Understanding Battery Runtime: mAh, Voltage, and Discharge Curves

The Basic Formula

A 3000mAh battery powering a device that draws 500mA should last 6 hours. But this idealized calculation ignores three critical real-world factors: safety margins, variable power draw, and battery chemistry.

1. Discharge Safety Margins

Draining a lithium battery to 0% permanently damages it. For lithium-polymer (LiPo) batteries used in drones and RC vehicles, discharging below 20% can cause irreversible capacity loss. For lithium-ion (Li-ion) cells in smartphones and power banks, the safe floor is around 10-15%.

This means your “3000mAh” battery effectively provides only 2400-2700mAh of usable capacity. Always factor in a 10-20% safety margin when estimating runtime.

2. Sleep Modes and Duty Cycling

Modern IoT devices rarely run at full power continuously. A temperature sensor might wake up for 3 seconds every 5 minutes to take a reading and transmit data, then enter a deep sleep state drawing mere microamps.

The effective average consumption is a weighted average:

For example, a sensor drawing 50mA while active (5% of the time) and 0.05mA while sleeping (95% of the time) has an average draw of just 2.5mA. That 3000mAh battery lasts 1,200 hours—50 days—rather than the 60 hours you'd estimate from the active-only draw.

3. Peukert's Law and Chemistry Effects

Batteries are not perfect linear energy sources. Peukert's Law describes how a battery's effective capacity decreases as the discharge rate increases. Drawing 2A from a battery rated at 2000mAh (1C rate) yields less total energy than drawing 200mA (0.1C rate).

Temperature also plays a major role. Battery capacity is rated at room temperature (~20°C). At freezing (0°C), effective capacity can drop 20-30%. If you're deploying an outdoor sensor in winter, factor this into your calculations.

Try It Yourself

Use our battery calculator with sleep mode support and configurable safety margins to get an accurate estimate for your specific device: