Inside a power bank there are four blocks: cells that store energy, a converter that changes the cell voltage into the voltage your device expects, protection circuitry that keeps the cells inside their safe operating range, and the connectors and indicator that connect all of it to the outside world. The gap between the number printed on the case and the amount of energy that reaches your phone comes from the second and third of those blocks, plus the efficiency of the charging path itself.
The four blocks inside
The cells are the storage, and they are almost always lithium-ion or lithium-polymer cells with a nominal voltage around 3.7 V. The converter is a switching circuit that raises that voltage to the 5 V a USB port expects, or to a higher level for fast charging. The protection circuit watches voltage, current and temperature and disconnects the cells if any of them leaves its safe range. And the ports, the indicator and the case are what make the assembly usable.
The comparison that makes the energy accounting intuitive is a water tank on a roof. The tank stores the water; a pump raises it to the pressure the taps need; and a float valve stops the tank overflowing or running dry. A power bank works the same way, except that the tank and the pump share a small plastic box and the pump runs whenever the box is in use — which is why the box gets warm and why not all the stored energy comes out of the port.
Cells: the storage
Two cell formats are common. Cylindrical cells, in sizes such as 18650 or 21700, look like thick batteries and are used where space allows a few large units. Pouch cells are flat and are used where the shape of the product is thin. The two have different thermal behaviour: a cylindrical cell has a metal can that resists swelling and conducts heat along its axis, while a pouch cell relies on the surrounding structure and the surface area for cooling.
Cell capacity is quoted in milliampere-hours, which is a unit of charge rather than of energy, and this is the source of most of the confusion about power bank ratings. To compare energy, the charge has to be multiplied by the voltage of the cell it was measured at. A pack rated at a given number of milliampere-hours at 3.7 V holds considerably less energy than the same number of milliampere-hours at 5 V would, which is why the watt-hour figure is the honest one to compare.
Converters: making the right voltage
When a power bank supplies a USB port it has to raise the cell voltage from around 3.7 V to at least 5 V, and this step is called a boost. When it is being charged from a wall adapter, the same board has to do the opposite and reduce the incoming voltage to the cell’s level, which is a buck. Some designs use one circuit for both jobs and some use separate paths, and that choice affects the efficiency and the number of components.
Each conversion loses energy, and the loss appears as heat in the case. A conversion that is 90 per cent efficient loses a tenth of what passes through it, and a power bank that both boosts and bucks will incur that penalty in both directions. This is the honest explanation for the gap between the label and the phone: energy is stored at one voltage, delivered at another, and converted twice in between.
| Block | Job | What to check |
|---|---|---|
| Cells | Store energy at around 3.7 V per cell | The watt-hour figure rather than the milliampere-hour figure |
| Boost and buck converter | Raise the voltage for the output and lower it for charging | Which fast-charging standards it supports and at what power |
| Protection and monitoring | Limit voltage, current and temperature | Whether the unit reports temperature protection at all |
| Ports and indicator | Connect to cables and show the state of charge | Port types, and whether power is shared between them |
Protection chips you never see
A lithium cell has a narrow band of conditions in which it is happy. Above a certain voltage it degrades and can become unstable; below another it is damaged and may not recover; too much current heats it internally; and too much heat from the outside does the same. The protection circuit exists to keep the cell in the middle, and it does that by measuring the cell voltage, the current through the pack and the temperature, and disconnecting the load when any of them leaves the safe range.
Three parts are worth naming because they appear in every design. A battery management circuit or charge controller supervises the whole process. A thermistor is placed against the cells to report temperature, because temperature is the variable that predicts trouble earliest. And a protection switch, usually a pair of MOSFETs in the current path, is the part that actually interrupts the circuit. Those MOSFETs are unglamorous and they carry the full current of the pack, which is why their resistance matters to the efficiency of the whole product.
Why capacity claims feel wrong
The number on the case is usually the sum of the cell capacities, quoted as charge at the cell voltage. The energy that reaches your phone is measured at 5 V or higher, after a boost that loses part of it. A rough rule of thumb used by buyers is that a substantial fraction of the nominal figure is lost in the two conversions and in the charging process inside the phone, which is why a pack described as holding a large number of milliampere-hours fills fewer phone charges than the arithmetic suggests.
Two practices make comparison easier. Compare in watt-hours, which is energy rather than charge, and compare like with like by checking whether the figure was measured at the cell voltage or at the output. A pack that states its watt-hours honestly is more informative than one that states a large charge figure without saying where it was measured. And treat a figure that exceeds the physical limit of the cell size with suspicion, because energy per unit volume in a lithium cell is bounded by chemistry.
Heat, aging and safety
A lithium cell ages faster when it is hot, when it is held at a high state of charge, and when it is cycled hard. That is why a power bank that is left in a hot car and kept topped up at 100 per cent will lose capacity faster than one that is stored cool and partly charged. It is a chemistry fact rather than a manufacturing defect, and it is the reason a pack that worked well for two years can feel tired in the third.
The safety rules follow the same logic. Do not charge a swollen or physically damaged pack, do not leave it in direct sun, do not use it in a place where it cannot lose heat, and stop using it if it becomes noticeably hot during normal discharge. A swelling cell has already changed shape because of internal pressure, and that is a condition to retire the product rather than to monitor.
How to choose one sensibly
Five points cover most of the decision. The rated output power, because a pack that cannot supply the power a device wants will charge it slowly regardless of capacity. The ports, and whether power is shared when several devices are connected. The supported fast-charging standards, because a pack that does not speak the same standard as your phone will fall back to a slow rate. The stated watt-hours, which lets you compare energy rather than charge. And the safety documentation and certifications for the market you are in.
One practical note about air travel belongs here, because it is the most common real consequence of the watt-hour figure. Lithium batteries may not travel in checked baggage, and the widely applied limits for carry-on allow a small allowance without approval and a larger one only with the airline’s consent. The thresholds are expressed in watt-hours, not milliampere-hours, so the label matters. Rules vary by carrier and by jurisdiction, so the airline is the authority to check before travelling rather than a general article.
FAQ
Why do I have to charge a new power bank before first use?
Because it is usually shipped partly charged rather than full, which is the state lithium cells tolerate best in storage. Charging it fully before the first use also lets the fuel gauge calibrate itself, so the indicator percentages become meaningful. There is no need to discharge it completely first.
Why are power banks restricted on aircraft?
Because a lithium cell that is damaged or internally faulty can release a great deal of energy, and a fire in a confined space is difficult to manage. The rules restrict them to carry-on baggage and set limits in watt-hours, with larger packs allowed only with the airline’s consent and beyond a further threshold not permitted at all. The limits are expressed in energy, so the watt-hour figure on the label is the number to check, and the airline is the authority on the rules that apply to your flight.
What are the downsides of using a power bank?
Three are real. Charging through one adds conversion stages, so some of the stored energy is lost as heat rather than delivered. The charging rate is usually lower than a wall adapter, so a phone takes longer to fill. And repeated deep cycling plus heat ages the pack itself, so the capacity you paid for declines over a few hundred cycles.
What is the difference between a charger and a power bank?
A charger converts mains power into the low voltage a device needs. A power bank stores energy in cells and delivers it later, which means it contains a charger, a battery and a converter. The naming is loose in everyday use, and the specification is what tells you which product you are looking at.
How many times can a power bank charge a phone?
Fewer times than the label suggests, and the number depends on the two devices rather than on the pack alone. The stored energy has to pass through a boost conversion, the phone’s own charging circuit loses more, and the phone’s battery accepts less energy than it stores. Estimating from watt-hours instead of charge, and then allowing for conversion losses on both sides, gives a much more realistic figure.
What to do next
Read the watt-hour figure on your own power bank and compare it with the battery capacity of the device you charge from it. That comparison explains more than any claim on the packaging. The same subject from the adapter side is in how a power adapter works, the protection devices that keep small circuits safe are in transient suppressors in plain language, and the heat question is in why power devices get hot.
Background is available in our overviews of battery management systems, battery chargers, USB and switched-mode supplies, with safety standards from IEC and energy material from the US Department of Energy. Efficiency and safety background is published by NIST, ISO, the Semiconductor Industry Association and JEDEC.
Tell Good-Ark the cell configuration, the load current and the protection requirements, and we will point you at the MOSFETs, diodes and suppressors that suit the pack.
Email sales@goodark.com, or start from the application design centre.
This article is published by Good-Ark, a manufacturer of discrete semiconductor devices used in battery-powered products. It names and ranks no brand, quotes no capacity-retention figure, and advises checking the airline’s own rules before travelling with a battery pack.