Battery Capacity Converter

Overview

A power bank labelled 20,000 mAh, a laptop battery rated 56 Wh and a power tool pack marked 5 Ah all describe the same physical quantity in different units, and the numbers cannot be compared directly. The conversion that matters is watt hours equals milliamp hours times voltage divided by a thousand. A phone cell running at 3.7 volts therefore stores 74 Wh at 20,000 mAh, while the carry-on limit published by airlines is expressed as 100 Wh rather than in milliamp hours at all. This converter starts from any one of mAh, Ah or Wh, takes the cell voltage you supply, and fills in the other two. It also prints a table showing how much charge the same energy represents at the common 3.7, 5, 9 and 12 volt rails, which is the quickest way to judge how many laptop charges a power bank can realistically deliver.

Features

• Three units in one pass: milliamp hours, amp hours and watt hours, where any one input derives the other two
• The core relation is fixed as Wh equals mAh times voltage divided by 1000, with 3.7 volts as the editable default
• Energy also reported in kilojoules so it lines up with safety data sheets that use joules
• A voltage table showing the equivalent mAh at 3.7, 5, 9 and 12 volts for the same stored energy
• Invalid input such as a blank, a negative value or zero volts clears the result instead of printing a misleading figure
• Copy the result as 5000 mAh at 3.7 V equals 18.5 Wh for a spec sheet or a support reply

How to use

1. Enter the capacity value and pick whether it is mAh, Ah or Wh
2. Enter the cell voltage: 3.7 volts for phones and power banks, 5 volts for USB output, 11.1 or 15.4 volts for many laptops
3. Read the headline and the five detail rows covering mAh, Ah, Wh and kilojoules
4. Check the voltage table to see the same energy expressed at other rails
5. Copy the one line summary into a product description or a support answer

Notes

• The voltage must be the real one: the same 10,000 mAh is 37 Wh at 3.7 volts and 50 Wh at 5 volts, so a wrong rail shifts the watt hours by a third
• mAh is charge, not energy: comparing packs across voltages requires watt hours, which is exactly why airlines and couriers quote Wh
• Rated capacity is computed at the cell voltage, while real output passes through boost conversion losses, so usable charge into a phone is typically 60 to 70 percent of the rating
• Series and parallel arrangements change the totals: two 3.7 volt cells in series give 7.4 volts at the same capacity, while parallel doubles capacity at the same voltage
• Airline carry-on rules usually draw the line at 100 Wh, allow 100 to 160 Wh with carrier approval and prohibit anything above 160 Wh, subject to the operator
• The tool converts units only and makes no health assessment; degradation needs internal resistance and discharge curve measurements
• Fast charging ratings describe power delivery, not stored energy, so a 65 W charger does not imply a larger battery
• Watt hours equal milliamp hours times volts divided by one thousand, so the same 5,000 mAh cell reads 18.5 Wh at 3.7 V but 25 Wh at 5 V; airline cabin limits are usually quoted in watt hours, which is why the voltage field matters.