Battery Runtime Calculator
Estimate battery runtime from capacity (mAh/Wh) and device power consumption. Enter values for instant results with step-by-step formulas.
Formula
Runtime (hours) = (Capacity_mAh / 1000 * Voltage * Efficiency) / Power_W
First converts mAh to Wh by multiplying capacity by voltage, then applies efficiency factor to get usable energy, and finally divides by load power in watts to get runtime in hours.
Worked Examples
Example 1: Smartphone Battery Life Estimation
Problem: A smartphone has a 5000 mAh battery at 3.7V. The average power consumption during mixed use is 4W. Assume 90% efficiency. How long will the battery last?
Solution: Energy = (5000 / 1000) * 3.7 = 18.5 Wh\nUsable energy = 18.5 * 0.90 = 16.65 Wh\nRuntime = 16.65 / 4 = 4.16 hours\nCurrent draw = (4 / 3.7) * 1000 = 1081 mA\nC-rate = 1081 / 5000 = 0.216C
Result: Runtime: 4 hours 10 minutes | Current draw: 1081 mA | C-rate: 0.216C
Example 2: IoT Sensor Node on AA Batteries
Problem: An IoT sensor runs on 2 AA batteries in series (3V total, 2500 mAh each). It draws 50mW average. Assume 85% efficiency from the voltage regulator. What is the expected runtime?
Solution: Energy = (2500 / 1000) * 3.0 = 7.5 Wh\nUsable energy = 7.5 * 0.85 = 6.375 Wh\nRuntime = 6.375 / 0.05 = 127.5 hours = 5.31 days\nCurrent draw = (0.05 / 3.0) * 1000 = 16.7 mA\nC-rate = 16.7 / 2500 = 0.0067C
Result: Runtime: 127.5 hours (5.3 days) | Current draw: 16.7 mA | C-rate: 0.007C
Frequently Asked Questions
How is battery runtime calculated from mAh and power consumption?
Battery runtime is calculated by first converting the battery capacity from mAh to watt-hours (Wh) using the formula Wh = (mAh / 1000) * voltage. Then you divide the available energy by the power consumption of the device in watts. For example, a 5000 mAh battery at 3.7V has 18.5 Wh of energy. If a device draws 5 watts, the theoretical runtime would be 18.5 / 5 = 3.7 hours. However, real-world runtime is always less due to conversion efficiency losses, voltage regulator overhead, and battery degradation over time.
What is the difference between mAh and Wh for battery capacity?
Milliamp-hours (mAh) measures electric charge capacity and does not account for voltage, while watt-hours (Wh) measures total energy and includes voltage in the calculation. Two batteries can have the same mAh rating but different energy capacities if they operate at different voltages. A 3000 mAh battery at 3.7V has 11.1 Wh, but a 3000 mAh battery at 7.4V has 22.2 Wh, meaning twice the actual energy. Wh is the more meaningful metric for comparing batteries of different chemistries and voltages. Phone manufacturers typically advertise mAh because all phones use similar 3.7V lithium cells, making mAh a reasonable comparison.
What is C-rate and why does it affect battery runtime?
C-rate describes how fast a battery is being discharged relative to its total capacity. A 1C rate means the battery is fully discharged in one hour, 0.5C means two hours, and 2C means 30 minutes. C-rate matters because batteries deliver less total energy at higher discharge rates due to internal resistance losses and chemical reaction limitations. A battery rated at 5000 mAh might only deliver 4500 mAh at a 1C rate and even less at 2C. This phenomenon is described by Peukerts law for lead-acid batteries and similar effects occur in lithium-ion cells. For maximum runtime, keep the C-rate as low as practical.
How does efficiency factor into battery runtime estimates?
Efficiency accounts for energy losses in the power delivery chain between the battery and the actual load. These losses come from voltage regulators (which convert battery voltage to the required device voltage), DC-DC converters, power management ICs, and heat dissipation in the battery itself due to internal resistance. Typical efficiency values are 85 to 95 percent for well-designed switching regulators and 50 to 70 percent for linear regulators. The calculator multiplies the raw battery energy by the efficiency percentage to get usable energy. For example, at 90 percent efficiency, a 20 Wh battery provides only 18 Wh of usable energy to the device.
How does temperature affect battery runtime?
Temperature significantly impacts battery performance and runtime. Most lithium-ion batteries perform best between 20 and 25 degrees Celsius. In cold temperatures (below 0 degrees C), the chemical reactions slow down, internal resistance increases, and available capacity can drop by 20 to 40 percent. At minus 20 degrees C, some batteries may deliver only half their rated capacity. High temperatures above 45 degrees C can temporarily increase available capacity slightly but accelerate permanent degradation and capacity loss. Extreme heat can also trigger thermal runaway, a dangerous condition. For outdoor or automotive applications, always account for the expected temperature range when sizing batteries.
What is battery degradation and how does it affect long-term runtime?
Battery degradation is the gradual, permanent loss of capacity and increase in internal resistance that occurs over a battery lifecycle. Lithium-ion batteries typically retain 80 percent of their original capacity after 300 to 500 full charge cycles, depending on the chemistry and usage conditions. After that, degradation accelerates. Factors that speed up degradation include deep discharges (below 20 percent), charging to 100 percent regularly, high temperatures, and fast charging. A phone with a 5000 mAh battery might effectively have only 4000 mAh after two years of daily charging. To maximize battery longevity, keep the charge between 20 and 80 percent and avoid extreme temperatures.