Why a 10,000mAh Power Bank Delivers Less Than It Says

A 10,000mAh powerbank does not give your phone 10,000mAh, and nothing has gone wrong. The number printed on the case is measured inside the battery, at the cells' own voltage. Your phone charges at a higher voltage, through a converter, down a cable, while it is also running. Four separate losses sit between the printed figure and the charge that reaches your battery.

Each of those losses has a published figure attached, from the manufacturer's own documentation rather than a rule of thumb. They add up to more than most people expect.

There are two capacity numbers, and they are not the same thing

Look at the small print on the back of a powerbank and you will usually find both.

Cell capacity is what the cells hold. Anker's own documentation on what the printing on a powerbank means works through an example: "a detail reading 'Cell Capacity: 5,000mAh, 14.4VDC, 72Wh' means each cell can store 5,000mAh, operates at 3.6 volts nominally, with the total voltage for multiple cells being 14.4 volts, equating to 72 watt-hours of energy."

Rated capacity is the headline number. The same document defines it: "Rated capacity refers to the battery's capacity at the rated voltage, usually measured in milliampere-hours (mAh). For example, 'Rated Capacity: 10,000mAh' indicates that the battery has a capacity of 10,000mAh at the rated voltage of 3.6V."

That last clause is the whole story. Ten thousand milliamp-hours at 3.6 volts. Milliamp-hours are only comparable at the same voltage, which is why watt-hours are the figure that actually travels between devices. If you have never converted between the two, the mAh to Wh method is the same arithmetic airlines use.

The four losses, itemised

Anker's document names five causes for the gap between rated and actual capacity. Four of them carry numbers.

1. Voltage conversion

"The typical voltage of a phone is 3.87V. When charging, the capacity might adjust to approximately 9,300mAh from 10,000mAh at 3.6V."

This is not a loss so much as a unit change. The same energy, expressed at a higher voltage, is fewer milliamp-hours. About 700mAh of your 10,000 disappears here before anything has been wasted at all.

2. Conversion efficiency

"Not all energy converts during charging; typically, 10-15% is lost."

This one is a real loss, and it leaves as heat. The circuitry that steps the cells' voltage up to what USB delivers is not free, which is also why a powerbank gets warm while it works.

3. Your phone's own consumption

"The phone consumes some energy during charging, even when in StandBy mode."

The phone does not stop being a phone while it charges. Screen on, navigation running, and a meaningful share of what arrives is spent rather than stored.

4. Cable loss

"The cable used for charging also incurs some energy loss; the longer the cable, the greater the loss, typically around 5%."

The fifth cause has no figure attached, and the document is honest about it: "Temperature and the number of charge cycles can also cause discrepancies between the rated capacity and the actual output capacity."

Working it through

Take those published figures and compound them on a nominal 10,000mAh unit. This is arithmetic from the manufacturer's stated percentages, not a bench measurement, so treat it as an estimate of the right shape rather than a spec:

Then subtract whatever the phone burns while it is plugged in, which nobody can put a single number on because it depends entirely on what you are doing with it. A phone sitting in a pocket on standby loses very little. A phone navigating with the screen at full brightness in the sun can consume a large fraction of what is arriving.

So a 10,000mAh powerbank realistically moves somewhere in the region of 7,000 to 8,000mAh into a phone that is not being used hard. Against a 4,000mAh phone battery, that is not three charges. It is closer to two. The same arithmetic on a 5,000mAh unit explains why a pocket-sized powerbank is a one-charge device.

The cells are not the problem

It is tempting to read all this as the battery being poor quality. It is not. Battery University's technical reference on charging lithium-ion puts the cell's own charge efficiency at "about 99 percent", with the cell remaining cool during charge, and gives the traditional lithium-ion nominal cell voltage as 3.60V.

The chemistry is close to lossless. Almost everything you lose is lost in the conversion, the cable and the phone, which are the parts outside the battery. That is a useful thing to know, because it means a more expensive powerbank with the same rated capacity will not fix it. A shorter, better cable makes more difference than a better brand.

What this changes if you rent instead of buy

Renting moves the question. When you own a powerbank, the rated capacity is a one-time purchase decision you live with. When you rent one from a station, what matters is whether the unit you are handed is actually full, and whether you can swap it when it is not.

Brick operates powerbank stations in 4,000 venues across 25 or more countries, and a rented unit returns to the station to be recharged rather than living in a drawer at 40%. The practical difference for the person renting is that the relevant number stops being the printed capacity and becomes how long the thing lasts in the hand, which is covered in how long a rented power bank actually lasts. If the whole model is new to you, start with what power bank sharing is.

What to read on the label

• Find the watt-hours. Wh is voltage-independent, so it is the only figure that compares two powerbanks honestly. It is also the number airlines care about, per the current airline rules.
• Check which number is which. A case printed only with a large cell-capacity figure and no rated capacity is telling you the flattering one.
• Discount by roughly a quarter. Assume you will see somewhere around 70 to 80% of the rated figure in practice, before your phone's own consumption.
• Blame the cable before the brand. A long, thin cable is a measurable loss you control. If charging feels slow as well as short, the causes overlap.

FAQ

Why does my 10,000mAh power bank only charge my phone twice?

Because roughly 20 to 30% of the rated capacity is consumed by voltage conversion, conversion efficiency losses of 10 to 15%, cable loss of around 5%, and the phone's own power use while charging. A 10,000mAh unit moves closer to 7,000 to 8,000mAh into a phone, which against a 4,000mAh battery is about two charges.

Is the capacity on a power bank false advertising?

No. The rated capacity is a genuine measurement of the cells at their nominal 3.6V. It is accurate at the voltage it is quoted at. The confusion comes from comparing it directly to a phone battery quoted at a different voltage, which is not a like-for-like comparison.

What is the difference between cell capacity and rated capacity?

Cell capacity is what the individual cells hold, and it is quoted with a voltage and a watt-hour figure. Rated capacity is the pack's capacity at the rated voltage, usually 3.6V, and it is the headline number on the case. They measure different things.

Does a better brand of power bank lose less capacity?

Marginally at best. The cell itself charges at about 99% efficiency, so the losses sit in the voltage conversion, the cable and the phone rather than in the battery. A shorter, thicker cable changes the outcome more than the brand does.

Should I compare power banks by mAh or Wh?

Watt-hours. Milliamp-hours are only comparable between products quoted at the same voltage, and watt-hours already account for voltage, which is why airlines use Wh thresholds rather than mAh.

Does a power bank lose capacity as it gets older?

Yes. Anker's documentation names temperature and the number of charge cycles as further causes of the gap between rated and actual output. A powerbank that has been through hundreds of cycles delivers less than a new one of the same rating.

The short version

The printed number is real, it is just measured somewhere other than where you use it. Read the watt-hours, expect about three quarters of the rated figure, and use a short cable. If you would rather not carry one at all, Brick runs the stations that let you pick one up and drop it off somewhere else.