The 3.7V to 5V Conversion Loss: Why Your Power Bank Delivers 30% Less Charge Than Advertised

When the USB Implementers Forum finalized the USB Power Delivery (PD) 3.1 specification in May 2021, it pushed charging limits to 240 watts, fundamentally changing how fast portable devices could draw power. But while charging speeds multiplied, the underlying math governing battery capacity remained anchored to a physical bottleneck: the voltage mismatch between internal lithium-ion cells and USB outputs. Buying a 10,000 milliampere-hour (mAh) power bank today still does not guarantee 10,000 mAh of charge delivered to a phone.[7]

The discrepancy stems from how manufacturers measure and market energy storage. The headline figure printed on the retail box—typically 10,000 mAh or 20,000 mAh—represents the total capacity of the internal battery cells at their native voltage. It does not represent the amount of power that actually exits the USB port.[1][2]

Inside almost every modern portable charger sits a cluster of lithium-ion or lithium-polymer cells. These chemical batteries operate at a nominal voltage of 3.7 volts. However, the baseline USB standard dictates that power must be delivered at 5.0 volts to safely charge external devices.[4][6]

To bridge this 1.3-volt gap, the power bank relies on an internal step-up converter circuit. This component boosts the 3.7V current to the required 5V before it travels down the cable. Because total energy must remain constant during this conversion, increasing the voltage proportionately decreases the available milliampere-hours.[1][6]

The mathematical formula for this conversion is straightforward. Multiplying the advertised capacity by the internal voltage, then dividing by the output voltage, reveals the theoretical maximum output. For a standard 10,000 mAh unit, the calculation is 10,000 multiplied by 3.7, divided by 5.0.[4][5]

That calculation yields 7,400 mAh. Simply by stepping the voltage up to the USB standard, 26 percent of the advertised milliampere-hours vanish from the usable pool. But the physical reality of electrical engineering introduces a second deduction: heat loss.[5][7]

No electrical conversion is perfectly efficient. The step-up circuit generates heat as it operates, bleeding energy into the surrounding air. According to technical documentation from JOWAY, "The conversion rate of a regular power bank is usually between 80% and 90%."[3]

Applying a generous 90 percent efficiency rating to the theoretical 7,400 mAh leaves approximately 6,660 mAh of actual, usable charge. A consumer who purchases a 10,000 mAh battery pack to charge a smartphone with a 3,300 mAh battery will get exactly two full charges, not the three charges the retail packaging implies.[5][6][7]

This reality forces a distinction between two critical terms in battery manufacturing. As AOHi engineers explain in their 2023 technical brief, "Battery capacity is the amount of power the battery itself can store, while rated capacity is the amount of power the power bank can actually output."[2]

Regulatory bodies and consumer protection agencies require manufacturers to print the "Rated Capacity" on the device itself, usually in small text on the bottom casing. This number—often listed around 6,000 mAh to 6,500 mAh for a 10,000 mAh unit—is the only figure a buyer should use when calculating how much travel gear they can keep powered.[1][2][4]

The math becomes even more complex when utilizing modern fast-charging protocols. When a device requests 9V, 12V, or 20V through a USB-C PD connection, the step-up converter must work harder, further reducing the mAh figure at that specific voltage, even though the total energy delivered remains roughly the same.[5][6]

This shifting math is why electrical engineers advocate for measuring portable power in Watt-hours (Wh) rather than milliampere-hours. Watt-hours calculate total energy independent of voltage. A 10,000 mAh battery at 3.7V holds 37 Watt-hours of energy, a figure that remains constant regardless of whether it outputs at 5V or 20V.[4][5]

Aviation authorities already rely exclusively on this metric. The Federal Aviation Administration (FAA) limits passengers to carrying lithium-ion batteries that do not exceed 100 Watt-hours without special airline approval, ignoring mAh entirely because it fails to accurately describe the battery's total energy density.[7]

Until the consumer electronics industry transitions to marketing Watt-hours on retail packaging, buyers must perform the 30 percent deduction themselves. Identifying the rated capacity rather than the battery capacity remains the only reliable method for ensuring a portable charger will actually survive a long-haul flight.[2][7]