Calculator Inputs
Use realistic current draw values. Rated C values can vary by brand, temperature, age, and testing method.
Formula Used
Capacity in amp-hours: Ah = mAh ÷ 1000 × parallel packs
Maximum continuous current: I = Ah × continuous C
Maximum burst current: Iburst = Ah × burst C
Required C rating: Crequired = load current ÷ Ah
Required C with headroom: Csafe = load × (1 + headroom ÷ 100) ÷ Ah
Runtime: minutes = usable Ah ÷ load current × 60
Voltage sag: sag = current × pack resistance
Heat loss: watts = current² × resistance
How to Use This Calculator
- Enter the battery capacity shown on the pack label.
- Add the continuous and burst C ratings from the battery label.
- Choose the series cell count, such as 3S, 4S, or 6S.
- Enter your expected continuous and peak current draw.
- Add a safety headroom value for heat, age, and hard throttle use.
- Use internal resistance if you have charger test data.
- Press the calculate button and review the pass or fail result.
- Download the CSV or PDF for build notes.
Example Data Table
| Use Case | Capacity | C Rating | Cells | Load | Estimated Current Limit |
|---|---|---|---|---|---|
| FPV freestyle quad | 1500 mAh | 100C | 6S | 85 A | 150 A |
| RC truck | 5000 mAh | 50C | 2S | 120 A | 250 A |
| Electric glider | 2200 mAh | 35C | 3S | 45 A | 77 A |
Advanced Guide to LiPo C Ratings
What the Rating Means
A LiPo C rating describes how fast a battery can safely discharge compared with its capacity. The number is not a fixed current by itself. It becomes useful only when capacity is included. A 2200 mAh pack is 2.2 Ah. If it has a 35C continuous rating, its theoretical current limit is 77 amps. This calculator turns that label into practical build data.
Why Headroom Matters
Real packs do not always deliver their printed rating. Temperature, age, wiring, connectors, and cell balance can reduce performance. High throttle bursts also create sharp current spikes. A safety margin helps prevent overheating and voltage collapse. Many builders use 15% to 30% headroom for general setups. Racing and heavy lift systems may need more.
Runtime and Voltage Sag
Runtime depends on usable capacity and average current. The calculator subtracts your reserve first. That keeps the estimate closer to safe field use. Voltage sag estimates how much pack voltage drops under load. Sag is affected by internal resistance. Lower resistance usually means stronger punch and less heat. Older packs often sag more than new packs.
Continuous Versus Burst
Continuous current is the safer number for planning. Burst current is only for short moments. It should not be used as the main design limit. If your build needs burst rating to survive normal operation, the pack is too small. Choose a larger capacity, higher quality cells, or a higher C rating.
Choosing a Better Pack
A good pack should pass continuous current, burst current, and sag checks. It should also fit the model weight target. Bigger packs supply more current, but they add mass. Higher C packs can help, but labels can be optimistic. Use this tool as a planning guide. Then confirm performance with temperature checks after real runs.
FAQs
1. What is LiPo C rating?
LiPo C rating shows how quickly a battery can discharge compared with its capacity. Multiply capacity in amp-hours by the C rating to estimate maximum current.
2. Is continuous C rating more important than burst C rating?
Yes. Continuous rating is better for normal planning. Burst rating is only for short spikes and should not support your average load.
3. How much safety headroom should I use?
Use at least 15% to 30% for many hobby builds. Use more for racing, heavy vehicles, old packs, hot weather, or unknown current draw.
4. Why does capacity affect current limit?
C rating is a multiplier. A larger capacity pack has more amp-hours, so the same C rating produces a higher current limit.
5. Does a higher C rating always mean better performance?
Not always. Cell quality, internal resistance, weight, temperature, and honest labeling also matter. A reliable lower C pack can outperform a weak high C pack.
6. What causes voltage sag?
Voltage sag is mainly caused by internal resistance under load. High current, weak cells, cold packs, thin wires, and poor connectors can increase sag.
7. Can I use burst current for motor sizing?
Use burst current only for brief peaks. For motor and ESC sizing, continuous current with safety headroom is the safer planning value.
8. Why is my real runtime different?
Real runtime changes with throttle, propeller load, terrain, temperature, pack age, and reserve level. The calculator gives a planning estimate, not a guarantee.