Enter the battery capacity and load information into the calculator to estimate battery runtime, required capacity, or average current draw. Note: mAh means milliamp-hours (battery capacity), not “milliamps per hour” (mA/h).
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Milliamp-hour (mAh) / Runtime Formula
C_{mAh}=I_{mA}\cdot t_{h}\\
t_{h}=\frac{C_{mAh}}{I_{mA}}\\
I_{mA}=\frac{C_{mAh}}{t_{h}}Variables:
- CmAh is the battery capacity in milliamp-hours (mAh)
- ImA is the average current in milliamps (mA)
- th is the time in hours (h)
To estimate runtime (in hours), divide the battery capacity (mAh) by the average current draw (mA). To estimate the required capacity, multiply current (mA) by time (hours).
How to Calculate Battery Capacity (mAh) or Runtime
The following steps outline how to use mAh, current draw, and time together.
- First, determine the average current draw in milliamps (mA).
- Next, determine the time in hours (h).
- Next, gather the formula from above (for capacity: CmAh = ImA × th; rearrange as needed for runtime or current).
- Finally, calculate the value you need (capacity, runtime, or average current).
- After inserting the variables and calculating the result, check your answer with the calculator above.
Example Problem :
Use the following variables as an example problem to test your knowledge.
average current draw (mA) = 74
time (hours) = 20
battery capacity = 74 × 20 = 1480 mAh
Frequently Asked Questions
What is the significance of calculating milliamp-hours (mAh)?
mAh is a common way to express a battery’s capacity (electric charge). If you know a device’s average current draw in mA, you can estimate runtime with runtime (h) ≈ capacity (mAh) ÷ current (mA) (and adjust for losses such as converter efficiency).
Can the mAh formula be used for any type of battery?
The basic relationships between capacity, current, and time apply broadly, but real-world runtime can vary with temperature, discharge rate, cutoff voltage, aging, and efficiency of the power circuitry.
How can I improve the accuracy of my mAh/runtime estimate?
Measure the device’s average current during typical use (not just peak current), and account for system efficiency and the fact that some batteries deliver less usable capacity at higher discharge rates or low temperatures.
Is it possible to convert mAh to watts?
Not directly. You can convert mAh to watt-hours (Wh) if you know voltage: Wh = (mAh × V) / 1000. To get watts (W) (average power), you also need time: W = Wh / hours.
