Battery Life Calculator
Calculate how long a battery will last based on its capacity, load current, and efficiency factor. Estimate runtime for any battery-powered device or project. See also our Battery Capacity Calculator and Electricity Cost Calculator.
How to Calculate Battery Life
Battery life estimation is essential for designing portable electronics, IoT devices, wearables, and any battery-powered system. The basic calculation divides the battery capacity by the average current draw, but real-world factors like voltage regulator efficiency, temperature effects, and discharge characteristics mean the actual runtime is always less than the theoretical maximum.
To estimate battery life, you need three values: the battery capacity in milliamp-hours (mAh), the average load current in milliamps (mA), and an efficiency factor that accounts for real-world losses. The efficiency factor typically ranges from 0.7 to 0.9 depending on the voltage regulator type, discharge rate, temperature, and battery chemistry.
The efficiency factor accounts for several losses: voltage regulator efficiency (switching regulators: 85-95%, linear regulators: Vout/Vin), battery capacity derating at high discharge rates (Peukert effect), temperature effects (capacity drops 10-20% in cold weather), and aging (batteries lose 20-30% capacity over their lifetime). For conservative estimates, use 0.7; for optimistic estimates with efficient regulators, use 0.9.
Battery Life Formula
Basic Formula:
Life (hours) = Capacity (mAh) × Efficiency / Current (mA)
With Peukert Effect (lead-acid):
Life = Capacity / I^k (k = Peukert exponent, 1.1-1.3)
Energy-Based:
Life = Battery Energy (Wh) / Power Consumption (W)
Energy (Wh) = Capacity (Ah) × Nominal Voltage (V)
Average Current (duty-cycled):
I_avg = I_active × D + I_sleep × (1-D)
D = duty cycle (active time / total time)
Example Calculation
A GPS tracker uses a 2000mAh Li-ion battery and draws 100mA average current. The boost converter has 80% efficiency:
Given: Capacity = 2000 mAh, Current = 100 mA, Efficiency = 0.80
Life = (2000 × 0.80) / 100 = 1600 / 100 = 16 hours
= 0.67 days = 960 minutes
With duty cycling (10% active, 5mA sleep):
I_avg = 100×0.1 + 5×0.9 = 10 + 4.5 = 14.5 mA
Life = (2000 × 0.80) / 14.5 = 110.3 hours = 4.6 days
Energy: 2000mAh × 3.7V = 7.4 Wh
Power: 100mA × 3.7V = 0.37W
Battery Life Reference Table
| Capacity (mAh) | Load (mA) | Efficiency | Life (hours) |
|---|---|---|---|
| 500 mAh | 50 mA | 0.8 | 8.00 h |
| 1000 mAh | 100 mA | 0.8 | 8.00 h |
| 1500 mAh | 50 mA | 0.85 | 25.50 h |
| 2000 mAh | 100 mA | 0.8 | 16.00 h |
| 2000 mAh | 200 mA | 0.8 | 8.00 h |
| 2000 mAh | 500 mA | 0.7 | 2.80 h |
| 3000 mAh | 150 mA | 0.85 | 17.00 h |
| 4000 mAh | 200 mA | 0.8 | 16.00 h |
| 5000 mAh | 100 mA | 0.9 | 45.00 h |
| 5000 mAh | 500 mA | 0.8 | 8.00 h |
| 10000 mAh | 200 mA | 0.85 | 42.50 h |
| 20000 mAh | 1000 mA | 0.8 | 16.00 h |
Frequently Asked Questions
What does mAh mean for batteries?
mAh (milliamp-hours) is a unit of electrical charge that represents battery capacity. A 2000mAh battery can theoretically deliver 2000mA for 1 hour, 1000mA for 2 hours, or 100mA for 20 hours. However, actual capacity varies with discharge rate, temperature, and age. Higher mAh means more stored energy and longer runtime at the same current draw.
Why is actual battery life less than calculated?
Several factors reduce actual runtime: voltage regulator losses (5-30%), increased internal resistance as the battery discharges, capacity reduction at high discharge rates (Peukert effect), temperature effects (cold reduces capacity), battery aging (capacity fades with charge cycles), and the cutoff voltage — you cannot use 100% of the rated capacity before the voltage drops too low for your circuit.
What efficiency factor should I use?
For switching regulators (buck/boost): 0.85-0.90. For linear regulators: multiply Vout/Vin × 0.9. For direct battery connection (no regulator): 0.90-0.95. For cold environments: reduce by 10-20%. For aged batteries: reduce by 20-30%. For conservative design estimates: use 0.7. A good general-purpose value for most portable electronics is 0.8.
How do I measure average current for variable loads?
For duty-cycled devices: I_avg = I_active × duty_cycle + I_sleep × (1 - duty_cycle). For complex profiles, measure current over a full operating cycle and calculate the time-weighted average. Use a current logger or oscilloscope with current probe. Many modern microcontrollers have energy profiling tools that calculate average consumption across different operating modes.
How does temperature affect battery life?
Cold temperatures increase internal resistance and reduce available capacity. Li-ion batteries lose about 10-20% capacity at 0°C and may not function below -20°C. Hot temperatures accelerate degradation and reduce cycle life. Optimal operating range is 20-25°C. For outdoor applications, factor in seasonal temperature variations when estimating battery life.
How can I extend battery life in my design?
Use sleep modes aggressively (reduce duty cycle), choose low-quiescent-current regulators, use efficient switching regulators, minimize LED indicators, reduce wireless transmission power and frequency, use interrupt-driven wake instead of polling, disable unused peripherals, and optimize code for speed (faster execution = shorter active time). Even small reductions in sleep current can dramatically extend battery life for low-duty-cycle applications.
Battery Chemistry Comparison
Different battery chemistries have different characteristics affecting runtime calculations. Lithium-ion (Li-ion) batteries have high energy density (150-250 Wh/kg), flat discharge curves, and 3.7V nominal voltage. Lithium polymer (LiPo) offers similar performance in flexible form factors. Alkaline batteries (1.5V) have sloping discharge curves and reduced capacity at high currents. NiMH rechargeable batteries (1.2V) offer good cycle life but lower energy density. Lead-acid batteries are heavy but inexpensive for stationary applications.
Optimizing for Long Battery Life
For IoT and sensor applications requiring months or years of battery life, the key is minimizing average current. A device that sleeps at 1μA and wakes for 10ms every minute at 10mA has an average current of only 0.0117mA — a 2000mAh battery would last over 19 years theoretically. Real-world factors like self-discharge (2-3% per month for Li-ion) and aging become the limiting factors for ultra-low-power designs rather than the load current itself.