Battery Charge Time Calculator
Our other calculator computes battery charge time instantly. Get useful results with practical tips and recommendations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Charge Time = (Capacity Wh x Charge%) / (Charger W x Efficiency%)
Where Capacity Wh is battery capacity in watt-hours (mAh x 3.7V / 1000), Charge% is the percentage of charge needed, Charger W is charger output wattage, and Efficiency% accounts for energy lost as heat during charging. The result is in hours.
Worked Examples
Example 1: Smartphone Fast Charging
Problem:Charge a 5,000 mAh phone battery from 20% to 100% using an 18W fast charger at 85% efficiency.
Solution:Battery capacity = 5,000 / 1,000 x 3.7 = 18.5 Wh\nCharge needed = 80% x 18.5 = 14.8 Wh\nEffective charger power = 18 x 0.85 = 15.3 W\nCharge time = 14.8 / 15.3 = 0.97 hours = 58 minutes\nEnergy from wall = 14.8 / 0.85 = 17.41 Wh\nEnergy wasted as heat = 17.41 - 14.8 = 2.61 Wh
Result:Charge time: 58 minutes | Energy used: 17.41 Wh | 2.61 Wh lost as heat
Example 2: Laptop Battery Slow Charge
Problem:Charge a 15,000 mAh laptop battery (56 Wh) from 10% to 80% using a 45W charger at 90% efficiency.
Solution:Battery capacity = 15,000 / 1,000 x 3.7 = 55.5 Wh\nCharge needed = 70% x 55.5 = 38.85 Wh\nEffective charger power = 45 x 0.90 = 40.5 W\nCharge time = 38.85 / 40.5 = 0.96 hours = 58 minutes\nEnergy from wall = 38.85 / 0.90 = 43.17 Wh
Result:Charge time: 58 minutes | Energy used: 43.17 Wh | Effective rate: 40.5W
Frequently Asked Questions
How is battery charge time calculated?
Battery charge time is calculated by dividing the energy needed to reach the target charge level by the effective power delivered by the charger. The energy needed equals the battery capacity in watt-hours multiplied by the percentage of charge required. The effective charger power accounts for charging efficiency, which is typically 80 to 90 percent because some energy is lost as heat during the charging process. For a 5,000 mAh phone battery at 3.7V nominal voltage, the total capacity is 18.5 Wh. Charging from 20 to 100 percent requires 14.8 Wh, and with an 18W charger at 85 percent efficiency delivering 15.3 effective watts, the charge takes approximately 58 minutes. Real-world times may differ because charging slows significantly above 80 percent to protect battery longevity.
Why does charging slow down when the battery is nearly full?
Batteries slow their charging rate above 80 percent due to a process called constant voltage charging, which is the second phase of the standard CC-CV charging protocol. During the first phase of constant current, the charger pushes maximum power into the battery at a steady current rate. Once the battery reaches approximately 80 percent charge, the voltage reaches its maximum safe level and the charger must reduce current to prevent overcharging and overheating. This taper phase can take as long as the first 80 percent combined, which is why going from 80 to 100 percent often feels disproportionately slow. This design is essential for battery safety and longevity because lithium-ion batteries can become unstable or degrade rapidly if forced to charge at high rates near full capacity. Many modern devices report reaching 80 percent in 30 minutes but require another 30 to 45 minutes for the final 20 percent.
What does mAh mean and how does it relate to charging time?
Milliamp-hours (mAh) is a measure of electrical charge capacity that tells you how much current a battery can deliver over time. A 5,000 mAh battery can theoretically deliver 5,000 milliamps for one hour, or 1,000 milliamps for five hours. To calculate energy in watt-hours, multiply mAh by the nominal voltage (typically 3.7V for lithium-ion) and divide by 1,000, giving 18.5 Wh for a 5,000 mAh battery. Higher mAh batteries store more energy and take longer to charge at the same charger wattage. When comparing devices, mAh alone is misleading because voltage differs, which is why watt-hours is a more accurate measure of total energy. A laptop battery rated at 50 Wh stores about 2.7 times more energy than an 18.5 Wh phone battery, regardless of how each manufacturer reports their mAh ratings at different voltages.
Does fast charging damage the battery over time?
Fast charging does cause slightly more battery degradation than slow charging, but modern battery management systems minimize this impact significantly. The primary mechanism of degradation is heat, as fast charging generates more thermal stress in the battery cells, which accelerates chemical side reactions that reduce capacity over time. Studies show that batteries consistently fast-charged retain about 80 percent capacity after 500 to 800 cycles, compared to 80 percent after 800 to 1,000 cycles with standard charging. However, manufacturers design their fast charging systems with safety margins that keep degradation within acceptable limits for the expected device lifespan of 2 to 3 years. Practical tips to minimize degradation include avoiding fast charging when the battery is already warm, not fast charging above 80 percent, and using standard charging overnight when speed is unnecessary. The convenience benefit of fast charging typically outweighs the marginal battery life reduction for most users.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy