Why your 10,000 mAh power bank isn’t actually 10,000 mAh

If you’ve ever felt even a little bit cheated best power banksyou are not alone. At first glance, a 10,000 mAh battery pack looks like it should charge your 5,000 mAh. flagship phone without breaking a sweat twice. But somehow, the numbers never work out quite so neatly. One and a half or so is more realistic.

The good news is that your power bank isn’t lying to you. The bad news is that battery capacity is a bit more complicated than the box’s marketing suggests. The milliampere-hour (mAh) number you’re looking at doesn’t reflect the amount of charge your phone will last, and that’s where most of the confusion begins.

When shopping for a new power bank, we all pay attention to the mAh number on the hood – a metric that we probably have a rough idea of ​​based on our day-to-day experiences. While our 5,000 mAh phones last most of the day, an equivalent battery pack is definitely enough for a day trip, and a 10,000 mAh or 20,000 mAh pack is definitely enough for a weekend camping trip.

However, mAh is a relative, not an absolute, measure of capacity. The listed value refers to the capacity of the internal lithium battery based on its “nominal” operating voltage. For a Li-Ion battery pack, this is typically 3.7-3.85V, but this varies slightly depending on the exact battery chemistry. However, some manufacturers consider the mAh rating to be equivalent to 5V, 20V, or other output depending on the intended use case.

So the first real complication is that the reference voltage of the pack may be different from the device you are trying to charge. Your phone may have a nominal operating voltage of 3.6V to 3.9V for lithium-ion. silicon-carbon batteriesbut your laptop may be between 11V and 16V depending on the number of cells packed and its age. Tablets, smartwatches and other devices have their own internal voltage requirements.

Hopefully this clarifies why a 20,000mAh, 3.7V battery will go further in your low-voltage smartphone than your high-voltage laptop. However, this may lead some to inaccurately conclude that the charging voltage is the reason why battery packs do not provide as much “real” capacity as they think, but this is not really true.

Before we get there, we need to determine the actual capacity of our power banks. Thankfully, it’s easy: we simply multiply the mAh rating by the cell’s nominal voltage to get the total average power output per hour – watt-hours (Wh). I say “average” because a battery pack’s voltage and power capabilities vary based on its charge level.

For example, a 5,000 mAh, 3.7 V battery pack can provide 18.5 Wh of power, while a 20,000 mAh, 3.6 V battery pack offers 72 Wh. A good battery pack will display this value somewhere on its box, while cheaper, weaker models probably hide it in the hope that you won’t notice or understand its significance.

Now, in an ideal theoretical experiment, we can convert this Wh value to any required voltage and calculate how many mAh a battery pack can provide for a given gadget. There is no inherent power loss in the raw math of converting between voltage levels. The fact that your phone charges at 9V or your laptop at 20V is largely coincidental.

Although the battery voltage itself does not directly cause problems, it needs to be stepped up and pulled down, and this conversion costs us energy. Voltage conversion is not a lossless process in reality. DC-DC converter chips are 85% to 98% efficient, depending on factors such as chip and inductor quality, load voltage, and converted currents. Efficiency is a curve, not a constant.

Converting the 3.7V battery voltage to the charging voltage of whatever USB charging specification your device uses (5V, 9V, 20V, etc.) causes power losses due to the converter and heat. Likewise, when your phone converts power over USB-C back to battery voltage, doubling the less-than-perfect efficiency of these power ICs, a second conversion is required at the other end of the wire. This is part of the problem USB Power Delivery PPS and AVS charging protocols aim to solve, but that’s another discussion entirely.

Batteries themselves also exhibit some internal resistance during charging, meaning that even “perfectly” efficient power transfer does not translate into 100% of stored energy. This Coulomb efficiencyor in this case the inefficiency causes energy to be wasted as heat as the battery voltage rises, especially during fast charging and after the cell is above 80% full.

Then there are the voltage drops that occur in the USB-C cable from the pack to your phone. It’s usually small, but here it adds up to a percent or two, and a cheaper, lower-quality cable (like the 100W that goes into your laptop) combined with high power levels can result in higher losses.

When it comes to wireless charging, this is the biggest waste of energy. Wireless charging reduces energy both from heat generated in the coils and from conduction through the air (a natural insulator). Even the latest Qi2 and MagSafe accessories is not immune and may result in a conversion loss of 20% or more for added convenience of wiring.

Putting it all together, it’s easy to see how frustrating it can be to charge a battery from another battery. The cold reality of physics means that we lose some power to heat and other inefficiencies during the conversion between battery and charging voltages and then gain it back.

Unfortunately, the 5,000 mAh battery pack can’t fully charge a smartphone of equivalent size. Adding in converter losses, cable losses, and the charging IC on the phone side, even a good setup rarely ends up at 85-90%, and cheaper models can only lead to 70 or lower efficiency. As a very rough playing field, I would aim for 75% additional capacity to avoid disappointment. That is, a 5000 mAh pack should at worst give the phone 3750 mAh of juice, or you’ll need a 6700 mAh pack to charge a 5000 mAh phone. Give or take.

Unfortunately, there’s no way to avoid a bit of math to figure this all out. If you really want to make sure your battery pack can give you those two full charges that weekend, the best way to go is to overspec. Grab a 70Wh or higher capacity battery pack and put that battery worry firmly to rest.