Every shopper has had the experience: the box says 10,000 mAh, your phone is 4,000 mAh, so you expect 2.5 charges, but you get closer to 1.7. The difference is not cheating; it is physics. A power bank's cell capacity is measured at 3.7 V, while the phone charges at 5 V, 9 V or higher through a conversion circuit, and every conversion step loses some energy as heat. Wireless charging loses even more.
This FAQ explains how to read the two capacity numbers on a bank, how to estimate real charging times, and how to compare products honestly. Once you understand the math, you will no longer overpay for headline numbers.
Q: What is the difference between cell capacity and rated capacity?
Cell capacity (often printed prominently, e.g. 10,000 mAh) is the energy stored inside the internal lithium cells at their native 3.7 V. Rated capacity (usually printed smaller, e.g. 6,500 mAh at 5 V) is the energy the bank can actually deliver through its USB output after conversion. The rated capacity is the number that tells you how many times you can charge your phone; the cell capacity is more of a spec-sheet figure. A bank that honestly prints both is transparent; one that prints only the cell number is relying on shoppers to overestimate.
Q: How do I calculate real charging times?
Use the rated capacity, not the cell capacity. Divide the rated capacity by your phone's battery capacity, then multiply by the real-world efficiency. For wired charging, assume 80 to 85 percent efficiency; for wireless, 60 to 70 percent. For example, a 10,000 mAh bank with a 6,500 mAh rated output charging a 4,000 mAh phone: 6,500 divided by 4,000 = 1.62, then 1.62 times 0.85 is about 1.38 charges wired, and times 0.65 is about 1.06 charges wireless. This is why a 10,000 mAh bank usually gives one to one and a half full charges to a modern phone.
Q: Why is wired charging more efficient than wireless?
Wired charging delivers energy through a direct electrical connection with conversion in one step (3.7 V cell to 5 V or 9 V USB). Wireless charging adds two extra stages: the bank converts energy to a magnetic field, transmits it across an air gap, and the phone converts the field back to electricity. Each stage loses a few percent, and heat dissipation through an air gap is worse. Expect wireless efficiency about 15 to 20 percentage points lower than wired.
Q: Does charging speed affect how many times I can charge?
Only indirectly. Fast charging (18 W, 27 W, 65 W) transfers energy faster but loses more as heat because the conversion circuits work harder. A bank that delivers 22.5 W to a phone may show slightly lower overall efficiency than one delivering 10 W, meaning you get a little less total runtime. The trade-off is worth it for speed, but do not expect the same total number of charges at maximum speed as at medium speed.
Q: Why does my phone stop at 80 percent when charging from a power bank?
Modern phones stop fast charging around 80 percent battery and switch to trickle charging to protect the battery. This is the phone's behavior, not the bank's. It means the bank takes longer to finish the last 20 percent, but it does not reduce total capacity. If you compare advertised charges, assume the bank can top your phone up from zero to full, not from zero to 80 percent.
Q: Does the phone's battery capacity printed on the box match the real capacity?
Phone batteries are typically measured at 3.85 V or similar, while the bank output is 5 V. If you do a direct mAh comparison without adjusting for voltage, you overestimate charges. The rated capacity on the bank is already adjusted to 5 V output, so the cleanest comparison is rated bank capacity divided by phone battery mAh, then applying efficiency. Do not try to compare cell mAh directly to phone mAh without the voltage correction.
Q: How much does heat reduce charging efficiency?
When a bank or phone runs hot, the control chips reduce power to protect the cells, which lowers efficiency and extends charge time. This is why wireless charging on a hot day can feel slower. If you want the maximum number of charges from a bank, charge at moderate speed, in a cool place, and avoid running heavy apps on the phone while it charges.
Q: Can I trust the advertised "X full charges" claim on the box?
Treat it as a best-case figure. Manufacturers typically calculate charges at 5 V wired, with the phone screen off, at room temperature, and using the rated capacity. In real use, with heat, fast charging and screen-on time, expect 10 to 20 percent fewer charges. If a box claims "2.5 charges for iPhone," expect roughly two under real conditions.
Q: How do I compare two power banks fairly?
Compare three numbers. First, rated output capacity (not cell capacity) in mAh at 5 V. Second, output wattage, which tells you how fast it charges. Third, weight and size, because a bank with the same capacity but better cells is lighter. Ignore marketing claims that only show mAh. Zishine prints both cell and rated capacity on its products and packaging, so buyers can calculate real charging times directly rather than relying on estimates.
Q: How does Zishine help buyers get accurate capacity expectations?
Zishine labels every power bank with both cell capacity and rated output capacity, measured at a standard 5 V output, and publishes typical charge counts for popular phone models on its product pages. The Shenzhen R&D team verifies capacity under both wired and wireless conditions before mass production, and for OEM customers can supply independent test reports showing conversion efficiency. Buyers in Indonesia, Japan, the United States and other markets can request samples to measure real charging times on their own device list, so product listings match real-world experience rather than optimistic marketing.
Q: Why does my 20,000 mAh bank feel like it charges my phone fewer times than a 10,000 mAh bank?
This usually happens when the 20,000 mAh bank is also delivering fast charging (22.5 W or higher) to a phone that accepts high wattage. The higher speed generates more heat and conversion loss, so per mAh you get slightly less delivered energy. Also, some high-capacity banks run their own circuits (LED display, wireless coil) that drain a small amount of standby power. The difference is small, but if you notice it, charge at medium speed rather than maximum.
