We've all been there. You buy a new power bank rated at 10,000 mAh, and logically, you expect to fully charge your 5,000 mAh smartphone twice. Yet, when you use it, the power bank conks out halfway through the second charge.

The math seems flawless: 10,000 ÷ 5,000 = 2. But in reality, this equation almost never holds up.

First, rest assured—with reputable brands, you are not being "scammed." You are simply running into the unyielding laws of physics and an industry-standard metric that can be misleading.

Imagine your power bank is a water tank and your phone is a bucket. The problem is, the pipe connecting them leaks. This "leakage" is primarily due to voltage conversion.

1. "Boost" Conversion Loss Inside the Power Bank

   • The heart of a power bank is a lithium-ion battery, which operates at 3.7V.

   • However, a USB port (including Type-C) must output a standard 5V.

   • Therefore, an internal circuit must "boost" the voltage from 3.7V to 5V. This process isn't 100% efficient; approximately 10-15% of the energy is lost as heat.

2. "Buck" Conversion Loss Inside Your Phone

   • Once the 5V power enters your phone, the phone's charging circuitry cannot feed 5V directly to its battery without damaging it. It must "buck" or reduce the voltage down to the battery's 3.7V.

   • This "buck" conversion also incurs a 10-15% loss, which is a key reason your phone gets warm while charging.

These two voltage conversions alone waste a combined 20-30% of the power bank's energy! This is the single biggest reason your power bank dies sooner than expected.

You may have already spotted the issue: the mAh (milliamp-hour) unit itself is part of the problem.

• mAh is not a unit of energy. It describes "current × time" but ignores the critical factor of voltage.

• The true unit of energy is Wh (Watt-hour), calculated using the formula: Wh = Ah × V.

Let's use Wh to re-examine our example:

• Power Bank's Total Energy: 10,000 mAh (10 Ah) × 3.7V = 37 Wh

• Phone Battery's Total Energy: 5,000 mAh (5 Ah) × 3.7V = 18.5 Wh

Now you see, in terms of total energy, the power bank (37 Wh) does have twice the capacity of the phone (18.5 Wh). But when we account for the conversion losses, the story changes.

Let's factor in the voltage conversion losses (assuming a realistic combined efficiency of 80%):

1. Usable Energy from Power Bank: 37 Wh × 80% = 29.6 Wh

2. Energy Needed per Full Phone Charge: ~ 18.5 Wh

3. Realistic Number of Charges: 29.6 Wh ÷ 18.5 Wh ≈ 1.6 charges

So, a 10,000 mAh power bank will, in the real world, deliver approximately 1.5 to 1.6 full charges for a 5,000 mAh phone. This is far from the ideal "2 charges," but it is a fact of physics.

A New Complication from Emerging Tech:
Some modern power banks now use safer, more durable Lithium Iron Phosphate (LiFePO4) cells. However, their nominal voltage is around 3.2V. If a 30,000 mAh LiFePO4 power bank is placed next to a 30,000 mAh standard lithium-ion power bank, the former actually holds less total energy. Judging by mAh alone leads to confusion. Only by comparing the Wh rating can you see the true picture.

To avoid disappointment and make an informed purchase, follow these guidelines:

1. Always Look for the "Wh (Watt-hour)" Rating: This is the gold standard for comparing energy capacity, regardless of the internal battery chemistry or voltage. Reputable brands, especially for larger power stations, always state the Wh.

2. If Only mAh is Given, Do the Math Yourself: Multiply the advertised mAh by the battery's voltage (typically 3.7V for Li-ion, 3.2V for LiFePO4) to get a rough estimate of the total energy (Wh).

3. Trust the Manufacturer's "Real-World Charge Estimates": Reputable brands provide estimated charge counts based on specific phone models (e.g., "Charges an iPhone 15 three times"). These figures are derived from real-world testing and already account for efficiency losses, making them far more reliable than the raw mAh number.

4. Be Wary of Implausibly Large, Cheap Power Banks: If a small, inexpensive power bank claims a massive capacity like 50,000 mAh, it is almost certainly fraudulent. These are the real scammers.

Your power bank isn't broken; it's working honestly within the laws of physics. mAh is an imperfect industry habit, and energy loss is an unavoidable natural tax. The next time you wonder why you're not getting all the charges you expected, remember: the "missing" 20-30% of energy was the necessary cost of safely delivering power to your device.

Understanding this will allow you to see through the numbers and become a truly savvy consumer.

Longhehui – 22 Years of Battery Excellence, Powering Your World One Cell at a Time.

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