About the Battery Charge Time Calculator
This battery charge time calculator estimates how many hours a charger needs to top up a battery. Enter the battery capacity in mAh (1 Ah = 1,000 mAh), the charging current, the current state of charge and the level you want to reach, plus the charging efficiency, and it returns the charging time, the C-rate, the charge added and the energy drawn.
It works for phone and laptop cells, 18650 packs, power banks, drone and RC LiPo packs, NiMH AA cells and 12 V lead-acid car or leisure batteries. Hobbyists use it to pick a charger, campers to plan solar or generator time, and anyone wondering whether a battery will be full by morning.
The estimate assumes the charger delivers its rated current for the whole charge. Lithium-ion chargers switch to constant-voltage mode near full and the current tapers, so the last 10–20% takes noticeably longer than the linear estimate; lead-acid batteries behave similarly during absorption. Charging to 80% is therefore much quicker than charging to 100%.
With the default inputs, the charging time is 2.82 h. Change any value above to recalculate instantly.
How to use the battery charge time calculator
- 1Enter the battery capacity in mAh (Ah × 1,000).
- 2Enter the charger output current in mA.
- 3Set the current charge level and the level you want to reach.
- 4Adjust charging efficiency for your battery chemistry.
- 5Read the charging time, C-rate and energy figures.
Formula and method
The charge a battery needs is its capacity multiplied by the fraction you want to add: going from 20% to 100% of a 3,000 mAh cell means 2,400 mAh. Dividing by the charging current gives the ideal time, but some energy is lost as heat and in side reactions, so only part of the current is stored. Dividing by the efficiency (for example 0.85) accounts for that.
The C-rate is charging current ÷ capacity: 1,000 mA into a 3,000 mAh battery is 0.33C. Energy figures multiply the charge added (in Ah) by the nominal voltage. The model is constant current throughout; real chargers taper current near full charge, which lengthens the final stage.
- Capacity
- Rated battery capacity in mAh
- Start, Target
- Initial and final state of charge (%)
- Charging current
- Current delivered to the battery in mA
- Efficiency
- Fraction of input charge actually stored
- C-rate
- Charging current divided by capacity
Worked examples
3,000 mAh 18650 cell on a 1 A charger, 20% to 100%
The cell needs 80% × 3,000 = 2,400 mAh. At 85% efficiency only 850 mA of the 1,000 mA is stored, so 2,400 ÷ 850 = 2.82 h, about 2 h 49 m. That stores 2.4 Ah × 3.7 V = 8.88 Wh and draws 10.45 Wh.
60 Ah car battery from 50% with a 10 A charger
Half of 60 Ah is 30,000 mAh. Ten amps at 85% efficiency stores 8.5 A, so 30,000 ÷ 8,500 = 3.53 h. The battery gains 30 Ah × 12 V = 360 Wh, drawing about 424 Wh from the charger.
5,000 mAh phone battery to 80% at 2 A
80% of 5,000 mAh is 4,000 mAh. With 90% efficiency the battery receives 1,800 mA, so 4,000 ÷ 1,800 = 2.22 h (2 h 13 m) at 0.4C, storing 4 Ah × 3.85 V = 15.4 Wh.
Frequently asked questions
How do I calculate battery charging time?+
Divide the charge needed (capacity × fraction to add) by the charging current, then divide by the efficiency. A 2,000 mAh battery charged from empty at 500 mA with 80% efficiency takes 2,000 ÷ 500 ÷ 0.8 = 5 hours.
Why does the last 20% take longer to charge?+
Lithium-ion chargers use constant current until the cell reaches about 4.2 V, then hold the voltage constant while the current gradually falls. That constant-voltage phase can take an hour or more, so 0–80% is far quicker than 80–100%.
What charging efficiency should I use?+
Lithium-ion cells store almost all the charge they receive (coulombic efficiency above 99%), but charger, wiring and heat losses mean 90–95% is a sensible overall allowance. Lead-acid batteries are typically around 80–85% and NiMH lower still, which is why NiMH chargers often put in about 1.2–1.4 times the rated capacity.
What is a C-rate?+
It is the charge or discharge current relative to capacity. 1C charges a battery in about an hour, 0.5C in about two hours. Many Li-ion cells are rated for 0.5C standard charging and 1C fast charging.
Does a higher-wattage charger always charge faster?+
Only if the device accepts the extra current. Phones and laptops negotiate power with the charger and the battery management system limits current by temperature and charge level, so a 100 W charger may deliver far less to a small battery.