How to Choose a Power Bank Without Paying for Fake mAh

Your phone hits 8 percent on the train home, you pull out the power bank you bought last year, and it gives you barely one full charge even though the box promised 20,000 mAh. That is not necessarily a fake. It is how battery capacity is measured, and once you understand it, learning how to choose a power bank becomes much simpler than the spec sheets make it look.

I have tested power banks for this site since 2021, mostly by draining them into the same phone and laptop and timing the results. The numbers below come from that bench, and they explain why so many buyers feel short changed.

How to Choose a Power Bank by Real Capacity

The mAh figure on the box refers to the battery cells inside, measured at around 3.7 volts. Your phone charges over USB at 5 volts or more, and converting between the two wastes energy as heat. In practice you get roughly 60 to 70 percent of the printed number delivered to your device.

A more honest number is watt hours (Wh), which many good brands print on the back. Multiply mAh by 3.7 and divide by 1,000: a 10,000 mAh pack is about 37 Wh. If the Wh figure on the label does not match that maths, treat the listing with suspicion.

Spotting capacity that really is fake

Some cheap marketplace power banks claim 30,000 or 50,000 mAh in a body the size of a deck of cards. Battery cells have physical limits. A genuine 20,000 mAh pack weighs roughly 330 to 450 grams. If a "40,000 mAh" model weighs 220 grams, the capacity is invented. Weight is the quickest lie detector you have.

Charging Speed and Ports

Capacity tells you how much; speed tells you how fast. Look for these terms:

• USB Power Delivery (USB-PD): the standard most modern phones, tablets and laptops use over USB-C. 20 W is enough for fast phone charging. 45 W or more is needed for most laptops.
• Total output vs per port output: a pack rated "65 W" might only give 65 W from one port and split down to 30 W plus 30 W when two devices are plugged in.
• Input speed: how fast the power bank itself recharges. A 20,000 mAh pack with 18 W input takes about 5 to 6 hours to refill; with 45 W input, under 2.5 hours.
• Pass through charging: lets you charge the bank and a phone from one wall socket overnight. Handy on trips.

For most people, one USB-C port with PD at 20 to 30 W and one USB-A port is plenty. More ports mean more weight and price for features you may never use.

Cables matter more than people think

A 45 W power bank cannot deliver 45 W through a thin 3 dollar cable. On my bench, the same 20,000 mAh pack charged a laptop at 43 W with a rated 60 W USB-C cable and at only 15 W with a generic cable from a drawer. If your power bank feels slow, swap the cable before you blame the battery. Look for a cable rated at least 60 W for laptops, and keep it short; a 30 cm cable is easier to carry and loses less power than a 2 metre one dangling from your bag.

Why Bigger Is Usually the Wrong Buy

The default advice is to buy the biggest capacity you can afford so you never run out. In my experience that leads to a 500 gram brick that stays at home in a drawer because it is too heavy to carry every day. A power bank you leave behind has zero capacity.

Think about your actual worst day instead. If your phone usually ends the day at 20 percent, you need half a charge, not three. A 10,000 mAh pack around 180 to 220 grams covers that with room to spare and fits in a jacket pocket. Buy a 20,000 mAh model only for multi day trips, camping or laptop charging.

There is a travel angle too. Airlines generally allow power banks up to 100 Wh in cabin luggage without approval, and between 100 and 160 Wh only with airline approval. A 27,000 mAh pack sits just under 100 Wh. Anything bigger can be taken off you at security.

Safety Checks Before You Pay

A power bank is a dense lithium battery you will carry next to your body and leave charging near your bed, so this is not the place to save 8 dollars on an unknown brand.

1. Look for a known brand with a real support page and a warranty of at least 12 months. Our piece on whether an extended warranty is worth paying for explains why the standard one usually suffices here.
2. Check for safety certification marks relevant to your region printed on the casing, not just in the listing photos.
3. Read recent one star reviews for words like "swollen", "hot" or "smell".
4. Avoid secondhand or open box power banks with no history. Battery health is invisible from the outside, unlike the labels we cover in open box, refurbished or renewed labels.
5. Stop using any pack that bulges, gets hot while idle or makes a crackling sound.

What a Fair Price Looks Like

From my testing this year, a good 10,000 mAh pack with 20 W USB-PD costs around 20 to 30 dollars. A solid 20,000 mAh model with 30 to 45 W output lands around 35 to 55 dollars. Laptop capable 65 W packs with 20,000 to 25,000 mAh start near 60 dollars. Prices well below those ranges usually mean slow charging, inflated capacity or both. The general rules in our guide to buying tech without overpaying apply well here: compare the spec that matters, not the headline number.

FAQ

How many mAh power bank do I need for my phone?

For one full phone charge with a little spare, 10,000 mAh is enough for nearly every current phone. Choose 20,000 mAh only if you need two or more charges or travel for several days without a socket.

Why does my 10,000 mAh power bank not fully charge my phone twice?

Because voltage conversion and heat losses mean you get about 60 to 70 percent of the printed capacity. A 10,000 mAh bank typically delivers around 6,500 mAh to your phone.

Can I take a power bank on a plane?

Usually yes, in hand luggage only, up to 100 Wh without special approval. Check the Wh rating printed on the pack and your airline's own rules before you fly.

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

A good quality bank stops charging when full, so it is generally fine. Still, charge it on a hard surface away from bedding, and unplug any pack that feels unusually hot.