How do I calculate battery run time?

Divide battery capacity (mAh) by device current draw (mA): Run time (hours) = mAh ÷ mA. For example, a 10,000 mAh battery powering a 500 mA device gives 10,000 ÷ 500 = 20 hours. Multiply by an efficiency factor (typically 0.85) to account for real-world losses.

Milliamp-hours (mAh) is a measure of a battery's charge capacity. A 10,000 mAh battery can theoretically deliver 10,000 milliamps for 1 hour, or 1,000 mA for 10 hours, or 500 mA for 20 hours. The higher the mAh rating, the longer the battery lasts at a given current draw.

Why does the calculator use an efficiency factor?

Real-world battery run time is 10–25% shorter than theoretical calculations suggest. This is due to internal resistance, the Peukert effect (higher discharge rates reduce efficiency), voltage cutoff (devices stop before the battery is truly empty), and temperature. The default 85% efficiency factor is a realistic estimate for most lithium-ion batteries.

How long does a 10,000 mAh power bank last?

At 500 mA draw with 85% efficiency: 10,000 × 0.85 ÷ 500 = 17 hours. At 1,000 mA (typical smartphone fast charging): 10,000 × 0.85 ÷ 1,000 = 8.5 hours. Keep in mind USB conversion losses mean a real power bank delivers about 6,000–7,000 mAh to the device, not the full 10,000 mAh.

What affects battery life?

Temperature, discharge rate (Peukert effect), battery age and cycle count, and device efficiency all reduce run time. Cold temperatures (below 0°C) can cut capacity by 20–30%. Discharging at high rates reduces efficiency. Batteries lose 20–30% capacity after 300–500 charge cycles.

How do I find my device's current draw?

Check the device's spec sheet or the charger rating label. If the charger shows watts (W), divide by voltage: amps = watts ÷ volts (e.g., 5W ÷ 5V = 1A = 1,000 mA). A USB multimeter or power meter can measure actual draw in real-time. Smartphone screens, radios, and processors are the biggest current consumers.

What does mAh mean on a battery?

Milliamp-hours (mAh) is a measure of a battery's total charge capacity — how much electrical charge it can store and deliver. The "h" stands for hours: a 3,000 mAh battery can theoretically supply 3,000 mA (3 amps) for 1 hour, or 300 mA for 10 hours, or 100 mA for 30 hours. It is the most common rating for rechargeable batteries in smartphones (3,000–5,000 mAh), tablets (6,000–12,000 mAh), and portable power banks (10,000–30,000 mAh).

Why doesn't my battery last as long as the rated capacity suggests?

Several real-world factors reduce battery run time below the theoretical maximum. Internal resistance converts some energy to heat. The Peukert effect means drawing current faster than the C-rate (the rate at which the battery is discharged in 1 hour) reduces usable capacity — a battery drawn at 2× the C-rate delivers only about 75% of rated capacity. Voltage cutoff stops the device before the battery is fully empty. Temperature reduces lithium-ion capacity by 20–30% below 0°C. Battery aging after 300–500 cycles degrades capacity by 20–30%. An 85% efficiency factor accounts for typical real-world losses.

How long will 10,000 mAh last on my device?

It depends on current draw. At 500 mA (typical for a small device), 10,000 mAh at 85% efficiency lasts about 17 hours. At 1,000 mA (moderate smartphone charging), about 8.5 hours. At 2,000 mA (fast charging a phone), about 4.25 hours. For a power bank charging a phone: expect 2–3 full phone charges from a 10,000 mAh bank, accounting for USB conversion losses and the phone's typical 3,000–4,000 mAh battery.

Battery Run Time Calculator — Hours & Minutes

How to Calculate Battery Run Time

The battery run time calculator on this page estimates how long a battery lasts from capacity (mAh) and current draw (mA). The fundamental formula is: run time in hours = capacity ÷ current draw. For example, a 10,000 mAh battery powering a device that draws 500 mA gives a theoretical run time of 20 hours. Real-world run time is always shorter — which is why this calculator applies an efficiency factor (default 85%) to produce a realistic estimate.

Formula: Run Time (hours) = (Battery Capacity in mAh × Efficiency Factor) ÷ Current Draw in mA

What Is mAh and How Does It Affect Battery Life?

Milliamp-hours (mAh) measures how much total charge a battery holds — it is the product of current (mA) and time (hours). A 5,000 mAh battery can deliver 5,000 mA for 1 hour, 500 mA for 10 hours, or 100 mA for 50 hours. Capacity is printed on most batteries and is available in manufacturer spec sheets. Larger mAh ratings mean longer run times at the same current draw. Common ranges: AAA cells 1,000–1,200 mAh, smartphone batteries 3,000–5,000 mAh, laptop batteries 40,000–80,000 mAh (40–80 Wh ÷ nominal voltage).

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Understanding the Efficiency Factor

Theoretical battery calculations assume perfect efficiency — no energy is lost to heat, internal resistance, or conversion overhead. In practice, several factors reduce usable capacity:

Internal resistance — all batteries waste some energy as heat during discharge
Peukert effect — drawing current faster than the C-rate rating reduces total capacity
Voltage cutoff — devices stop running before the battery is completely empty
Temperature — cold temperatures reduce capacity by 20–30%
Age and cycles — batteries lose 20–30% capacity after 300–500 full charge cycles

For most lithium-ion batteries in consumer electronics, an 85% efficiency factor is a realistic default. Lead-acid batteries (car batteries, UPS systems) are typically 70–80% efficient. Adjust the efficiency input in the calculator to match your specific application. For a deeper look at device power usage, use the download time calculator to understand data-rate trade-offs in connected devices. When tracking how long employees operate battery-powered equipment on a job site, pair this tool with our time card calculator to log and total hours worked.

Common Device Current Draw Reference

Understanding how much current your device draws is essential for accurate run time estimates. Here are typical ranges for common devices:

Bluetooth earbuds: 20–60 mA
LED light (low power): 50–150 mA
Smartphone (screen off): 50–100 mA
Smartphone (screen on): 150–400 mA
Tablet: 200–600 mA
Raspberry Pi: 500–1,000 mA
Laptop (light use): 1,000–3,000 mA

For accurate measurements, use a USB power meter or bench multimeter to measure actual draw in real use. Spec sheet values are often maximum (peak) draw, not average draw — actual run time may be longer than calculated.

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Power Bank and Portable Battery Considerations

Power banks have a rated capacity (e.g., 20,000 mAh) but deliver less usable charge to devices due to USB voltage conversion overhead. A 5V USB power bank using a 3.7V lithium cell must step up voltage, which is typically 85–92% efficient. A 20,000 mAh power bank at 3.7V holds 74 Wh — charging a phone at 5V yields 74 Wh ÷ 5V × 0.9 efficiency ≈ 13,300 mAh at the USB port. This is why power bank manufacturers quote the internal cell capacity, not the delivered capacity. For phones and tablets, a practical rule of thumb is that a power bank delivers about 60–70% of its rated mAh to the device.

Tips for Maximizing Battery Run Time

Reduce current draw — lower screen brightness, disable Bluetooth and Wi-Fi when unused
Keep batteries warm — avoid use in temperatures below 0°C (32°F)
Avoid deep discharge — lithium-ion batteries last longer when kept between 20–80% charge
Use energy-efficient modes — low-power sleep modes can cut draw by 80–90%
Match battery to load — use high-capacity cells for high-draw applications

Sources & References

IEC 62133: Safety Requirements for Portable Sealed Secondary Lithium Cells — International Electrotechnical Commission
Battery University: How to Estimate Battery Life — Cadex Electronics

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