Calculate battery drain time, capacity, or power consumption from two known values with mAh, Ah, mA, A, hours, or minutes to find the missing value.
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Battery Drain Time Formula
The basic battery drain time calculation uses battery capacity and current draw. Use capacity in milliamp-hours and current in milliamps to get time in hours.
- T = battery drain time in hours
- C = battery capacity in mAh
- I = power consumption or current draw in mA
If you need to solve for battery capacity instead of time, use:
If you need to solve for power consumption instead of capacity or time, use:
The calculator accepts battery capacity in mAh or Ah, power consumption in mA or A, and drain time in hours or minutes. Internally, it converts capacity to mAh, current to mA, and time to hours before applying the formula.
- Calculate drain time: enter battery capacity and power consumption.
- Calculate battery capacity: enter power consumption and desired drain time.
- Calculate power consumption: enter battery capacity and drain time.
Common Battery Capacity and Current Unit Conversions
| Conversion | Equivalent Value | Used For |
|---|---|---|
| 1 Ah | 1000 mAh | Battery capacity |
| 1 mAh | 0.001 Ah | Battery capacity |
| 1 A | 1000 mA | Current draw |
| 1 hour | 60 minutes | Drain time |
Typical Current Draw Examples
| Device or Load | Typical Current Draw | Note |
|---|---|---|
| Small LED indicator | 5 to 20 mA | Depends on LED and resistor |
| Microcontroller board | 20 to 100 mA | Sleep mode can be much lower |
| USB-powered small device | 100 to 500 mA | Varies by device activity |
| Phone charging load | 1 to 2 A | Equals 1000 to 2000 mA |
Example Calculations
Example 1: Calculate battery drain time
You have a 2000 mAh battery and a device that draws 250 mA.
The estimated battery drain time is 8 hours.
Example 2: Calculate required battery capacity
You want a device drawing 0.5 A to run for 3 hours. First convert 0.5 A to 500 mA.
The required battery capacity is 1500 mAh, or 1.5 Ah.
FAQ
Why is the real battery life shorter than the calculated drain time?
The result is an ideal estimate. Real battery life can be shorter because of battery age, temperature, voltage cutoffs, discharge rate, converter losses, and the fact that many devices do not draw a perfectly constant current.
Should I use mAh or Ah for battery capacity?
Either unit works if you keep the units consistent. For small batteries, mAh is common. For larger batteries, Ah is common. The conversion is simple: 1 Ah equals 1000 mAh.
Does this calculation work for watts?
This calculator uses current, not watts. If you know power in watts, you need voltage to convert it to current. The formula is current equals watts divided by volts. After finding the current in amps, you can use it as the power consumption value.
