Why can't a 20000 mAh power bank charge my 5000 mAh phone four times?

Power-bank mAh is quoted at the internal cell voltage of 3.7 V, so 20000 mAh is 74 Wh. The bank boosts that to 5 V, and the phone steps it back down to charge its 3.85 V battery, losing energy at each stage, so overall efficiency is often only 60–75%. A 5000 mAh phone battery holds about 19 Wh, giving roughly 2.5–3 full charges. Comparing batteries in Wh avoids this confusion because Wh already includes the voltage.

How do I convert mAh to Wh?

Wh = mAh × voltage ÷ 1000. A 3000 mAh 18650 cell at 3.6 V stores about 10.8 Wh, and a 12 V 60 Ah car battery about 720 Wh. You can only compare mAh directly at the same voltage: a 1.2 V 2000 mAh NiMH cell holds just 2.4 Wh, less than a third of a lithium cell with the same mAh rating.

What does C-rate mean?

C-rate is current divided by capacity: 1C empties the battery in one hour, which is 3 A for a 3 Ah cell; 0.5C is 1.5 A and 2C is 6 A. Higher rates mean more voltage drop and heat from internal resistance, less usable capacity and shorter life. Phone fast charging runs at about 1–2C and power-tool cells discharge at 3–10C. Lead-acid capacity is usually rated at 0.05C (the 20-hour rate) and shrinks noticeably at high current, which is Peukert's law.

Why is the last 20% of charging so slow?

Lithium batteries charge in two phases, constant current then constant voltage (CC-CV). The charger pushes a steady current until the cell reaches its maximum voltage (4.2 V for NMC), which happens at around 70–85% charge. It then holds that voltage while the current tapers off to about 0.05C, and this tail can take a third or more of the total time. The faster the charge, the earlier the taper starts, which is why '80% in 30 minutes' is common but a full charge takes over an hour.

What size power bank can I take on a plane?

Under the common IATA rules, lithium batteries and power banks up to 100 Wh can go in your carry-on but not in checked bags; 100–160 Wh needs airline approval and is usually limited to two per person; above 160 Wh is not allowed at all. Use the cell voltage (usually 3.6 or 3.7 V) to convert: at 3.7 V, 100 Wh is about 27000 mAh. Some countries add rules, for example China has required the CCC mark on power banks for domestic flights since June 2025, so check with your airline before you travel.

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🔋Battery Capacity & Runtime Calculator - mAh to Wh, Charge Time, Battery Packs

Enter a capacity and voltage, or pick a preset (AA alkaline, 18650 cell, phone, laptop, power bank, EV pack or lead-acid car battery), to get the energy, load current, C-rate and runtime, plus a CC-CV estimate of the charge time. The discharge curve tab compares the voltage shapes of NMC, LFP, NiMH, lead-acid and alkaline cells and shows Peukert's law and the effect of cold. You can also build an nSmP pack from real cells with a wiring sketch, compare energy density against gasoline and hydrogen, and check whether a power bank may fly.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

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How to use Battery Capacity, Runtime & Pack Calculator

1Choose a battery preset, or enter the capacity in mAh, nominal voltage and chemistry
2Set the load as battery current, power through a converter, or output voltage × current (for example USB 5 V at 2 A)
3Read the energy, C-rate and runtime, and enter a charge current or power below to estimate the time to full
4Use the other tabs for discharge curves, series/parallel packs, energy density and the airline check

FAQ

Why can't a 20000 mAh power bank charge my 5000 mAh phone four times?: Power-bank mAh is quoted at the internal cell voltage of 3.7 V, so 20000 mAh is 74 Wh. The bank boosts that to 5 V, and the phone steps it back down to charge its 3.85 V battery, losing energy at each stage, so overall efficiency is often only 60–75%. A 5000 mAh phone battery holds about 19 Wh, giving roughly 2.5–3 full charges. Comparing batteries in Wh avoids this confusion because Wh already includes the voltage.
How do I convert mAh to Wh?: Wh = mAh × voltage ÷ 1000. A 3000 mAh 18650 cell at 3.6 V stores about 10.8 Wh, and a 12 V 60 Ah car battery about 720 Wh. You can only compare mAh directly at the same voltage: a 1.2 V 2000 mAh NiMH cell holds just 2.4 Wh, less than a third of a lithium cell with the same mAh rating.
What does C-rate mean?: C-rate is current divided by capacity: 1C empties the battery in one hour, which is 3 A for a 3 Ah cell; 0.5C is 1.5 A and 2C is 6 A. Higher rates mean more voltage drop and heat from internal resistance, less usable capacity and shorter life. Phone fast charging runs at about 1–2C and power-tool cells discharge at 3–10C. Lead-acid capacity is usually rated at 0.05C (the 20-hour rate) and shrinks noticeably at high current, which is Peukert's law.
Why is the last 20% of charging so slow?: Lithium batteries charge in two phases, constant current then constant voltage (CC-CV). The charger pushes a steady current until the cell reaches its maximum voltage (4.2 V for NMC), which happens at around 70–85% charge. It then holds that voltage while the current tapers off to about 0.05C, and this tail can take a third or more of the total time. The faster the charge, the earlier the taper starts, which is why '80% in 30 minutes' is common but a full charge takes over an hour.
What size power bank can I take on a plane?: Under the common IATA rules, lithium batteries and power banks up to 100 Wh can go in your carry-on but not in checked bags; 100–160 Wh needs airline approval and is usually limited to two per person; above 160 Wh is not allowed at all. Use the cell voltage (usually 3.6 or 3.7 V) to convert: at 3.7 V, 100 Wh is about 27000 mAh. Some countries add rules, for example China has required the CCC mark on power banks for domestic flights since June 2025, so check with your airline before you travel.

Features

mAh ↔ Wh conversion with runtime, C-rate, converter efficiency and depth of discharge
Power-bank conversion from 3.7 V cell capacity to 5 V output, explaining the missing charges
CC-CV charge time with charge and current curves; Peukert capacity curve for lead-acid
Typical discharge curves, mean voltage and cold-weather capacity for five chemistries
nSmP pack builder: voltage range, capacity, energy, max current, mass and a wiring sketch
Energy density comparison including gasoline and hydrogen, cycle life table and a 100/160 Wh airline check

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