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Convert battery energy from watt-hours (Wh) to milliamp-hours (mAh). Useful for understanding and comparing battery capacity ratings.
Standard Li-ion cell: 3.7V, 2-cell: 7.4V, USB output: 5V
Where mAh is capacity in milliamp-hours, Wh is energy in watt-hours, and V is voltage in volts.
A 100 Wh laptop battery at 11.1V:
mAh = (100 × 1000) ÷ 11.1 = 9,009 mAh (9 Ah)
| Wh | @ 3.7V | @ 5V | @ 7.4V |
|---|---|---|---|
| 10 Wh | 2,703 mAh | 2,000 mAh | 1,351 mAh |
| 20 Wh | 5,405 mAh | 4,000 mAh | 2,703 mAh |
| 37 Wh | 10,000 mAh | 7,400 mAh | 5,000 mAh |
| 50 Wh | 13,514 mAh | 10,000 mAh | 6,757 mAh |
| 74 Wh | 20,000 mAh | 14,800 mAh | 10,000 mAh |
| 100 Wh | 27,027 mAh | 20,000 mAh | 13,514 mAh |
| 160 Wh | 43,243 mAh | 32,000 mAh | 21,622 mAh |
Converting watt-hours (Wh) to milliamp-hours (mAh) translates a battery's energy capacity into the charge capacity unit most commonly used for portable electronics. Watt-hours measure total energy independent of voltage, while milliamp-hours measure charge flow at a specific voltage over time. The formula mAh = (Wh × 1000) ÷ V derives from the power relationship P = V × I. Because mAh is voltage-dependent, the same Wh value produces different mAh ratings at different voltages – a 37 Wh battery equals 10,000 mAh at 3.7V but only 5,000 mAh at 7.4V. This conversion is particularly important for smartphone users comparing batteries, travelers checking airline lithium battery limits (100 Wh maximum for carry-on), and engineers designing portable power systems where both Wh and mAh ratings appear in component datasheets.
Locate the watt-hour rating on the battery label or product specifications. Laptop batteries typically list Wh prominently. For power banks, it may be on the bottom label or in fine print alongside the mAh rating.
Determine the nominal voltage of the battery. Phone batteries use 3.7V (single lithium cell), power banks internally use 3.7V but output 5V via USB, laptops use 7.4V, 11.1V, or 14.8V depending on cell configuration.
Calculate: mAh = (Wh × 1000) ÷ V. For example, a 74 Wh power bank at 3.7V: (74 × 1000) ÷ 3.7 = 20,000 mAh. A 56 Wh laptop battery at 11.1V: (56 × 1000) ÷ 11.1 = 5,045 mAh.
If comparing a power bank to a phone, note that the power bank mAh is often rated at 3.7V internally, but it outputs at 5V USB. The effective mAh at 5V is lower. A 20,000 mAh power bank at 3.7V delivers about 14,800 mAh at 5V, minus 10–15% for conversion losses.
Phone and power bank mAh ratings can be misleading when voltages differ. Converting to Wh first, then comparing, reveals which battery truly holds more energy and will provide more charges.
Airlines enforce lithium battery limits in Wh. Converting your power bank's mAh to Wh ensures you know whether it meets the 100 Wh carry-on limit before heading to the airport.
Electronics engineers need both units when designing battery-powered devices. Component datasheets may list Wh while marketing materials use mAh, requiring accurate conversion between the two.
| Device | Voltage | Typical mAh | Wh Equivalent |
|---|---|---|---|
| Smartwatch | 3.7V | 300 mAh | 1.11 Wh |
| Wireless Earbuds (case) | 3.7V | 500 mAh | 1.85 Wh |
| Smartphone | 3.7V | 5,000 mAh | 18.5 Wh |
| Tablet | 3.7V | 10,000 mAh | 37 Wh |
| Power Bank (large) | 3.7V | 26,800 mAh | 99.2 Wh |
| Laptop (3-cell) | 11.1V | 5,045 mAh | 56 Wh |
| Laptop (4-cell, max carry-on) | 14.8V | 6,750 mAh | 99.9 Wh |
Divide the watt-hours by the voltage, then multiply by 1000: mAh = (Wh ÷ V) × 1000. For example, a 37 Wh battery at 3.7V equals (37 ÷ 3.7) × 1000 = 10,000 mAh. The voltage is essential because the same Wh value produces very different mAh ratings at different voltages.
Power banks advertise mAh at the internal cell voltage of 3.7V. When outputting at 5V USB, the effective capacity is lower. A 20,000 mAh power bank at 3.7V holds 74 Wh, which equals only 14,800 mAh at the 5V output. After 10–15% conversion efficiency losses, you effectively get about 12,500–13,300 mAh at 5V.
The FAA limits carry-on lithium batteries to 100 Wh without airline approval. At 3.7V, this equals about 27,027 mAh. Many manufacturers sell 26,800 mAh power banks (99.2 Wh) that are just under this limit. To check your power bank: mAh × V ÷ 1000 = Wh.
Compare in Wh, not mAh, for accuracy. A 74 Wh power bank charging a phone with an 18.5 Wh battery: 74 ÷ 18.5 = 4 theoretical charges. After conversion losses (about 85% efficiency), expect roughly 3.4 full charges. Do not simply divide mAh values unless both are at the same voltage.
Fast charging does not change a battery's rated mAh or Wh capacity. It only changes the charging speed by delivering higher wattage (more volts or amps). However, repeated fast charging generates more heat, which can degrade battery chemistry over time, gradually reducing the actual usable capacity by a few percent per year compared to slow charging.