Why does my battery last less than calculated?

Internal resistance, regulator losses, temperature, and high current draw reduce effective capacity.

The percentage of time a device is active. Average current = active current x duty cycle.

Milliamp-hours measures battery capacity. 1000mAh = 1000mA for 1 hour theoretically.

Electronics

Battery Life Calculator

Estimate battery runtime from capacity and current draw. Supports multiple loads with duty cycles for accurate IoT and embedded system estimates.

Quick Answer

Battery Life (hours) = Capacity (mAh) / Current Draw (mA). A 3000mAh battery at 100mA lasts about 30 hours. Factor in ~85% discharge efficiency for real-world estimates.

Battery Specifications

Capacity (mAh)

Voltage (V)

Efficiency (%)

Common:

Current Draw

Name

Duty %

Results

2d 21h

Estimated Runtime

37.0 mA

Avg. Current Draw

11.10 Wh

Battery Energy

2550 mAh

Usable Capacity

68.9 h

Total Hours

About This Tool

The Battery Life Calculator estimates how long a battery will power your device. It goes beyond the simple capacity-divided-by-current formula by accounting for discharge efficiency and multiple loads with different duty cycles, making it useful for IoT devices, embedded systems, and portable electronics.

The Basic Formula

Battery life in hours equals capacity in milliamp-hours divided by average current draw in milliamps. A 2000mAh battery powering a 100mA device lasts 20 hours in theory. In practice, you never get the full rated capacity because internal resistance, temperature, and discharge rate all reduce the usable energy.

Discharge Efficiency

Real batteries deliver less than their rated capacity due to internal resistance losses, voltage regulator inefficiency, and the fact that the battery voltage drops as it discharges. Using 80-90% as a discharge efficiency factor gives more realistic estimates. Lithium-ion batteries are among the most efficient at 85-95%, while alkaline batteries can be as low as 50-70% efficiency under high current loads.

Duty Cycle Matters

Many electronic devices do not draw constant current. A wireless sensor might sleep at 10 microamps for 59 seconds, then wake up and transmit at 150 milliamps for 1 second. The average current is what matters for battery life calculation. The duty cycle field in this calculator lets you account for these intermittent loads without doing the math manually.

Battery Types Compared

Lithium-ion (18650, LiPo) batteries offer the best energy density at 3.6-3.7V nominal. Alkaline AA cells provide 1.5V at about 2000-2500mAh. Coin cells like CR2032 have 220mAh at 3V. Rechargeable NiMH AAs offer 1.2V at 1900-2800mAh. Choose based on your voltage requirements, physical size constraints, and whether rechargeability matters.

Frequently Asked Questions

Why does my battery last less than the calculated time?

Real-world battery life is shorter than theoretical calculations for several reasons: internal resistance wastes energy as heat, voltage regulators have efficiency losses, temperature affects capacity, and high current draw reduces effective capacity. Using the 85% efficiency factor helps account for these losses.

What is duty cycle and why does it matter?

Duty cycle is the percentage of time a device is active versus sleeping. A 10% duty cycle means the device is on 10% of the time. For battery life calculations, the average current (active current times duty cycle) determines runtime. IoT devices with aggressive sleep modes can last months or years on a coin cell.

How does temperature affect battery life?

Cold temperatures significantly reduce battery capacity. At 0 degrees C, a lithium-ion battery may deliver only 70-80% of its rated capacity. At -20 degrees C, it drops to 50% or less. Hot temperatures can temporarily increase capacity but accelerate degradation. Alkaline batteries are especially sensitive to cold.

What does mAh mean?

Milliamp-hours (mAh) is a unit of electric charge that measures battery capacity. A 1000mAh battery can theoretically deliver 1000mA for 1 hour, 500mA for 2 hours, or 100mA for 10 hours. Higher mAh means more stored energy. Note that capacity ratings assume specific discharge conditions that may not match your application.

Can I compare batteries with different voltages using mAh?

Not directly. A 3000mAh battery at 3.7V stores more energy than a 3000mAh battery at 1.5V. To compare different voltage batteries, convert to watt-hours: Wh = mAh x V / 1000. The 3.7V battery stores 11.1Wh while the 1.5V battery stores only 4.5Wh.

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