The discrepancy between the nominal and rated capacities of a power bank, such as a 10,000mAh power bank displaying a typical rated capacity around 6,000mAh, can be attributed to two main factors: capacity conversion loss and circuit loss.
- Capacity Conversion:
The 10,000mAh rating refers to the total charge capacity of the power bank's internal battery cells, which typically operate at a voltage of around 3.7V. However, the rated output capacity of the power bank is often calculated at a standard output voltage of 5V. Using the energy conservation formula (Energy in Watt-hours = Capacity in Ampere-hours × Voltage in Volts), we can calculate the adjusted capacity:
- 10Ah × 3.7V = 37Wh
- 37Wh / 5V = 7.4Ah = 7,400mAh
- Circuit Loss:
To boost the voltage from 3.7V to 5V, power banks use a conversion circuit. This process incurs energy loss due to inefficiencies in the circuit, typically around 82% efficiency. This means that not all the stored energy can be efficiently converted and used at the higher voltage.
Calculation of Final Rated Capacity:
Given the efficiency and the adjusted capacity:
- Rated Capacity = 7,400mAh × 0.82 = 6,068mAh
Thus, the effective rated capacity that can be used by devices is about 6068mAh, which explains why a 10,000mAh power bank might typically display a rated capacity around 6,000mAh on its label.
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