Here's why power banks aren't as big as they seem to be
But since the average Joe does not have a degree in electrical engineering, the capacity of power banks is often misinterpreted by consumers. Many assume that if a power bank can hold, let's say, 10,000 mAh of charge, it will be able to fully recharge a hypothetical 2,000 mAh smartphone battery exactly five times. This would have been true in a perfect universe, but in the real world, the math involves quite a few more variables.
For one to understand what causes the inevitable difference between a power bank's advertised and effective capacities, they have to be familiar with the device's inner workings. In a nutshell, there are two essential components inside of a power bank – a battery for storing charge and an electrical circuit in control of the charging process. When a power bank is advertised as having a charge capacity of, for example, 10,000 mAh, that means it contains one or several battery cells with a combined charge capacity of 10,000 mAh. Makes sense, doesn't it?
Here's where it gets tricky and technical. Battery cells based on lithium-ion chemistry, such as the one inside the bank and your smartphone, have a nominal or average voltage of around 3.7 volts. We said "average" because a battery's voltage does not remain constant – it drops as energy reserve is used and increases as the battery is being recharged. Modern smartphone batteries are considered 100% full at about 4.3 volts and empty as they get drained to around 3 volts, hence the 3.7 V figure above. Unfortunately, this low and varying voltage is of no use to your smartphone.
To get the charging juices flowing, a power bank provides 5 volts of boosted voltage output, not 3.7 volts. Why? Because that's the standard voltage a smartphone's charger (or a computer's USB port) delivers, and that's what the charge control circuitry inside a smartphone expects to receive. These 5 volts are then precisely stabilized by the phone's circuitry and reduced to match the maximum voltage of the phone's battery, which is somewhere around 4.3 volts. This may all seem like overkill, but it is not. All this precise control over the charging process is implemented for safety reasons because a battery may explode if it is fed more voltage than it can handle. Conversely, a battery will not charge to its best potential if it is provided with less voltage than it is designed to receive.
Another factor that has to be taken into account is the rate at which the power bank is discharged – drawing more current decreases efficiency. In most cases, there's nothing you can do about this as most power banks are designed to provide 2 amps of output current. However, some models offer both 1-amp and 2-amp outputs. Using the 1-amp output would charge your smartphone slowly, but will provide a few percent more charge.
We must also add that batteries hate extreme temperatures. Your power bank will lose charge if it is stored in a chilly place or on your car's dashboard under the scorching sun. Extreme heat or cold may even cause permanent damage to its battery.
Even if you treat your power bank well, it will lose charge over time. This is due to a phenomenon called self-discharge – a full battery may be empty in a matter of months even if not used. That's why it is a good idea to discharge and recharge a power bank on a monthly basis.
TL;DR: The actual mAh capacity of a power bank is smaller because the 3.7 V nominal voltage of its battery has to be boosted to 5 V, and this results in about 10% losses. Also, power bank capacity is affected by many factors, including ambient temperature, charging current, the health of the bank and the phone's batteries, and the efficiency of your phone's charge regulating circuitry. To fully charge a smartphone's battery, you should aim for a power bank with a mAh rating of at least 25-30% more than the mAh capacity of your smartphone's battery.