# A bit about portable batteries

Hello dear friends! When choosing a portable battery, you may encounter a lot of negative reviews about the discrepancy between their declared capacity and the number of charged gadgets. It would seem that having bought a charge for 13,000 mAh, we should charge our smartphone with a 2300 mAh battery about 5.5 times! But not so simple.

### A little background

As a lover of gadgets and modern technology, I have a smartphone and other good things. And on a certain path, I ran into one, in my opinion, a serious problem of advanced devices - they have a relatively short battery life. Yes, I won’t argue, there are “monsters” of telephone construction with 4000 mAh and more batteries. But, often, such devices are extremely rare and have other disadvantages. In any case, even if your gadget is able to hold out until the evening (and my Nexus 5 with 2300 mAh is not on this list), sooner or later the question arises of buying a portable battery.

Like many geeks, my hands for buying this type of device have long been scratched. I was considering options with the purchase of a box for 18650 format batteries, as well as a ready-made device (in which the very same 18650, as in laptop batteries, are with a huge probability). As a result, there was a need to have a charged phone at work in the absence of an outlet, and a portable battery DF TRIO-02 was purchased .

Frankly - there was not much time to choose and read reviews. It was just that I quickly went through one well-known online store (the one that is in a group of companies along with a bank and jewelry) and was selected by the following criteria:
• required capacity
• price quality
• appearance (yes, you need to strive not only for ergonomics, but to enjoy aesthetically pleasing)

Pros:
1. good capacity
2. two outputs of 5V, 1 A; single output 5V, 2.1 A
3. microUSB battery charging input

Minuses:
1. Marky glossy case

### Capacity calculation arithmetic

For ease of calculation, we introduce the following assumptions :
1. take the efficiency of the voltage converter for 100%
2. we accept all indicated capacities as real values
3. we consider constant values ​​of current and voltage during charging
4. Charging the phone comes from the ideal 0% to 100% (excluding the residual charge that manufacturers lay, etc.)

To eliminate inaccuracies, take a look at Wikipedia :

The maximum possible useful charge of the battery is called the charging capacity, or simply capacity. Battery capacity is the charge given by a fully charged battery when discharged to the lowest allowable voltage. In the SI system, the capacity of batteries is measured in coulombs; in practice, an off-system unit, ampere-hour, is often used. 1 Ah 36 = 3600 C Battery capacity is indicated by the manufacturer. Not to be confused with the electric capacitance of a capacitor.

At present, more and more often on batteries the energy capacity is indicated - the energy given off by a fully charged battery when discharged to the lowest permissible voltage. In the SI system, it is measured in joules; in practice, an off-system unit is used - watt-hour. 1 Wh ⋅ h = 3600 J.

On the packaging we have a proud inscription: "13000 mAh." This is our charging capacity.
Carefully looking at the sticker on the reverse side, we see the following.

Voltage: 3.7 V.
Charging capacity: 13000 mAh.
Energy capacity: 48.1 Wh

It turns out that many manufacturers indicate the stored charge in mAh (mAh), but the voltage of this device is also important. To the fullest extent, “capacity” characterizes the stored energy .

Often people confuse the concepts of stored charge and stored energycalling it "capacity." If greater accuracy is not needed, then we can assume that the stored energy (in Wh) is approximately equal to the product of the stored charge (in Ah) and the average voltage (in volts).

1 W · h = 1 V · 1 A · h.

Now, after understanding the concepts, let's move on to our example: a 48.1 Wh battery, this is 13 Ah (13000 mAh) times 3.7 V. So far, everything is converging. But, our device is charged from an output of 5 V. Therefore, the charge that our device is capable of delivering is found as a quotient from the stored energy and output voltage.
48.1 Wh / 5 V = 9.62 Ah (9620 mAh).

### Analyze

Now you can easily count "how many times can I charge my device." So, the same Nexus 5 can be charged:
9620 mAh / 2300 mAh = 4.18
Or, in other words, a little more than 4 times . What is against 5.5

### Draw conclusions

The calculated reserve charge of 9620 mAh turned out to be 26% less than the 13000 mAh that we see on the box. And 26% less than the user who is inexperienced in calculations expects. Although, in fact, the manufacturer did not deceive us at all. Just such a marketing ploy.

Useful articles and sources:
» Electric capacity
» Electric battery
» Amp-hour