Battery Life Calculator
Estimate how long your battery will last given capacity, load current, and efficiency.
Estimated runtime (real)
1.4 days
34.0 h at 85% efficiency
Ideal runtime (100% eff)
1.7 days
Formula
runtime (h) = capacity (mAh) / load (mA) × efficiency
Real-world runtime is lower than the ideal because of converter losses and ageing.
Estimating runtime
Divide battery capacity by average current draw for an ideal runtime, then de-rate it by an efficiency factor for regulator losses, self-discharge and capacity that isn't usable at your cut-off voltage. The gauge above fills green when runtime is comfortable and red when it's short.
Make estimates realistic
- Use the average current over a duty cycle, not the peak — sleep modes hugely extend battery life.
- 80–90% is a sensible efficiency for a switching regulator; linear regulators can be far worse.
- Cold temperatures and high discharge rates both reduce usable capacity.
- Leave margin: a pack rarely delivers its full rated mAh down to a useful voltage.
Where it's used
Battery-powered IoT and ESP32 nodes, wearables, remote sensors, and any portable build where you need to know how often you'll recharge.
Frequently Asked Questions
Why is the real runtime lower than ideal?+
Batteries can't deliver 100% of their rated capacity due to internal resistance, temperature, and the cut-off voltage of the circuit. Typical efficiency for Li-ion is 80–90%.
How do I reduce current draw?+
Use sleep modes on microcontrollers, turn off peripherals when idle, switch to lower-power radios (LoRa vs. Wi-Fi), and use a low-quiescent-current LDO regulator.