Find out how long a battery takes to charge from its current level to full (or any target). Works for phones and power banks in mAh, and for car, deep-cycle and LiFePO4 batteries in Ah, with charge losses and the slower final stage included.
Last updated:
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Adds energy in Wh
Chargers reduce the current near full (CV / absorption stage)
Enter your values to see the result.
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How to use the Battery Charge Time Calculator
Enter the battery capacity and choose mAh or Ah.
Enter the charger current in amps or milliamps (from the charger label).
Pick the battery type to set the charge efficiency, or enter your own.
Set the current and target charge level in percent.
Read the charge time, the ready-at time and the table for other levels.
Battery charge time formula
Charge time (h) = Capacity (Ah) × (Target % − Current %) ÷ 100 ÷ (Charger current (A) × Efficiency)
To convert mAh to Ah divide by 1,000, and mA to A also divide by 1,000. If you know the battery voltage, the energy added is Ah × V in watt-hours.
Example: a 20,000 mAh power bank charged from empty with a 3 A input at 90% efficiency: 20 Ah ÷ (3 × 0.9) = 7.4 hours in constant current. With the slower final stage modelled, the estimate rises to about 8.9 hours.
Typical charge efficiency and charge rates
Battery type
Charge efficiency
Usual charge rate
Charging method
Li-ion / Li-polymer
about 90–95%
0.5–1C (fast charge up to 2C+ if rated)
CC-CV, stops at low current
LiFePO4
about 95%+
0.2–1C
CC-CV to about 3.6 V per cell
Lead-acid (AGM, flooded, gel)
about 80–85%
0.1–0.3C
Bulk, absorption, float
NiMH / NiCd
about 65–75%
0.1C (14–16 h) to 1C smart charge
Constant current with −ΔV or temperature cut-off
Always follow the battery manufacturer's maximum charge current and voltage. Charging very cold lithium batteries (below 0 °C) can damage them.
Frequently asked questions
How do I calculate battery charge time?
Charge time (hours) = capacity to add (Ah) ÷ (charger current (A) × charge efficiency). A 5,000 mAh phone battery at 20% needs 4 Ah; with a 2 A charger at 90% efficiency that is 4 ÷ 1.8 ≈ 2.2 hours, plus extra time for the slower final stage.
Why does the last 20% take longer?
Lithium batteries charge at constant current up to about 80%, then switch to constant voltage, where the current gradually falls. Lead-acid batteries have a similar absorption stage. This calculator models that stage at about half the current on average, which is a common approximation.
What charge efficiency should I use?
Typical values are about 90–95% for lithium-ion and LiFePO4, 80–85% for lead-acid and 65–75% for NiMH. The rest is lost as heat in the battery and charger.
What is the C-rate?
The C-rate is the charge current divided by the capacity. Charging a 100 Ah battery at 20 A is 0.2C. Lead-acid batteries are usually charged at 0.1–0.3C, and most lithium cells at 0.5–1C unless they are rated for fast charging.
How long does it take to charge a car battery?
A 60 Ah car battery at 50% needs 30 Ah. With a 6 A charger at 85% efficiency that is about 30 ÷ 5.1 ≈ 5.9 hours, and the final absorption stage adds more. A 2 A maintainer would take well over a day.
Does a higher-watt charger always charge faster?
Only if the device and battery accept the higher current. Phones and laptops negotiate the charge rate, and many slow down when hot or nearly full, so the real time can be longer than the calculation.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.