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Battery Runtime & Charge Converter

Calculate how long a power bank or battery will run your devices and estimate charge cycles for phones, laptops, and tablets.

Configure Battery & Load

Battery Specifications

V

3.7V is standard for Li-ion/power banks.

%

Typically 80-90% due to voltage conversion losses.

Device Load

W

Quick Presets

Common Capacities

Common Device Loads

Conversion Results

Estimated Device Runtime

6

hrs

16

mins

Total Energy

74.00 Wh

Real Usable Energy

62.90 Wh

Estimated Device Charge Cycles

~4.2

Smartphone Charges (4000mAh)

~2.1

Tablet Charges (8000mAh)

~1.0

Laptop Charges (60Wh)

~56

Smartwatch Charges (300mAh)

Discharge Simulator Timeline

100% Battery

0 hrs elapsed

75% Capacity

1.5 hrs elapsed

50% Capacity

3.1 hrs elapsed

25% Capacity

4.7 hrs elapsed

0% Depleted

6.2 hrs elapsed

What is the Battery Runtime Calculator?

A battery runtime calculator is a utility that allows you to calculate how many hours or minutes a power bank, lithium battery, or portable power station will operate your electronics before running out of power. Because hardware companies label battery capacities in milliamp-hours (mAh) while devices consume power in Watts, this tool acts as an essential mAh to Wh converter to standardise the electrical units.

Our converter also functions as a charge estimator, letting you determine how many complete charge cycles a portable battery will provide for common appliances like smartphones, laptops, tablets, and smartwatches. We perform calculations entirely client-side, ensuring your device specifications and power requirements remain confidential.

How to Calculate Battery Runtime

1

Set Battery Capacity

Input the battery capacity and select the appropriate unit. Standard phone accessories are measured in milliamp-hours (mAh), whereas larger stations specify Watt-hours (Wh).

2

Input Nominal Voltage

Nominal voltage is necessary to convert milliamp-hours to Watt-hours. Most portable lithium battery cells operate at 3.7V nominal voltage internally.

3

Enter Device Load

Specify the power load in Watts. You can also switch input mode to enter Voltage and Amps if you do not know the direct wattage rating of your hardware.

4

View Discharge Timeline

The interactive discharge simulator immediately updates to reveal the exact runtime curve and expected charge counts for daily electronic devices.

The Mathematical Formulas Behind Battery Capacity

Converting electric charge capacity (mAh) to energy (Wh) is critical because electric current values change when voltages are converted by boosters or regulators. The mathematical formula for converting milliamp-hours to energy is:

Watt-hours (Wh) = [Capacity (mAh) * Nominal Voltage (V)] / 1000

Once the total capacity is established in Watt-hours, the runtime is determined by dividing by the device's average power draw, taking into account the efficiency rating:

Runtime (Hours) = [Energy (Wh) * (Efficiency (%) / 100)] / Power Consumption (Watts)

Why Do Power Banks Lose Energy During Conversion?

When you use a power bank to charge accessories, the system does not deliver 100% of its stored chemical energy. This discrepancy is caused by thermodynamic and electrical design challenges:

  • Voltage Conversion: The internal battery cell voltage is typically 3.7V, but standard USB connections require 5.0V, 9.0V, or higher. Boosting this voltage releases thermal energy, creating loss.
  • Internal Resistance: Charging cables, circuit boards, and connectors possess electrical resistance, which consumes some energy as heat.
  • Device Recipient Efficiency: The destination device (such as your phone) also contains internal charging circuits that experience efficiency losses as they step the voltage back down to match their internal cells.

Standard external battery packs operate with a real-world energy efficiency of 80% to 90%. This application uses a conservative baseline of 85% to ensure the calculated durations remain realistic.

Frequently Asked Questions

How do you calculate battery runtime in hours?

To calculate battery runtime, convert the battery capacity from milliamp-hours (mAh) to watt-hours (Wh) using the formula: Wh = (mAh * Voltage) / 1000 (standard internal lithium voltage is 3.7V). Then multiply by the efficiency factor (typically 85% for power banks) and divide by the device's power consumption in Watts. Formula: Runtime = (Wh * Efficiency) / Watts.

Why does my 20,000mAh power bank not charge my 5,000mAh phone 4 times?

When a power bank charges a phone, it must boost the voltage from the internal lithium battery level (3.7V) to the USB output level (5V or higher), and then the phone bucks it back down to its own battery voltage. This double voltage conversion, combined with heat generation and cable resistance, results in a energy loss of 15% to 30%. Thus, a real-world efficiency of 70% to 85% is normal.

How do you convert mAh to Wh?

Milliamp-hours (mAh) measure electric charge capacity, while Watt-hours (Wh) measure total stored energy. To convert between them, you must know the battery's nominal voltage (typically 3.7V for phones and power banks). The conversion formula is: Wh = (mAh * Nominal Voltage) / 1000.

What is the nominal voltage of most rechargeable batteries?

Standard lithium-ion and lithium-polymer batteries used in smartphones, power banks, and laptops have a nominal voltage of 3.7V or 3.8V. Lithium Iron Phosphate (LiFePO4) cells have a nominal voltage of 3.2V, while standard alkaline batteries are 1.5V and rechargeable NiMH batteries are 1.2V.

Can I take a 100Wh power bank on a commercial airline flight?

Yes, FAA and TSA guidelines permit passengers to carry power banks with a capacity up to 100 Watt-hours (Wh) in their carry-on baggage. Power banks between 100.1Wh and 160Wh require airline approval, and carrying any power bank in checked baggage is strictly prohibited due to fire safety concerns.

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