But then, reality hits. After the first charge, the power bank’s indicator lights have dropped significantly. By the third charge, it’s sputtering on its last legs. You barely manage to get a little over three charges before the power bank is completely drained. You’re left confused and frustrated. Did the manufacturer lie on the packaging? Is your device a cheap knock-off? Are you being scammed?
The short answer is: probably not. While shady manufacturers do exist, this frustrating discrepancy is usually not a lie, but a fundamental misunderstanding of physics, electronics, and the numbers advertised on the box. The “missing” capacity isn’t gone; it’s lost in translation—an electronic and thermal tax you pay every time you charge. This article will demystify the numbers and explain exactly why your 20,000mAh power bank doesn’t deliver 20,000mAh to your phone.
The Milliamp-Hour Misunderstanding
The heart of the issue lies in what the “mAh” number on your power bank actually represents. A milliamp-hour (mAh) is a unit of electric charge, and it measures capacity. However, a capacity rating in mAh is only meaningful when you also know the voltage at which that capacity is measured. This is the crucial piece of information most consumers miss.
A power bank is essentially a case filled with one or more lithium-ion (Li-ion) battery cells, similar to what’s inside your smartphone. These internal cells have a standard nominal voltage, which is typically 3.7 volts (V).
So, when a manufacturer labels a power bank as “20,000mAh,” they are stating the total capacity of the internal battery cells measured at their native 3.7V.
20,000mAh is the capacity at 3.7 volts.
However, what voltage does your phone charge at? The universal standard for USB ports, and therefore for charging, is 5 volts (V). Newer fast-charging technologies can even use higher voltages like 9V or 12V, but 5V is the baseline. Your phone’s battery doesn’t want 3.7V; it needs the standard 5V from the USB port to charge correctly.
Think of it like converting currency. Imagine you have 20,000 Japanese Yen. If you travel to the United States, you can’t spend it directly. You must convert it to US Dollars. After the exchange rate, you won’t have 20,000 US Dollars; you’ll have a much smaller number (around $135). Voltage works in a similar way. The 20,000mAh capacity at 3.7V must be “converted” to the 5V charging standard, and just like with currency, the number will change.
The Math of Conversion: Calculating the “Real” Capacity
To accurately compare capacities at different voltages, we need to use a universal unit of energy: the Watt-hour (Wh). A Watt-hour is a measure of the actual energy stored in the battery, and its value doesn’t change regardless of the voltage. The formula to calculate Watt-hours is simple:
(mAh * V) / 1000 = Wh
Let’s apply this to our 20,000mAh power bank. We know its capacity is measured at the internal cells’ voltage of 3.7V.
Step 1: Calculate the Total Energy of the Power Bank in Wh
(20,000mAh * 3.7V) / 1000 = 74 Wh
This 74Wh is the true, fixed amount of energy the power bank holds when fully charged. Now, let’s see what that capacity looks like when it’s being output at the 5V required for USB charging. We can rearrange the formula to find the new mAh value:
(Wh * 1000) / V = mAh
Step 2: Calculate the Theoretical mAh at the 5V Charging Voltage
(74Wh * 1000) / 5V = 14,800mAh
This is the first major revelation. Due to the fundamental physics of voltage conversion, your 20,000mAh power bank can only ever deliver a theoretical maximum of 14,800mAh to a 5V device. We’ve already “lost” 5,200mAh before the electricity has even left the power bank’s port. This single conversion step is the biggest reason for the discrepancy and proves that getting five full charges on a 4,000mAh phone was never possible in the first place.
The Efficiency Tax: Where the Rest of the Energy Goes
The 14,800mAh figure is still a theoretical maximum. In the real world, no energy transfer is 100% efficient. Every time energy changes form or voltage, some of it is lost, primarily as heat. This is the “efficiency tax,” and you pay it at multiple stages of the charging process.
1. The Power Bank’s Conversion Circuit
Inside the power bank, there is a circuit board with a “boost converter.” This component is responsible for stepping up the 3.7V from the internal batteries to the 5V (or 9V/12V for fast charging) needed for the USB output. This process is not perfect. The electronics on the circuit board generate heat as they work, and that heat is wasted energy. A high-quality power bank might have a conversion efficiency of around 90-95%. A cheaper, lower-quality one could be as low as 80-85%.
Let’s assume we have a good quality power bank with 90% efficiency. We apply this loss to our new theoretical capacity:
14,800mAh * 0.90 (90% efficiency) = 13,320mAh
So, the amount of charge that can actually be delivered out of the USB port is now down to around 13,320mAh.
2. The Charging Cable and Connectors
The cable you use to connect the power bank to your phone also plays a role. Every cable has some level of electrical resistance. This resistance causes a small amount of energy to be lost as heat along the length of the cable. A short, thick, high-quality cable will have minimal loss. A long, thin, or poorly made cable will have higher resistance and waste more energy. While this loss is typically smaller than the conversion loss, it still contributes to the overall inefficiency.
3. Your Phone’s Charging Circuit
The efficiency tax doesn’t stop at the power bank. Once the electricity enters your phone, it is managed by your phone’s own internal charging circuit. This circuit takes the incoming 5V and converts it to the voltage required by your phone’s battery (which, ironically, is also around 3.7-4.2V). This process also generates heat—which is why your phone gets warm while charging. Your phone’s charging circuit also has its own efficiency rating, typically in the range of 85-95%.
Assuming your phone is also 90% efficient at storing the power it receives, we calculate the final amount of charge that actually makes it into your phone’s battery:
13,320mAh (delivered by the power bank) * 0.90 (phone’s efficiency) = 11,988mAh
After this final step, our initial 20,000mAh is now effectively just under 12,000mAh of usable charge that gets stored in your phone’s battery.
The Final Verdict: How Many Charges Can You Really Get?
Let’s recap the journey of our 20,000mAh power bank:
- Advertised Capacity: 20,000mAh at 3.7V
- Energy Stored: 74Wh
- Theoretical Capacity at 5V Output: 14,800mAh
- Actual Deliverable Capacity (after 90% power bank efficiency): ~13,320mAh
- Final Usable Capacity (after 90% phone efficiency): ~11,988mAh
Now we can perform our original calculation with this new, realistic number. How many times can we charge our 4,000mAh phone?
11,988mAh / 4,000mAh = 2.997 charges
Suddenly, the real-world result of getting just about three full charges makes perfect sense. It’s not a scam; it’s a series of unavoidable energy conversions and efficiency losses. The “missing” 8,000mAh wasn’t a lie—it was dissipated as heat throughout the charging process.
How to Be a Smart Power Bank Buyer
Now that you’re armed with this knowledge, you can make much more informed decisions. Here’s what to look for:
- Look for the Watt-hour (Wh) Rating: Because Wh is a constant unit of energy, it’s a more honest and direct comparison tool. Many reputable brands like Anker, Belkin, and RavPower print the Wh rating in small text on the device. A 74Wh bank is a 74Wh bank, regardless of the mAh marketing.
- Check for “Rated Capacity”: To be more transparent, some manufacturers have started listing the “Rated Capacity” or “Output Capacity” on their products. This figure represents the deliverable capacity at a specific voltage (e.g., “13,000mAh at 5V”). This is the number you should use for your calculations.
- Stick to Reputable Brands: Well-known brands are more likely to use higher-quality battery cells and more efficient circuitry. This means you’ll get a higher percentage of the advertised energy compared to cheap, unbranded alternatives.
- Consider Fast Charging: Fast charging uses higher voltages (9V or 12V), which requires another level of voltage conversion that can sometimes be slightly less efficient. However, the convenience of faster charging often outweighs the very minor difference in energy loss.
Conclusion
The “power bank lie” is less of a lie and more of a complex truth rooted in physics. The number on the box isn’t what you get in your phone, and it was never meant to be. The advertised mAh is the raw capacity of the internal cells, not the final delivered charge after voltage conversions and efficiency losses. By understanding the role of voltage, Watt-hours, and the unavoidable “efficiency tax,” you can set realistic expectations and appreciate the incredible technology packed into these portable lifesavers. You are now an informed consumer, capable of seeing past the big numbers and choosing a device based on its true energy and real-world performance.
