Today, battery life equals social life. If your smartphone's battery dies, your social life dies, or at least it sleeps until you can charge your phone. But with a charging case or a powerbank, you can keep your phone – and your social life – longer.
That said, charging cases and power banks may not always work exactly as their specifications or marketing would suggest. While a typical powerbank or charging case can come with an advertised capacity of 20,000 mAh, that doesn't mean it can charge your Motorola Edge Plus or Samsung Galaxy S20 Ultra exactly four times. In practice, its actual (ie transferable) capacity is probably about two-thirds of this, meaning it can only charge a smartphone with a 5,000 mAh battery twice (or two and a half times) before charging itself .  In this article, we explain how exactly charge drops and power banks work. The most important thing is that we provide a general rule of thumb to estimate how much real impact they actually have, so that you have a more realistic idea of what to expect from your electricity bank or charging case. Hopefully this will help your phone to live longer, longer.
Battery and electric bank capacity explained
Power banks are basically portable batteries for your smartphone. If your iPhone or Android runs out of its own battery power, you can use a powerbank to charge it. As such, they are good if you travel frequently or are on the move often.
However, a 20,000 mAh power bank does not mean that you can transfer exactly 20,000 mAh to your smartphone at a single charge. In practice, your phone will get less out of your electricity bank than 20,000 mAh. Generally, your power bank can transfer about two-thirds (66%) of its own battery power to your smartphone, and there are two main reasons for this.
Reason 1 : Power bank output of 3.7 volts, while USB technical standards charge smartphone batteries at 5 volts. This creates an imbalance between the output from the power bank and the input on your phone.
For example, if your power bank has a capacity of 20,000 mAh, multiply this by 3.7 to reveal that it has total energy – measured in mWh – of 74,000 mWh. But it will need to be output at 5 volts to charge a smartphone, so dividing 74,000 mWh by 5 – to convert back to mAh – will correspond to a smartphone battery charge of 14,800 mAh.
As I said, in practice, your 20,000 mAh Power Bank will not even provide a total charge of the 14,800 mAh smartphone battery, as there is another factor that reduces its actual total output.
Reason 2 : Inefficiency in the charging process also means that 20,000 mAh power bank charge corresponds to noticeably less than 20,000 mAh smartphone battery charge. For example, electrical resistance in the USB cable can reduce the total amount of energy transferred to your smartphone. Similarly, a certain percentage of the power bank's energy can be converted to heat during the charging process, which of course means that it is not stored as energy in your smartphone's battery. In addition, environmental factors such as temperature can affect the charging efficiency.
In total, inefficiency can be expected to reduce your power bank's transferable energy by about 10%. If your 20,000 mAh power bank can be assumed to have 14,800 mAh of real, transferable power, inefficiency will mean that it actually has 13 3200 mAh of power, it can transfer to your phone in total.
As a result, 20,000 mAh Power Bank can actually charge a 5,000 mAh smartphone about 2.66 times before it needs to charge itself. Of course, this figure will vary depending on your powerbank or phone, so expect more charges depending on if you have a larger powerbank and / or a phone with a smaller battery. In general, as we said above, we will multiply the battery capacity by 3.7, divide it by 5 and reduce by 10% a rough estimate.
Much the same goes for charge cases. They also produce 3.7 volts, while, again, smartphone batteries are powered at 5 volts due to USB requirements. In combination with the inefficiency factor, they actually provide about 66% of their advertised charge.
As I said, they can be somewhat less inefficient than power banks, since they do not require the use of cables and can instead connect directly to your phone. On the other hand, due to size limitations, their batteries are generally smaller than at power banks, so you will probably get less juice out of them overall.