Understanding Battery Specs: mAh, C-Rating, and Voltage Explained
Battery datasheets are full of numbers — but what do they actually mean? We explain capacity, discharge rate, nominal voltage, and cutoff voltage in plain language.
When you look at a battery datasheet or product listing, you'll see a wall of numbers — 5000mAh, 3.6V nominal, 45A continuous, 2.5V cutoff. Understanding what these mean helps you choose the right cell for your application and avoid damaging your batteries. Here's a plain-language guide.
Capacity — mAh and Wh
Capacity is measured in milliampere-hours (mAh) or watt-hours (Wh). A 5000mAh cell can theoretically deliver 5000mA (5A) for one hour, or 1A for five hours. In practice, capacity decreases as discharge current increases — a cell rated at 5000mAh at 0.2A might only deliver 4500mAh at 5A.
Watt-hours (Wh) is a more useful measure when comparing cells of different voltages. Wh = mAh × nominal voltage ÷ 1000. A 5000mAh cell at 3.6V nominal = 18Wh.
Voltage — Nominal, Fully Charged, and Cutoff
| Voltage Point | Typical Value (Li-ion) | What It Means |
|---|---|---|
| Fully charged | 4.2V | Maximum safe charge voltage |
| Nominal | 3.6–3.7V | Average voltage during discharge — used for capacity calculations |
| Storage | 3.7–3.8V | Ideal voltage for long-term storage |
| Cutoff / minimum | 2.5–3.0V | Do not discharge below this — causes permanent damage |
Continuous Discharge Rating (CDR)
The continuous discharge rating (CDR) is the maximum current the cell can safely deliver indefinitely without overheating. It is measured in amps (A). A cell rated at 45A CDR can deliver 45A continuously — though in practice, sustained high-current discharge generates heat and reduces cycle life.
Pulse discharge ratings are higher than CDR — a cell might handle 60A for a 10-second pulse even if its CDR is 45A. Pulse ratings are relevant for applications with short bursts of high current (like a power tool trigger pull).
C-Rating
C-rating is a relative measure of discharge current as a multiple of capacity. 1C = discharge the full capacity in one hour. For a 5000mAh cell: 1C = 5A, 5C = 25A, 9C = 45A. C-ratings are more commonly used for LiPo batteries (RC cars, drones) than for cylindrical Li-ion cells, where absolute amp ratings are more common.
Internal Resistance (IR)
Internal resistance is measured in milliohms (mΩ). Lower IR means the cell can deliver high current with less voltage sag and less heat generation. A fresh, high-quality 21700 cell typically has an IR of 10–20mΩ. As cells age and cycle, IR increases — a cell with IR above 50mΩ is approaching end of life for high-drain applications.
When buying cells, always check that the specifications come from the manufacturer's official datasheet — not marketing claims. Battery Gulf lists manufacturer-spec CDR and capacity for every cell we sell.
Putting It Together
When choosing a cell, start with your application's peak current draw and make sure the CDR exceeds it with margin. Then look at capacity for the runtime you need. Finally, check the voltage range is compatible with your device's battery management system (BMS).