Why the Number on the Case Is Not Equal to What You Actually Get

The capacity of a power bank is indicated in milliampere-hours at the voltage of the battery cells themselves, which is approximately 3.7 volts. Your phone, meanwhile, receives energy through the connector at a voltage of 5 volts or higher if fast charging is being used. Between these two values, there is a voltage converter, and it consumes part of the energy by converting it into heat. In addition, losses occur in the cable itself and in the phone's charging controller. In total, approximately 80 to 90 percent of the stated capacity reaches the device, which is why a 10,000 mAh power bank charges a 5,000 mAh phone approximately one and a half to two times, rather than exactly twice.

The Unit You Should Use for Comparison

Milliampere-hours do not allow you to compare devices with different voltages, which is why specialists recommend looking at watt-hours. The formula is simple: capacity in milliampere-hours is multiplied by 3.7 and divided by 1,000. A 10,000 mAh power bank has approximately 37 watt-hours. The same device with a capacity of 20,000 mAh has approximately 74 watt-hours.

Now compare this with what you actually need. A phone with a 5,000 mAh battery has approximately 19 watt-hours. A laptop usually has between 40 and 80 watt-hours. This unit is also used by airlines, since restrictions on carrying power banks in hand luggage are usually set in watt-hours, and the typical limit is 100 watt-hours, which approximately corresponds to 27,000 mAh.

Capacity and Power Are Different Characteristics

Capacity in watt-hours answers the question of how long the device will last. Power in watts answers the question of what can actually be connected and how quickly it will charge. A power bank with a large capacity and low power output will charge a phone slowly and may not power a laptop at all. For a phone, 18 or 20 watts is sufficient, and this is enough for fast charging on most models. For a tablet, a comfortable range is from 15 to 30 watts. For a laptop, support for the Power Delivery standard through a USB-C port and an output of at least 45 or 65 watts are required. With lower power, the laptop may either not start charging at all or charge more slowly than it discharges during use.

This is why a power bank for a phone and a power bank for a laptop are different devices, even if both are labeled as 20,000 mAh.

The Charging Speed of the Power Bank Itself

Cheap models take eight hours or more to charge, and during schedules of power outages, this means that the device simply does not have enough time to recharge between periods when electricity is available. Therefore, you should look not only at the output power but also at the input power, and the availability of fast charging through USB-C for the power bank itself is often more important than anything else.

What to Choose for a Specific Task

For everyday carrying in a bag, 10,000 mAh is suitable because it is a compromise between weight and capacity. Such a device weighs approximately 200 grams.

For long power outages at home, 20,000 mAh or more makes sense, although carrying such a device every day is inconvenient.

For a laptop, you need 20,000 mAh or more together with support for 65-watt Power Delivery.

For powering a router, almost any model with sufficient capacity is suitable, since a router consumes approximately 10 to 20 watts, and even a small power bank can keep a home network running for many hours. Check whether your router has USB-C power input, or purchase an appropriate adapter.

What You Should Avoid

Unnamed devices claiming 50,000 mAh for a few hundred hryvnias. Such capacity in a case of that size and at that price is physically unrealistic, while the quality of the cells in a device that you carry in your backpack is a safety issue.

Models without a stated output power in their specifications. If the manufacturer indicates only the capacity, this is already a warning sign.

Devices without protection against overheating, overcharging, and short circuits, since these functions are not available in all cheap models.

About Automatic Shutdown

There is a practical problem that is rarely discussed.

Powerful power banks often automatically switch off when the connected device consumes very little energy, and this applies to wireless headphones, fitness trackers, and smartwatches.

The device considers charging complete and stops supplying power. If you plan to charge small electronic devices, look for models with a low-current mode.

How to Check the Real Capacity at Home

It is impossible to make an exact measurement without equipment, but you can make an estimate. Fully charge the power bank, then fully discharge it by charging your phone and count the number of full charges. Multiply your phone's battery capacity by the number of charges, and you will get the approximate actual output. If a phone with a 4,000 mAh battery was charged twice, the actual output was approximately 8,000 mAh. Compare this with the stated figure while taking into account natural losses of approximately 15 percent. If the difference is significantly larger, the device is either worn out or its specifications are overstated.

Care and Storage

Do not keep a power bank fully charged for months, since the optimal state for long-term storage is approximately half of its capacity, with a check once every few months. Do not leave the device in direct sunlight, in a car during summer, or near heat sources. Cover the contacts if you carry the power bank together with small metal objects. A swollen or damaged device must not be used, and it should not be stored at home either, since it presents a real fire hazard. I wrote separately about lithium battery care in more detail.


Sources

Channel 24 Publications, NV Techno, industry materials from portable power sources