You might get a power bank with a capacity of, for example, 10,000 mAh, but it will only charge your phone 2-3 times, and it turns out that the reality corresponds more to 80-85% of the value stated on the package. Don't worry, it's not a manufacturer's scam or a faulty unit. The reason behind the lower real capacity of power banks lies in technology and physics.
The number stated on the package, or directly on the power bank, does not correspond to how many times you will charge your phone. This applies to all power banks in the world. But what's behind it?
The difference in voltage plays a crucial role
The main essence of the mystery lies in the power bank's voltage.
Manufacturers (including us at FIXED) state the capacity of the battery cells inside the power bank, which have a voltage of 3.7 V. And under these circumstances, it is true that the capacity is 10,000 mAh, so manufacturers are doing everything correctly.
However, when you start charging a phone or other device from the power bank, its real capacity decreases. Two factors are responsible for this:
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USB output has a voltage of 5 V or more
Battery cells in a power bank typically have a voltage of 3 V, but the USB output you charge from requires 5 V (and even more for fast charging). Due to the need for higher voltage, the real capacity decreases; for a power bank with a declared capacity of 10,000 mAh, this means a drop to approximately 7,400 mAh.
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Conversion losses and heat
However, even 7,400 mAh is not the final value. Just like no device, a power bank does not operate with 100% efficiency; losses occur during charging in the cable or voltage converter, and some energy is converted into heat (which is why a power bank often feels warm to the touch).
When you factor in this, the real capacity of the power bank comes out to somewhere between 6,000–6,300 mAh at 5 V.
INTERESTING FACT: FIXED power banks are manufactured to have the highest possible efficiency, ranging at least between 80–85%.
Two different power banks can behave differently
Imagine you have two power banks of the same capacity, but from different manufacturers, on your desk. Each of them will likely provide a different amount of energy.
The difference lies in the quality of the internal electronics, energy conversion efficiency, or the degree of heating. Thus, power banks that appear identical at first glance will provide a different amount of "juice".
BMS system or protection against complete discharge
Every power bank contains a feature without which it could not function.
It's called the Battery Management System (BMS), and its task is to protect the battery from overcharging, complete discharge, short circuits, and overheating.
The biggest risk is the complete discharge of the power bank (it could destroy it), so the BMS ensures that the battery cells never completely run out of power.
When you see 0% on the display, it means you can no longer charge, but there's still a little reserve energy left inside the power bank. The same applies at 100% battery (there's still a reserve, but you can't charge the power bank further).
Thanks to the BMS and the safety reserve, the power bank will not be damaged, will last a long time, and you won't find it unable to turn on one day due to "under-discharge."
TIP: Also read the article on how to fly with electronics in 2026. This will help you avoid having your power bank and other devices confiscated at security checks or on the plane.
Don't leave your power bank completely discharged for too long
We have one more useful tip for you at the end.
When you return from a holiday or trip, do you leave a depleted power bank in a drawer and don't recharge it for a long time? That's not a good idea.
Even though it's turned off, it slowly continues to discharge internally. Once the charge level drops below a safe limit, these consequences can occur:
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significant reduction in capacity
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charging problems
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problems turning on the power bank
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in extreme cases, device malfunction
Therefore, we recommend maintaining a charge level of at least 40%, even if you know you won't be using the power bank for an extended period.
If you have further questions, do not hesitate to contact us. Or ask our AI assistant FIXIE in the chat.
Or simply choose a power bank from the Czech brand FIXED. We have classic ones with an LCD display as well as wireless power banks with MagSafe wireless charging.
