numbers.
Sink a fixed current (say 0.5A, or up to the cell's rated max) and log voltage, current, and time continuously down to a safe cutoff — typically 2.5V for Li-ion. Never over-discharge below the manufacturer's limit.
Integrate current over time for amp-hours (mAh), and current × voltage over time for energy (Wh). Record the cutoff and current so the result is reproducible.
Apply a known current step and divide the voltage drop by the current change (ΔV / ΔI), or use a 1kHz AC milliohm meter. Note the method — DC and AC values differ.
Recharge, rest, and run again at higher currents. One curve isn't enough — a cell that looks great at 0.5A can sag badly at 20A. Sweeping currents is what reveals real-world behavior.
You don't need a lab to start. Here's what each tier buys you in accuracy and detail.
e.g. Liitokala, Opus BT-C3100, SkyRC MC3000
Measures capacity by discharging at a low fixed current (0.5–1A) and showing the mAh. Great for grading cells and rough capacity checks; limited current range and no detailed curve.
Best for: Hobbyists, salvaged-cell grading, quick health checks
e.g. ZKETECH EBC-A20, benchtop USB loads
Programmable constant-current discharge up to ~20A, often with PC logging so you can capture an actual discharge curve. The sweet spot for serious DIY builders.
Best for: DIY pack builders who want real curves on a budget
e.g. Rigol DL3021, Korad, BK Precision — what CellBench uses
High-precision constant-current/-power sink with 4-wire Kelvin sensing, SCPI automation, and sub-millivolt resolution. Repeatable curves across many current levels — research-grade data.
Best for: Reviewers, manufacturers, and anyone publishing trustworthy data
Tier examples are illustrative, not endorsements. The right tool depends on the current range and data fidelity you need.
Full transparency on the lab method behind the data you see on cell pages.
Lithium cells store a lot of energy. Use a proper cell holder, never discharge below the manufacturer's cutoff, size your wiring for the test current, keep an eye on temperature at high currents, and test in a ventilated area away from flammables. A damaged, swollen, or hot cell should be retired safely, not tested.
Charge the cell fully to 4.2V, then discharge it at a known constant current down to a safe cutoff (usually 2.5V) while logging time. Capacity in amp-hours is the current multiplied by the discharge time (integrated if current varies). A smart charger/analyzer does this automatically at low current; a programmable load does it at any current with a full curve.
A discharge curve plots cell voltage against delivered capacity as it drains. Its shape reveals usable runtime, how flat the voltage stays under load, and how much capacity you actually get at a given current. Real curves differ from datasheet curves, especially at high current.
Two common ways: a DC method, where you apply a current step and divide the voltage change by the current change (ΔV / ΔI); or an AC method using a 1kHz milliohm meter. The two give slightly different numbers, so always note which you used. Lower internal resistance means less voltage sag and heat.
At minimum, a smart charger/analyzer ($20–150) that reports capacity. For real discharge curves, a programmable hobby load with PC logging ($50–200). For research-grade, repeatable data, a programmable DC electronic load with 4-wire sensing ($400–900) — the tier CellBench uses for its published data.
For most lithium-ion (NMC/NCA) 18650 and 21700 cells, 2.5V is a common test cutoff, though some manufacturers spec 2.0V. Going lower damages the cell and is unsafe. LiFePO4 cells use a lower cutoff (around 2.0V). Always follow the cell's datasheet.
Because a cell's usable capacity and voltage depend heavily on current. A cell might deliver its full rated capacity at 0.5A but lose 10–20% and sag hard at 20A. Testing across a range of currents (CellBench sweeps 0.5A up to 20A) is the only way to know how a cell performs for your actual application.
We've already tested popular 18650 and 21700 cells across the full current range. Compare real discharge curves or simulate them in a pack.