Power banks 101


Power banks are more popular than ever.

There are tens of models to choose from with different features, but the most important thing is obviously the capacity. We explain what the power bank capacity actually means in relation to your phone battery capacity.

You bought a 9000mAh power bank, which should charge your 3000mAh phone battery about three times, right ? Actually you’re lucky to get two full charges. Where did one third of the capacity disappear ?


The main reason for capacity loss is due to voltage conversion. The power bank typically has lithium batteries inside, which have a maximum voltage of 4.2V … 4.4V, and as the batteries gets discharged, the voltage gradually decreases to about  3.2V level. But for charging purposes, we need a stable voltage of about 5V. To accomplish this, the power bank employs a voltage conversion circuit. Besides converting the voltage, it also converts some of your mAh’s to heat. The efficiency of the voltage conversion is typically about 80%.


USB cable

Short cable is good for charging

The next problem is the USB cable you are using. The cable always has some resistance, which causes heat dissipation when charging current is applied. The physical appearance of the cable tells a lot about the resistance – longer, thinner cables are higher in resistance, and you should avoid them when charging. The cable is a major factor in charging efficiency and charging speed, when using high charging current. Make sure your charging cables are up for the task!


The final obstacle is the second voltage conversion we have to do, this time inside the device you are charging. The charging voltage of 5V needs to be down-converted to a proper level for the lithium battery. The side effect is, you guessed it, heat generation. For smartphones, the efficiency is typically  85% or more. If the phone employs fast charging, the efficiency can be over 90%.


We made a measurement, where we fully discharged several power banks through a USB cable with a conservative current of 1A. The results are in the table below.


Power bankCapacity [mAh]Real capacity [mAh]Efficiency [%]Efficiency with a smartphone [%]

As you can see, the efficiency of these power banks is close to 80%. By taking the voltage conversion in the smartphone into account, the efficiency is about 66%. In other words, 2/3 of energy goes into use and 1/3 of the energy turns into heat.


Finally, few words about safety. Power banks may have 10 times the energy of your smartphone battery, which is a massive amount of energy in your pocket or bag. Be careful with them! Some tips:

  1. Buy power banks from a source you can trust
  2. Treat the power banks with caution – don’t throw them around
  3. Protect power banks from rain, direct sunlight and extreme temperatures
  4. Disconnect the power bank from the charger after it is fully charged



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