How the Formula Works

This calculator estimates how long a LiPo battery will last under a given current draw and tells you the C-rating needed and safe voltage limits.

Discharge Time

Discharge Time (hours) = (Battery Capacity (mAh) / 1000) / Current Draw (A)

The capacity in ampere-hours is divided by the total current draw to give flight duration in hours. Multiply by 60 for minutes.

C-Rating Needed

C-Rating Needed = Current Draw (A) / Battery Capacity (Ah)

This tells you the minimum discharge C-rating your battery must have to safely supply the current.

Safe Voltage Limits

Storage Voltage = 3.8V × Number of Cells
Min Safe Voltage = 3.3V × Number of Cells
  • 3.8V/cell: Safe storage voltage to preserve battery health
  • 3.3V/cell: Absolute minimum before battery damage risk

Example

  • Battery: 2200 mAh, 4S
  • Current Draw: 40A
Discharge Time = 2.2 / 40 = 0.055 hours = 3.3 minutes
C-Rating      = 40 / 2.2 = 18.2C
Storage       = 3.8 × 4 = 15.2V
Min Safe      = 3.3 × 4 = 13.2V

FAQs

1. Why does my battery drain faster than calculated?
Discharge time assumes constant current. In reality, motors draw more current during acceleration, climbing, and wind resistance, draining the battery faster.

2. What does the C-rating mean?
The C-rating on a battery indicates its maximum safe discharge rate. A battery rated 25C can safely discharge 25 times its capacity in amps. Higher C-ratings are needed for high-current applications.

3. What happens if my C-rating is too low?
The battery overheats, voltage sags severely, and the battery may puff or catch fire. Always buy batteries with a C-rating exceeding your required C-rating.

4. What is the safe storage voltage?
Store LiPo batteries at 3.8V per cell. This is typically achieved by discharging to storage voltage after flight, using the storage mode on your charger.

5. What is the minimum safe voltage?
Never discharge below 3.3V per cell (3.0V absolute minimum). Below this, cells can be damaged permanently, reduced in capacity, or become unsafe.

6. How should I dispose of old LiPo batteries?
Discharge fully, then dispose at a battery recycling center. Never throw LiPos in regular trash.

7. How does temperature affect discharge?
Cold temperatures reduce battery performance and increase voltage sag. LiPos perform best between 60-90°F (15-32°C).

Reference Links