Why does my 20,000 mAh power bank only charge my phone three times?

Voltage conversion losses (3.7V cell to 5V output) consume around 30–35% of rated capacity. A 20,000 mAh bank realistically delivers ~13,000 mAh to a phone — enough for 3 charges of a 4,500 mAh phone.

Can I take a power bank on a flight?

Yes, in carry-on baggage only, up to 100Wh without approval. Most consumer banks under 27,000 mAh are safe; check the Wh rating printed on the bank.

Is it bad to leave a power bank plugged in overnight?

Modern banks have over-charge protection and will stop drawing power when full, so it is safe — but keeping a Li-ion cell at 100% for long periods slightly accelerates aging. Unplug when convenient.

Power Banks 18 February 2025 8 min read

Power Bank Buying Guide: Capacity, Wattage & PD Explained

How to choose a power bank in 2025 — mAh vs Wh, fast-charge wattage, port count, airline rules and what to actually look for in a daily carry power bank.

A power bank is the single most useful accessory you can carry — until it is too heavy, too slow or banned at the airport. Here is what to look for so you buy one that you will actually keep in your bag.

Capacity: mAh is marketing, Wh is reality

Phone batteries are measured at 3.7V; power banks are quoted in mAh at the same voltage. A 10,000 mAh power bank holds about 37 Wh, but real-world output to your phone is ~6,500–7,000 mAh after voltage conversion losses. Plan for ~65–70% of the rated capacity reaching your device.

10,000 mAh: ~2 full phone charges, airline-safe
20,000 mAh: ~4 full charges or one laptop top-up
27,000 mAh / 100Wh: airline cap — anything bigger is restricted

Output wattage matters as much as capacity

A 20,000 mAh power bank that outputs 10W will charge your phone slower than your wall socket. Look for at least 18W PD output for phones and 30W+ if you also charge a tablet. For laptops, 65W–100W PD is the threshold.

Ports and pass-through charging

Two ports (1×USB-C, 1×USB-A) is the minimum useful configuration. A USB-C port that supports both input and output (so the same port charges the bank and your phone) keeps your cable count down. Pass-through charging (using the bank while it is being charged) is convenient but heats the cells — use sparingly.

Airline rules: do not get caught out

Power banks must travel in carry-on, never checked baggage. Most airlines cap at 100Wh (about 27,000 mAh) per bank. 100–160Wh needs airline approval; above 160Wh is banned. Always check the Wh rating printed on the casing before flying.

Safety features to insist on

BIS certification (mandatory in India), short-circuit protection, over-charge protection and temperature cut-off. A flame-retardant shell and quality LiPo cells from a known cell supplier make the difference between a 3-year accessory and a 6-month one.

Aeromex power banks (coming soon)

Aeromex is launching its first power-bank range in 2025 with 10,000 mAh and 20,000 mAh models featuring 22.5W and 65W PD output, Type-C in/out and full BIS certification. Sign up via our distributor page to be notified at launch.

Frequently asked questions

Why does my 20,000 mAh power bank only charge my phone three times?: Voltage conversion losses (3.7V cell to 5V output) consume around 30–35% of rated capacity. A 20,000 mAh bank realistically delivers ~13,000 mAh to a phone — enough for 3 charges of a 4,500 mAh phone.
Can I take a power bank on a flight?: Yes, in carry-on baggage only, up to 100Wh without approval. Most consumer banks under 27,000 mAh are safe; check the Wh rating printed on the bank.
Is it bad to leave a power bank plugged in overnight?: Modern banks have over-charge protection and will stop drawing power when full, so it is safe — but keeping a Li-ion cell at 100% for long periods slightly accelerates aging. Unplug when convenient.