You buy a power bank with a capacity of, for example, 10,000 mAh, but it charges your mobile 2–3× and it turns out that the reality is closer to 80–85% of the value stated on the packaging. Don't worry, it's not a manufacturer's scam or a faulty unit. The reason for the lower actual capacity of power banks lies in technology and physics.
The number stated on the packaging, 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 is behind it?
The voltage difference plays a crucial role
The main essence of the mystery lies in the voltage of the power bank.
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 indeed 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 actual capacity decreases. This is due to two factors:
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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 (or more for fast charging). Due to the need for higher voltage, the actual 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 the value of 7,400 mAh is not final. Like no other 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 aspect as well, 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 with the same capacity but from different manufacturers on your desk. Each of them will likely provide a different amount of energy.
The difference is made by the quality of the internal electronics, the efficiency of energy conversion, or the degree of heating. And so, power banks that appear identical at first glance will provide a different supply of "juice."
BMS system, or protection against complete discharge
Every power bank hides a feature without which it could not function.
It is 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 complete discharge of the power bank (it could destroy it), which is why the BMS ensures that the battery cells are never actually completely depleted.
When you see 0% on the display, it means you can no longer charge, but there is still some reserve energy left inside the power bank. The same applies at 100% battery (there is still a reserve, but you cannot 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 during security checks or on the plane.
Do not leave your power bank completely discharged for a long time
We have one more useful tip for you at the end.
When you return from vacation or a trip, do you leave your depleted power bank lying in a drawer and don't recharge it for a long time? That's not a good idea.
Even when turned off, it slowly continues to discharge internally. Once the charge level drops below a safe threshold, 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 a long time.
If you have any 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 offer classic power banks with an LCD display as well as wireless power banks with MagSafe wireless charging.
