IQnewswire
If you’ve ever bought a 20,000mAh power bank and wondered why it doesn’t fully recharge your phone four or five times, you’re not imagining things. The number printed on the box is technically correct—but it doesn’t tell the whole story.
To understand where the missing capacity goes, you need to look beyond marketing numbers and into how power banks actually store, convert, and deliver energy.
What Does 20,000mAh Actually Mean?
The mAh rating refers to the capacity of the internal battery cells, not the usable energy delivered to your phone. Most power banks use lithium-ion cells rated at around 3.6–3.7 volts.
This means the 20,000mAh figure describes storage at the battery’s native voltage—not the higher voltages required to charge modern devices.
Voltage Conversion: Where the Capacity Goes
Before power reaches your phone, it must be converted to a usable output voltage. This process is necessary but inefficient.
From 3.7V Cells to 5V, 9V, or 12V Output
Phones don’t charge at 3.7V directly. A power bank must boost voltage to 5V for standard charging or higher levels like 9V or 12V for fast charging through a USB C charger.
Every voltage conversion step results in energy loss, usually as heat.
Energy (Wh) Matters More Than mAh
Watt-hours (Wh) are a better measure of true capacity. A 20,000mAh power bank at 3.7V equals roughly 74Wh. Once converted to 5V, the available capacity drops significantly.
This is why comparing Wh gives a more accurate picture than comparing mAh alone.
A Simple Example Calculation
If 74Wh is converted to 5V with 85% efficiency, only about 63Wh reaches the output. That alone reduces usable capacity by nearly 15%—before accounting for any other losses.
Conversion Losses Inside Power Banks
Beyond voltage boosting, internal components like regulators and control chips consume power themselves. These losses increase during fast charging, when higher currents and more complex protocols are used.
Higher-quality power banks use more efficient components, but no conversion process is ever 100% efficient.
Built-In Safety Reserves and Cutoff Limits
Not all stored energy is meant to be used. Some capacity is intentionally kept out of reach.
Why Power Banks Never Fully Discharge
Fully discharging lithium-ion cells dramatically shortens their lifespan. To prevent damage, power banks stop output before cells reach true zero.
Low-Voltage Cutoff for Battery Protection
Once cell voltage drops below a safe threshold, the power bank shuts down—even if energy remains. This protects the battery from long-term degradation and failure.
How Much Capacity Is “Locked Away”?
Depending on design, 5–15% of total capacity may be reserved for safety. This energy exists but is never delivered to your devices.
Cable, Port, and Device Losses
Even after power leaves the power bank, losses continue along the way.
Cable Resistance and Voltage Drop
Long or low-quality cables increase resistance, causing voltage drops and heat. This means less energy actually reaches your phone.
Port Efficiency Differences (USB-A vs USB-C)
USB-C ports generally support more efficient power delivery and smarter negotiation. USB-A ports often lose more energy, especially at higher currents.
Phone Charging Circuit Losses
Your phone’s internal charging circuitry also converts and regulates power. Wireless charging, including using a MagSafe wireless charger, introduces even more loss due to coil inefficiency and heat.
Common Myths About Power Bank Capacity
Misinformation around capacity ratings is widespread, but most claims don’t hold up under scrutiny.
“Manufacturers Are Lying About Capacity”
In most cases, manufacturers are listing cell capacity correctly. The confusion comes from misunderstanding how much of that energy is usable after conversion and protection limits.
“Bigger mAh Always Means Longer Battery Life”
A higher mAh number doesn’t guarantee better real-world performance. Efficiency, voltage, and design quality matter just as much.
“Fast Charging Is Less Efficient Overall”
Fast charging is slightly less efficient, but the difference is often modest. Modern power banks balance speed and efficiency better than older designs.
How to Compare Power Banks the Right Way
Instead of focusing only on mAh, look at watt-hours, conversion efficiency, port types, and supported charging standards. Consider how you actually charge—wired, fast charging, or wireless—and whether you frequently use accessories like a MagSafe wireless charger.
A well-designed 15,000mAh power bank can outperform a poorly designed 20,000mAh one in real use.
Final Takeaway
A 20,000mAh power bank doesn’t deliver 20,000mAh to your phone—and it’s not supposed to. Voltage conversion, safety reserves, internal losses, cables, and device charging circuits all reduce usable capacity.
Once you understand this, capacity ratings make much more sense. By choosing efficient designs, quality cables, and the right USB C charger setup, you’ll get far better real-world performance—no matter what number is printed on the label.
