How to Calculate Battery Charging Time
Last reviewed: 2026-06-14
Quick answer
Battery charging time starts with energy capacity in Wh and charger output in W, then adjusts for charging efficiency. For Ah or mAh ratings, voltage is needed before the capacity can be compared with charger watts.
Convert capacity to Wh
If capacity is in Ah or mAh, multiply by voltage to estimate Wh. A 20,000 mAh battery is 20 Ah; at 3.7 V, that is about 74 Wh before efficiency and device charging limits.
Use charger watts
Divide battery Wh by charger W and efficiency to estimate charging hours. A 500 Wh battery on a 100 W charger at 85% efficiency estimates to about 5 hours and 53 minutes.
Understand the limits
Battery chemistry, charge controllers, heat, and manufacturer limits can change real charging time. Many devices also reduce power near full, so small batteries may not charge at the advertised wattage for the entire session.
FAQ
Can every battery charge at the charger's full wattage?
No. The device or battery management system may limit charging power for safety, heat, battery health, or connector limits. Use the result as a planning estimate rather than a guarantee.