Charge your phone from a brand-new 20,000mAh power bank and do the arithmetic. If your phone has a 5,000mAh battery, that brick should fill it four times. Instead, it manages two and a half, perhaps three, before giving up. It’s easy to blame cheap cells or an inflated marketplace listing, but a legitimate power bank may genuinely contain the advertised capacity.
The problem is that the number describes its internal cells before energy is converted, transmitted, and stored in your phone. The lie, if there is one, is the implication that its 20,000mAh can be compared directly with your phone’s 5,000mAh.
A milliamp-hour is not a unit of energy
It measures charge, but voltage completes the picture
A milliamp-hour measures how much electric charge a battery can deliver. A 1,000mAh battery could theoretically supply 1,000 milliamps for one hour, although that figure says nothing by itself about the amount of energy involved.
Energy also depends on voltage. Multiplying amp-hours by voltage gives watt-hours, which combine both values into one more useful measurement. The relationship between voltage, current, and power also explains why chargers that look identical can deliver very different results. Comparing batteries by milliamp-hours alone works only when they operate at the same voltage.
Listing capacity without voltage is a little like quoting a salary without naming the currency. Twenty thousand sounds impressive, but the number remains incomplete until you know the electrical terms attached to it.
Your power bank counts in one currency and pays in another
The cells speak 3.7V, USB starts at 5V
Most power banks use lithium-ion cells with a nominal voltage around 3.6V or 3.7V. The 20,000mAh printed on the case usually represents the combined cell-equivalent capacity at that nominal voltage, even if the cells are arranged differently inside the pack.
Using 3.7V as an example, multiplying 20 amp-hours by 3.7V gives 74Wh of rated energy. That figure is legitimate, and it is also why power-bank limits for air travel are expressed in watt-hours rather than milliamp-hours.
USB output commonly starts at 5V, so the numerical mAh value changes when the same energy is expressed at the port’s voltage. Under perfectly efficient conditions, 74Wh at 5V would equal approximately 14,800mAh. No energy has disappeared in that calculation. Each milliamp-hour at 5V represents more energy than one at 3.7V, so the same 74Wh produces a smaller mAh figure. USB-C rechargeable AA batteries demonstrate the same voltage-versus-capacity relationship on a smaller scale.
However, that 14,800mAh figure still cannot be divided directly by the 5,000mAh printed on a phone battery because the two ratings are measured at different voltages. Using a 5,000mAh phone battery rated at 3.85V as an example, the battery stores about 19.25Wh of nominal energy. In a lossless system, a 74Wh power bank could theoretically fill it approximately 3.8 times.
The reduction to roughly two and a half or three real-world charges comes mainly from what happens between the power bank’s cells and the phone’s battery.
The converter takes a cut, and so does your phone
Every step of the journey pays a toll in heat
Conversion efficiency varies by model, output level, temperature, and load, but a power bank may deliver roughly 80 to 90 percent of its stored energy through its USB port. The remainder is lost mainly as heat in its conversion circuitry and internal resistance. From an original 74Wh, that would leave approximately 59 to 67Wh available at the port.
The cable usually adds a smaller loss through electrical resistance. A poorly constructed or unsuitable USB-C cable can waste power as heat and restrict charging performance. The phone’s charging circuitry must then convert the incoming power into the voltage and current required by its battery, losing additional energy. The phone may also consume some of the incoming power through its screen, radios, processor, and background activity while it charges.
The clearest way to estimate the result is to remain in watt-hours. Using an illustrative 70 percent end-to-end efficiency estimate, about 52Wh of the original 74Wh may ultimately end up stored in the phone. Dividing that by a 19.25Wh phone battery gives roughly 2.7 equivalent full charges.
Better hardware and an idle phone may do somewhat better. Heat, an inefficient cable, or using the phone heavily while it charges can push the result lower.
Fast charging changes the conversion path but does not eliminate these losses. A higher negotiated voltage can deliver the same power with less current through the cable, reducing some resistive loss. Technologies such as USB Power Delivery PPS can also let compatible phones request a voltage better suited to their charging circuitry. Whether that improves overall efficiency depends on the power bank, phone, cable, negotiated voltage, and charging rate.
The missing capacity is not secretly removed from the cells. Some energy gets lost during conversion and transmission, while some is used to power the phone as it charges. Meanwhile, the original 20,000mAh and the phone’s 5,000mAh are measured at different voltages and were never directly comparable.
How to actually read a power bank label
Read past the big number
The most useful specification for comparing how much energy power banks store is usually buried in the regulatory fine print: its watt-hour rating. Some manufacturers also list a separate rated output capacity, such as 12,000mAh at 5V, measured under stated test conditions.
A model marked 74Wh and rated to deliver 12,000 or 13,000mAh at 5V is not contradicting itself. The watt-hour figure describes the energy stored in its cells, while the rated output shows how much it can deliver through the port after conversion losses.
When comparing power banks, watt-hours per dollar and watt-hours per gram are useful for judging nominal storage value and energy density. They do not reveal how efficiently the power bank delivers that energy, however. Capacity is also only one detail worth examining when buying a laptop power bank, alongside output power, port compatibility, charging input, and size.
For real-world output, look for a separately stated rated capacity or rated energy figure, together with its test voltage and conditions. Once the capacity numbers make sense, practical considerations such as built-in cables, port selection, and pass-through charging may determine which features your next power bank should have.
The usual scam test still applies, but it needs to be aimed at physically implausible claims. A “50,000mAh” power bank the size of a deck of cards is almost certainly exaggerating its capacity. At 3.7V, that rating would represent about 185Wh of stored energy, far more than could realistically fit inside such a small enclosure.
At roughly 185Wh, it would also exceed common passenger-airline limits. IATA’s 2026 passenger guidance [PDF] limits power banks to 100Wh, while FAA rules allow batteries between 101 and 160Wh only with airline approval. A 185Wh pack falls outside both allowances.
The label tells the truth selectively
The 20,000mAh printed on a legitimate power bank can be technically accurate while still inviting a misleading comparison. It describes the cell-equivalent charge at the cells’ nominal voltage, not the amount that will ultimately arrive in a phone battery.
Compare watt-hours when judging stored energy, then check the rated output capacity and its stated test voltage for a more realistic idea of what the ports can deliver. The headline number may not be fabricated, but it rarely tells the whole story.
- Brand
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Baseus
- Battery Capacity
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20,000mAh
- Ports
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1x 65W USB-C Port, 1x micro-USB Port, 2x 30W USB-A Ports
- Weight
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1.02lbs
- Dimensions
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6.06 x 2.55 x 1.06 inches
- Battery Technology
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Li-ion
The Baseus Adaman 65W 20000mAh Power Bank features a sleek aluminum alloy shell and a multi-metric digital LED screen tracking capacity, voltage, and current. Equipped with four ports (USB-C, dual USB-A, Lightning), it delivers 65W max output to quickly charge laptops, Steam Decks, and smartphones while remaining fully TSA airline-safe.
