The entire Internet is wrong: in power banks there is no problem indicating mAh instead of Wh

Most owners of power banks sooner or later face a snag: an external battery with a conventional 20,000 mAh cannot charge a smartphone with a conventional 5,000 mAh battery four times, although a simple mathematical calculation suggests otherwise. This “phenomenon” is not new for a long time, and the Internet is full of articles explaining the reason for this phenomenon. When choosing a new power bank for myself, I visited a bunch of similar materials and got acquainted with the arguments in them – that very snag using mAh instead of Wh (which I will write about below) seemed very logical. But after sleeping with the new / refreshed information for myself the night, I realized that it is absolutely useless when assessing the capacity of a power bank – or rather, in most cases it doesn’t matter at all whether the capacity is indicated in mAh or in Wh.

⚠️ I am not a professor of physics or an expert in electronics at all, and this material is not scientific at all. I just found a discrepancy that is presented on every first site from the search results as a truth, and decided to share it – I can also make mistakes and be mistaken in some way, so I will be glad to see indications or any additions to errors in the comments, if any. eat.

How the whole Internet describes the problem of fake capacity

Every site I visit almost immediately indicates that power bank manufacturers mislead buyers by measuring the capacity of their models in milliamp-hours (mAh), and not in watt-hours (Wh). All authors of such carbon copy materials explain this by the fact that the mAh indicator characterizes the electric charge, that is, the current strength (measured in amperes) that the battery is capable of delivering in one hour. And the electrical energy that a battery can provide must be indicated in Wh – the number of watts in one hour.

This is explained with a simple “life” example:

Let’s say a person has a 20,000 mAh power bank, from which he wants to charge a smartphone with a 5,000 mAh battery. Simple math tells him that the portable battery will last for four full charges of the smartphone, but in practice, four recharges are not achieved.

The authors of the articles explain this by saying that the reason is the incorrect measurement of the capacitance and the peculiarities of charging: the milliamp-hour indicator is indicated at 3.7 volts, and the charging itself takes place at 5 volts. According to them, if you convert mAh to Wh, the mystery will be solved:

20,000 mAh × 3.7 V = 20 Ah × 3.7 V = 74 Wh (true power bank capacity)

74 Wh ÷ 5 V (after all, charging at 5 volts, not 3.7 volts) = 14.8 A⋅h = 14,800 mAh (supposedly the “correct” capacity in mAh)

At this point, many materials emphasize that manufacturers are cunning – they mislead buyers and deliberately use lower voltage in order to increase the capacity indicator. And it is indicated that from a 20,000 mAh power bank you can’t charge a smartphone at 5,000 mAh four times precisely because its “real” capacity is 14,800 mAh (with this, you can’t power your smartphone even three full times).

However, this is an erroneous opinion that has been spread over the Internet by those who do not understand the principle of operation of power banks.

In fact, it doesn’t matter what to evaluate the capacity: in mAh or Wh

It’s worth clarifying right away: it’s all the same in the case of charging smartphones and many other mobile devices from power banks. But, for example, when comparison battery capacities of different devices, it is really incorrect to compare them by mAh (since the voltage may differ).

Almost all sites do not specify the fact that lithium batteries are installed in power banks, the nominal (average) voltage of which is 3.7 volts. It may be slightly higher or lower depending on the level of charge of the batteries, but their voltage never reaches 5 volts. So where did this 5 volt voltage come from? From the USB standard, through the cables of which charging is carried out, it requires that the voltage be at least 5 volts.

Conventional lithium batteries are incapable of delivering 5 volts on their own – this voltage is provided by a converter built into the power bank, which raises the original 3.7 volts (or so) to the required 5 volts (or higher).

At the same time, lithium batteries are also used in smartphones, the nominal voltage of which is similar – 3.7 volts. Since the voltages are identical for both power banks and the smartphones they charge, there is, in fact, no problem in measuring the capacity in milliamp-hours (depending on voltage). This can be verified mathematically:

Power bank capacity at 20,000 mAh in Wh:
20,000 mAh × 3.7 V = 20 Ah × 3.7 V = 74 Wh

Smartphone capacity at 5,000 mAh in Wh:
5,000 mAh × 3.7 V = 5 Ah × 3.7 V = 18.5 Wh

74 Wh ÷ 18.5 Wh = 4

As you can see, there is really no problem measuring the capacity of power banks in milliamp-hours if they charge devices with the same lithium batteries at the same voltage. The arguments of Internet experts who calculate the “real” capacity of power banks during “real” operation have nothing to do with reality, since external batteries produce a voltage of exactly 3.7 volts (as indicated on the box), and output 5 volts (actually In fact, with fast charges it happens a lot more) it is the converter that provides.

So why, in practice, from a power bank at 20,000 mAh, it still doesn’t work to charge a smartphone at 5,000 mAh four times, if everything should converge? The answer is simple – because of that same converter (but not only).

The real problem lies in the efficiency – it is not 100%

The coefficient of performance (COP) of any converter is not 100% – if it were, then a conditional 1.5 V little finger battery could produce all 220 V or more, up to infinity.

In fact, the converter consumes part of the energy (due to the strength of the current), and its efficiency varies from gadget to gadget. For example, all three of my power banks from Baseus, among which there is almost the best brand model, have an efficiency of at least 75% (Energy Conversion Rate ≥ 75%) – I intentionally mentioned the manufacturer, since he is one of the most famous in chargers, which means that lesser-known power banks may have even lower efficiency due to the use of cheaper converters (often it is not indicated at all).

In practice, an efficiency of 75% means the following:

20,000 mAh x 3.7V x 75% = 20Ah x 3.7V x 0.75 = 55.5 Wh

55 Wh – this is exactly how much energy the power bank can give out when the converter is activated, that is, three quarters of the original capacity (74 Wh). In fact, the specified minimum efficiency threshold of 75% is hardly real and is clearly higher, the manufacturer simply guarantees that the converter will not work worse than this value.

A good example of non-ideal efficiency is familiar to everyone – this is the heating of the charger (the same power bank or power supply). Efficiency is not 100% because some of the energy goes into the operation of the converter itself, and some is released as heat – in fact, the more heat, the lower the efficiency of the charger (this is best seen when using fast charging standards).

However, there is a downside (in the literal and figurative sense of the word) – there are also losses in the smartphone battery power circuit, which is why it does not receive 100% of the energy “poured” into it. In 2015, a Habr user measured the efficiency of charging the batteries of four smartphones and one tablet, then the losses ranged from 29% to an unthinkable 43.1%. I would like to believe that since that moment technologies have stepped forward, but the fact remains that there are losses at the opposite end.

Conclusion – it’s always better to figure it out yourself

In fact, other factors also affect the number of recharges of a smartphone from one power bank: from the banal honesty of the manufacturer and the real capacity of the batteries installed inside (fake values ​​are often indicated in cheap models) to the quality of the cable, because any ordinary conductor has a resistance that reduces the transmitted energy .

I wrote this article, obviously aimed at a narrow circle of people (and very unlikely to claim high views and a large number of “pluses”), solely for the sake of our readers – so that numerous rewrites of the same incorrect information do not mislead you. This is another example of the fact that it is better to understand the technical nuances yourself, and not blindly believe information from the Internet, even if most sites duplicate it.