I see a bunch of one-star reviews, mostly from people who seem confused about battery capacity, energy, power, voltage, current, etc. People complain that this little pocket-size battery won’t fully charge their iPad. That’s like buying a little one-gallon gasoline can for emergencies, and then complaining that it when you poured it into your truck’s empty gasoline tank it didn’t fill the tank all the way to the top. The cell inside the Anker Nano Power Bank is advertised as having a capacity of 5000 mAh. That means the cell can (for example) supply 5 A for one hour, or 2.5 A for two hours, or 1 A for five hours. Battery engineers like talking about mAh because it is useful for their circuit designs, but for the rest of us it’s not very useful, because knowing the energy in 5000 mAh depends on knowing what voltage we are talking about. For end users, it is more useful to compare batteries based on the energy they store rather than the amount of current they use to deliver that energy. So that means talking about watt-hours (Wh) instead of mAh. For this battery the cell voltage is about 3.6 V, so 5000 mAh gives you 18 Wh. But phones aren’t designed to take power at 3.6 V. Phones require 5 V (or higher). Voltage conversion circuitry is not perfectly efficient. The text on the battery itself claims, “2600 mAh @ 5V x 2.4 A,” which means it can deliver 2.6 x 5  =  13 Wh of energy. This means that the voltage conversion circuitry inside the Anker Nano is about 72% efficient, so the 18 Wh at 3.6 V becomes 13 Wh at 5V (28% of the energy is wasted as heat). In practice, measuring a couple of Anker Nano Power Banks using a USB energy meter, I found they actually deliver 12 Wh (2.4 A @ 5V for one hour). 12 Wh is close to the 13 Wh that Anker claims, but not quite. My iPhone 15 Pro Max has a battery capacity of 17.32 Wh. Charging a battery is also not perfectly efficient — charging makes your phone get warm, and that heat is more wasted energy. If we assume that battery charging is also 72% efficient, then the 12 Wh from the Anker Nano becomes 8.64 Wh in the phone battery, or about half of the my iPhone 15 Pro Max battery capacity. In my experience the Anker Nano does indeed charge my phone from empty to around 50%. Because the Anker Nano supports USB C voltage negotiation, phones that support higher voltages can request up to 10V, allowing the Anker Nano supply up to 22 W for faster charging than a normal USB 5 V supply. The Anker Nano Power Bank is great for unplanned situations where you unexpectedly find your phone battery is drained. The Anker Nano is a small battery that easily fits in a pocket or a bag, and you don’t need to remember to carry a cable with it, because the built-in USB C plug connects directly to the device being charged. If you prefer to use a cable, it also has a USB C socket for that. Last year at the end of a day of skiing I found myself trapped for two hours in a broken-down gondola with four other people. All of their phone batteries were drained, and they were concerned about how they could tell their friends where they were. I pulled the little Anker Nano out of my pocket and passed it around. Within a few minutes everyone had charged their phone 10-15%, enough to call their friends and let them know what was happening.