Charging Time Calculator
How long a phone, power bank or battery takes to charge — capacity, charger wattage and real-world efficiency included.
1,189 views
How Charging Time Is Calculated
Start with the energy actually stored in the battery, not just the mAh printed on the box. Capacity in watt-hours (Wh) = mAh × voltage ÷ 1000; phones and most lithium cells run at a nominal 3.7-3.85 V, so a 5,000 mAh phone battery holds roughly 18.5-19 Wh. Divide that by the charger's real output — its rated watts multiplied by an efficiency factor of about 80-90%, since some energy is always lost as heat in the charging circuit and the cable — and you get the theoretical time to 100%.
Take a 10,000 mAh power bank (about 37 Wh) charged through an 18 W adapter at 85% efficiency: 37 ÷ (18 × 0.85) ≈ 2.4 hours if charging were perfectly linear from 0 to 100%. In practice it never is. Manufacturers push current hard for the first 80% of charge — that portion can finish in well under half the total time — then deliberately throttle the last 20% into a slow "trickle" or constant-voltage phase, because ramming current into a nearly full lithium cell generates heat and accelerates long-term wear. So the last fifth of the charge can take almost as long as the first four-fifths combined, which is why real-world charging always runs longer than a naive Wh ÷ W division suggests.
What's Worth Knowing
- mAh vs Wh: mAh alone doesn't tell you energy — a 5,000 mAh battery at 3.7 V and one at 4.2 V store different amounts, so Wh (or mAh × V) is the only fair way to compare capacity across devices.
- Efficiency loss is real: even a "100 W" charger rarely delivers 100 W to the cell; 10-20% is normal loss, more if the cable or port is undersized for the wattage.
- Wattage isn't everything: a 65 W charger through a cheap 15 W-rated cable still only delivers 15 W — both ends of the chain must support the same power.
- The 80% rule: if you only need a quick top-up, unplugging at 80% is far more time-efficient than waiting for the slow final stretch to 100%.
Frequently Asked Questions
Why does my phone always take longer to charge than the number this calculator gives?
The calculation gives the average-case theoretical time. Real chargers slow down deliberately above ~80% (constant-voltage tapering) to protect the battery, and using the phone while charging, ambient heat, and an aging battery all add extra minutes on top.
Why is the last 20% so much slower than the first 80%?
Lithium-ion batteries charge in two phases: constant current (fast, roughly 0-80%) and constant voltage (slow taper, 80-100%). Pushing full current into a nearly-full cell would overheat and degrade it, so the charger intentionally throttles down — that's why the last fifth can take nearly as long as the rest combined.
What's the real difference between mAh and Wh for this calculation?
mAh (milliamp-hours) measures charge, not energy — it only tells you the real energy stored once you multiply by voltage. Two batteries with the same mAh but different voltages hold different amounts of energy, which is why the calculator converts to Wh before dividing by charger wattage.
Does a 65 W charger really charge three times faster than a 20 W one?
Only up to the point the battery itself can accept that much power — every device has a maximum charging rate set by its battery and controller. Beyond that ceiling, extra charger wattage does nothing; the cable and port must also support the full wattage for it to matter at all.
Does regular fast charging damage battery health long-term?
Heat, not wattage itself, is the main driver of long-term capacity loss. Modern phones manage the charging curve and cut current if temperature rises, so charging in a cool spot with the screen off — and avoiding leaving it at 100% for extended periods — protects battery health more than the charger's rated speed does.
Similar Tools
Report a Problem
Charging Time Calculator
Comments
No comments yet — be the first to write one!