Why Does Charging Time Vary So Much by Charger Wattage?
The same smartphone can take dramatically different amounts of time to charge depending on the charger used. Anyone who's felt the difference between a "20W fast charger" and a "5W standard charger" knows how much wattage affects charging speed. This calculator takes battery capacity, device type, and charger wattage, then estimates the time to a full charge.
The Relationship Between mAh and Wh
Battery capacity is usually listed in mAh (milliamp-hours), but calculating actual charging time requires the real energy amount in Wh (watt-hours), which factors in voltage. Smartphones run around 3.85V, tablets around 7.6V, and laptops around 11.4V — so the same mAh rating represents a different amount of energy depending on the device.
Why the Last 20% Takes Longer
Once a battery passes roughly 80%, protection circuitry enters a trickle-charge phase that gradually reduces current. This design protects battery longevity — the first 80% fills up quickly, but the last 20% typically takes noticeably longer. This calculator accounts for that charging curve to give a more realistic estimate.
Frequently Asked Questions
Only up to your device's maximum input power — beyond that, no difference.
The trickle-charge phase after 80% slows things down.
Battery voltage varies by device, so the same mAh means different energy amounts.