Calculating Power Bank Capacity

A Simple Example of Calculating Power Bank Capacity for Your Devices

Almost all manufacturers specify the battery capacity installed in a power bank, but this is not the same as the output capacity.

For example, consider a 40,000 mAh power bank rated at 3.7V. A power bank can operate in multiple modes, stepping up the voltage from 3.7V to 5V, 9V, 12V, and so on. The efficiency rate typically ranges from 70% to 90% (±), or sometimes even lower.

The battery in a power bank (the type commonly used today) should not be fully discharged to 0%, or it may become damaged. The circuitry that controls charging and discharging is designed to maintain a reserve of about 10-20% (±) and prevent overcharging. Ideally, the charge and discharge range should stay between 20% and 80% of the total capacity. In conclusion, if your power bank has a battery rated at 40,000 mAh at 3.7V. The actual output capacity, depending on the mode, could be reduced by 20-50% (±) or more from the stated capacity.

If it seems like your 40,000 mAh power bank delivers no more than 20,000 mAh—or even less—at the output, this is perfectly normal!

Prolonged storage of a power bank in a discharged state can lead to a reduction in its actual battery capacity. Unfortunately, this can happen not only at home but also in stores during long periods without charging. A well-designed power bank and its battery can last between 1 to 5 years (or more) of active use without significant capacity loss.

You may have also come across a capacity calculation formula online.

For example: Multiply 40,000 mAh by 3.7V and divide by 5.0V = 29,600 mAh. Here, a 40,000 mAh battery at 3.7V yields 29,600 mAh at 5V output. However, factoring in that the device’s circuitry won’t discharge the battery to 0%, the actual output might be slightly more than 20,000 mAh.

Calculating Power Bank Capacity. If your power bank supports 9V or 12V modes. The output capacity could drop even further, potentially to no more than 10,000 mAh. You can calculate the capacity yourself based on these principles.