Battery Charging Time Calculator

Step-by-Step Guide

1. 1Enter Battery Capacity: Input the battery's rated capacity in mAh or Ah.
2. 2Enter Charging Current: Input the charger's rated output current in amps (A).
3. 3Enter Charging Efficiency: Input expected efficiency — typically 85-90% for phone/tablet chargers, 75-85% for car chargers.
4. 4Set Start and End Charge Percentage: Input the battery's current level and target level, such as 20% to 80%.
5. 5Read the Charging Time Result: View charging time in hours plus a formatted hours-and-minutes figure.

What This Calculator Provides

• ✓Real-time calculation as you type
• ✓mAh and Ah unit support with automatic conversion
• ✓Custom start and end charge percentage
• ✓Charging speed rating (Very Fast to Very Slow)
• ✓Optional energy (Wh) calculation with voltage
• ✓Full step-by-step derivation
• ✓Six built-in presets (phone, tablet, car, more)
• ✓Calculation history
• ✓Export calculation as a text file
• ✓No signup required

Pro Tips

• 💡Add extra time margin beyond the calculated estimate for the final 10-20% of charge, since most lithium-based chargers taper their current as the battery approaches full, making the last portion slower than this calculator's constant-rate assumption.
• 💡Set the end percentage to 80% rather than 100% if you're optimizing for battery longevity rather than maximum runtime — many device manufacturers now recommend this range for daily charging to slow long-term capacity fade.
• 💡Use the charger's actual rated output current, not the cable's maximum rating, when estimating charging time — a high-current cable connected to a lower-current charger charges at the charger's rate, not the cable's.
• 💡For car batteries, use a lower charging current (like 2-4A trickle charging) if you have time, since this typically achieves higher charging efficiency and is gentler on battery health than fast high-current charging.
• 💡Cross-check your calculated estimate against the device manufacturer's stated charging time when available — this helps you calibrate a more accurate efficiency percentage for that specific device and charger combination going forward.

Common Mistakes to Avoid

• ✕Assuming charging time scales the same way across the entire 0-100% range. The final stretch toward 100% is typically slower per-percent than the earlier stages, due to the constant-voltage tapering phase most lithium chargers use.
• ✕Mixing up mAh and Ah when entering battery capacity. A 4,000mAh phone battery entered as "4,000" while the unit selector is set to Ah overstates the calculation by a factor of 1,000.
• ✕Ignoring charging efficiency entirely and using the ideal (100% efficient) time as the expected result. Real charging is always somewhat less efficient, and skipping this adjustment underestimates actual charging time.
• ✕Using the charger's maximum rated current when the battery or device can't actually accept that much current. Many batteries have a maximum safe charge rate lower than what a fast charger can supply, which effectively caps real-world charging speed below what this calculator predicts.
• ✕Forgetting to set a realistic starting percentage. Calculating charging time assuming a 0% start when the device is actually at 40% overstates the time needed by including energy that's already stored in the battery.