Battery Charge Time Calculator (phone, power bank & laptop)

Understanding the battery charge time calculator — concept and principle

How long will your phone, power bank or laptop take to charge? This calculator converts battery capacity in mAh and nominal voltage into watt-hours, works out the energy needed between your current and target charge levels, and divides it by the charging power that actually reaches the battery. It also shows energy drawn from the wall and the average charge rate.

Adding 60 % of a 19.25 Wh battery at 21.25 W effective takes about 33 minutes.

How it is calculated

Battery energy (Wh) = capacity (mAh) ÷ 1,000 × nominal voltage

Energy to add = battery Wh × (target % − start %) ÷ 100

Charging power = the lower of charger power and device maximum

Time (min) = energy to add ÷ (power × efficiency) × 60

Energy from the wall = energy to add ÷ efficiency

Average charge rate (C) = power × efficiency ÷ battery Wh

The principle

Energy equals power times time, so time equals energy divided by power. A phone and a charger negotiate a power level no higher than either supports, and some energy is lost as heat in the charger, cable and battery, captured by the efficiency. This gives the time for the fast, constant-current part of a lithium-ion charge. Above roughly 70%, chargers hold the voltage constant and the current gradually tapers (the saturation phase), so the last part to 100% takes noticeably longer than this simple estimate.

Worked example

Default inputs: 5,000 mAh, 3.85 V, from 20% to 80%, 25 W charger, device accepts 25 W, 85% efficiency.

Battery energy = 5 Ah × 3.85 V = 19.25 Wh.
Energy to add = 19.25 × 60% = 11.55 Wh.
Charging power = min(25, 25) = 25 W; effective = 25 × 0.85 = 21.25 W.
Time = 11.55 ÷ 21.25 × 60 = 32.6 min, shown as 33 minutes (0:32:37).
Energy from the wall = 11.55 ÷ 0.85 = 13.59 Wh; average rate = 21.25 ÷ 19.25 = 1.1 C. Since the target is above 70%, the slower top-up phase is reached (allow extra time).

Each point is the calculator re-run with only “Charger power (W)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Charger power (W)	Charging time (minutes)
6.25	130
12.5	65
18.8	43
25	33
31.2	33
37.5	33
50	33

How to read the result

Treat the result as a best case for the fast phase. Real phones also slow charging when they are warm or in use, and many manage charging to reduce battery wear. Charging to about 80% is much quicker than charging to 100% and is commonly recommended to reduce long-term wear.

Common mistakes

Using the charger's rating when the device accepts less.
Ignoring the slow phase above about 70%–80%.
Using mAh from a power bank as if it were at the battery's voltage; power banks quote cell capacity at about 3.6–3.85 V.
Expecting the same speed while gaming or navigating.

Keep in mind: Estimates assume steady conditions. Use the charger supplied or certified for your device, and avoid charging in hot places.

Frequently asked questions

Why does charging slow down near full?

Lithium-ion chargers switch to constant voltage and the current tapers to protect the battery.

Does a more powerful charger help?

Only up to the maximum power your device accepts.

What does 1 C mean?

A charging power equal to the battery's capacity per hour; 1 C would fill an empty battery in about an hour.

Why is energy from the wall higher?

Some energy is lost as heat in the charger, cable and battery.

Sources: Battery University (Cadex Electronics), BU-409 "Charging Lithium-ion"

Checked 9 Nov 2026. Battery University (Cadex Electronics), BU-409 “Charging Lithium-ion” (constant-current stage to about 70%, then saturation charge at constant voltage with falling current; full charge when current drops to 3–5% of rating); energy E (Wh) = Q (Ah) × V and t = E ÷ P.

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