Rita El Khoury / Android Authority

If you’ve ever felt slightly cheated by even the best power banks, you’re not alone. At first glance, a 10,000mAh battery pack looks like it should recharge your 5,000mAh flagship phone twice without breaking a sweat. Yet somehow, the numbers never seem to work out quite that neatly. A charge and a half, or thereabouts, ends up being much more realistic.

The good news is that your power bank isn’t lying to you. The bad news is that battery capacity is a little more complicated than the box’s marketing makes it seem. The milliamp-hour (mAh) figure you’re looking at doesn’t represent the amount of charge that will end up in your phone, and that’s where most of the confusion begins.

The big misunderstanding about power banks

Edgar Cervantes / Android Authority

When shopping for a new power bank, we’re all eyeballing that headline mAh figure — it’s a metric we probably have a rough gauge for based on our daily experiences. If our 5,000mAh phones last most of the day, then an equivalent battery pack would surely be plenty for a day trip, while a 10,000mAh or 20,000mAh pack would surely do for a weekend’s camping trip.

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

Sadly, a 5,000mAh battery pack can't fully-charge a 5,000mAh phone.

So the first real complication is that the pack’s reference voltage may differ from the device you’re trying to charge. Your phone might have a nominal operating voltage of 3.6V for lithium-ion to 3.9V for silicon-carbon batteries, but your laptop might be 11V to 16V, depending on the number of stacked cells and its age. Tablets, smartwatches, and other gadgets have their own specific internal voltage requirements.

Hopefully, that already makes it clear why a 20,000mAh, 3.7V battery might seem to go a lot further on your low-voltage smartphone than on your high-voltage laptop. However, this can lead some to inaccurately conclude that charging voltages are the reason battery packs don’t provide as much “real” capacity as they might think, but that’s not really true either.

Wh — not mAh — is what matters

Robert Triggs / Android Authority

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 — the watt-hour (Wh). I say ‘average’ because a battery pack’s voltage and power capabilities vary with its charge level.

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

A good battery pack will specify its Wh value on the case.

Now, in an ideal theoretical experiment, we can convert this Wh value to any required voltage and calculate how many mAh the battery pack could provide for a given gadget. There’s 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 incidental.

You just can’t beat physics

Robert Triggs / Android Authority

While the battery voltage itself doesn’t directly cause issues, stepping it up and back down does, and that conversion is what costs us energy. Voltage conversion is not a lossless process in reality. DC-to-DC converter chips are 85% to 98% efficient, depending on factors such as chip and inductor quality, load voltages, and the currents being converted. Efficiency is a curve, not a constant.

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

Batteries themselves also exhibit some internal resistance during charging, meaning that even “perfectly” efficient power delivery doesn’t translate into 100% of the energy stored. This Coulombic efficiency, or inefficiency in this case, causes power to be wasted as heat, especially when fast charging, and as the battery voltage increases once the cell is above 80% full.

Voltage conversion, transmission loss, and battery resistance all add to wasted power.

Then there are voltage losses that occur across the USB-C cable from the pack to your phone. This is usually minor, but a percent or two here adds up, and a cheaper, low-quality cable combined with high power levels (such as 100W into your laptop) could result in higher losses.

As for wireless charging, it’s by far the biggest energy waster. Wireless charging drops energy both from the heat generated in the coils and the transfer across the air (a natural insulator). Even the latest Qi2 and MagSafe accessories aren’t immune and could result in a conversion loss of 20% or more for the added convenience of no wires.

It’s best to over-spec your power bank

Kaitlyn Cimino / Android Authority

Putting it all together, it’s easy to see how charging up a battery from another battery can lead to disappointment. The cold reality of physics means we lose some power to heat and other inefficiencies when converting between battery and charging voltages, and then back again.

Sadly, a 5,000mAh battery pack just can’t charge an equivalent-sized smartphone back to full. Adding up converter losses, cable losses, and the phone-side charge IC, even a good setup rarely beats 85–90% end-to-end, and cheaper models could result in efficiencies of just 70s or below. As a very rough ballpark, I’d target 75% extra capacity to avoid disappointment. That’s to say, a 5,000mAh pack should provide 3,750mAh of juice to a phone in the worst case, or you’d need around a 6,700mAh pack to fill a 5,000mAh phone. Give or take.

Unfortunately, there’s no way to avoid doing a little bit of math to work all this out. If you want to be really sure your battery pack can give you those two full charges over that weekend away, the best way is to over-spec. Grab a 70Wh battery pack or higher and put that battery anxiety firmly to rest.