Amp-hours in a battery: what is it?

The battery life of a mobile phone, a portable tool, or the ability to supply current to the starter when starting a car engine - all this depends on such characteristics of the battery as capacity. It is measured in ampere hours or milliamp hours. By the size of the capacity, you can judge how long the battery will supply electrical energy to a particular device. The time it takes to discharge and charge the battery depends on it. When choosing a battery for a particular device, it is useful to know what this value means in ampere hours. Therefore, today’s material will be devoted to such a characteristic as capacity and its dimensions in ampere-hours.

In general, an ampere hour is a non-system unit of electrical charge. Its main use is to express the capacity of batteries.

One ampere-hour represents the electric charge passing in 1 hour through the cross-section of a conductor when passing a current of 1 ampere. You can find values ​​in milliamp-hours.

As a rule, this designation is used to indicate the capacity of batteries in phones, tablets and other mobile gadgets. Let's look at what ampere-hour means using real examples.

In the photo above you can see the capacity designation in ampere hours. This is a 62 Ah car battery. What does this tell us? From this value we can find out the current strength with which the battery can be uniformly discharged to the final voltage. For a car battery, the final voltage is 10.8 volts. Standard discharge cycles typically last 10 or 20 hours.

Based on the above, 62 Ah tells us that this battery is capable of delivering a current of 3.1 amperes for 20 hours.

In this case, the voltage at the battery terminals will not drop below 10.8 volts.

In the photo above, the laptop battery capacity is highlighted in red – 4.3 ampere-hours. Although with such values ​​the value is usually expressed as 4300 milliamp-hour (mAh).

How are the battery capacity (amp-hour) and its energy (watt-hour) related?

Many manufacturers do not indicate the capacity in ampere-hours on their batteries, but instead indicate the stored energy in watt-hours. Such an example is shown in the photo below. This is a Samsung Galaxy Nexus smartphone battery.

I apologize for the photo with small print. The stored energy is 6.48 watt-hours. The stored energy can be calculated using the following formula:
1 watt hour = 1 volt * 1 ampere hour.

Then for the Galaxy Nexus battery we get:

6.48 watt-hours / 3.7 volts = 1.75 amp-hours or 1750 milliamp-hours.

What other types of battery capacity are there?

There is such a thing as the energy capacity of a battery. It shows the ability of the battery to discharge over a certain time interval with constant power.

The time interval in the case of automobile batteries is usually set to 15 minutes. Energy capacity initially began to be measured in North America, but then battery manufacturers in other countries joined in. Its value can be obtained in ampere-hours using the following formula:

E (Ah) = W (W/el) / 4, where

E – energy capacity in ampere-hours;

W – power at 15 minute discharge.

There is another variety that came to us from the USA, this is a reserve tank. It shows the ability of the battery to power the onboard moving vehicle when the generator is not working. Simply put, you can find out how long the battery will allow you to drive your car if the alternator fails. You can calculate this value in ampere hours using the formula:

E (amp hours) = T (minutes) / 2.

Here we can also add that when batteries are connected in parallel, their capacity is summed up. When connected in series, the capacitance value does not change.

How to find out how many amp hours your battery actually has?

Let's look at the process of checking capacity using an example. But such a controlled discharge can be done for any battery. Only the measured values ​​will differ.

In order to check the actual amp hours of your battery, you need to fully charge it. Check the degree of charge by density. A fully charged battery should have an electrolyte density of 1.27-1.29 g/cm 3 . Then you need to assemble the circuit shown in the following figure.

You need to find out what discharge mode your battery capacity is specified for (10 or 20 hours). And discharge the battery with a current intensity calculated using the formula below.

I = E/T, where

E – nominal battery capacity,

This process requires constant monitoring of the voltage at the battery terminals. As soon as the voltage drops to 10.8 volts (1.8 on the bank), the discharge must be stopped. The time it takes for the battery to discharge is multiplied by the discharge current. This gives the actual battery capacity in ampere-hours.

If you do not have a resistor, you can use car light bulbs (12 volts) of suitable capacity. You select the power of the light bulb depending on what discharge current you need. That is, if you need a discharge current of 2 amperes, then the power will be 12 volts multiplied by 2 amperes. Total 24 watts.

Important! After the battery is discharged, immediately charge it so that it does not remain in such a discharged state. For such a discharge it is better not to do it at all. With such a deep discharge, they may lose part of their capacity.

Battery Capacity Units

When choosing a portable battery charger (ROM), many people ask questions: “What do the mAh and Wh characteristics mean?”, “And why are they needed?”

We answer. Both values: mAh (milliamp-hour) and Wh (watt-hour) characterize the capacity of the charger. But it is most correct to focus on capacity, measured in watt-hours. And that's why.

Wh is an absolute constant capacity that most accurately describes the potential of a device.

And the capacity indicated in mAh is a relative value that describes the capacity of the device in relation only to a specific selected voltage. That is, for one voltage there is one capacitance, and for another voltage there is another capacitance. Often you can also see the designation “Ah” (ampere-hour). 1 Ah = 1000 mAh. Thus, to get the Ah value, you need to divide the mAh value by 1000. Conversely, to get mAh, you need to multiply the Ah value by 1000.

For example, the CARKU E-Power-3 battery charger has a capacity of 29.6 Wh or 8000 mAh (8 Ah).

At the same time, 8000 mAh is the nominal capacity, and it is indicated relative to the nominal voltage of the batteries built into the body of the starter-charger. All lithium polymer (LiPo) and lithium ferrum phosphate (LiFePO4) batteries used in starter chargers have a nominal voltage of 3.7 V. Many will ask: “How so? If the nominal voltage = 3.7 V, then why are the ROM outputs marked with values ​​of 5V, 12V and 19V?” The answer is simple: the voltage increase for one or another ROM output occurs due to the electronic filling of the device.

Thus, for a nominal voltage of 3.7V, the CARKU E-Power-3 ROM has a nominal capacity of 8000 mAh. From this value of nominal relative capacity, expressed in mAh, it is easy to obtain the value of absolute capacity, expressed in Wh:

1) first, convert the capacity value expressed in milliamp-hours to ampere-hours

8 Ah x 3.7 V = 29.6 Wh

Thanks to this ratio, it is easy to calculate the actual capacity in mAh of the CARKU ROM and any other battery at a specific operating voltage of a specific electrical consumer.

Let's make calculations using the example of the CARKU E-Power-3 ROM. This model has 2 outputs:

1) USB output for charging mobile phones, tablets, etc. with an operating voltage of 5 V. To calculate the actual capacity for this operating mode, it is necessary to divide the absolute capacity of 29.6 Wh by the voltage of 5 V, and then we get 5.92 Ah:

29.6 Wh / 5 V = 5.92 Ah (or 5920 mAh).

2) Output for starting a motor with an operating voltage of 12 V. Here the same formula is used to calculate the actual capacity:

29.6 Wh / 12 V = 2.467 Ah (or 2467 mAh).

As we can see from the calculations, the most obvious and correct value characterizing the capacity of the ROM is precisely Wh. And based on it, it is easy to calculate the capacity in mAh for a particular voltage and, therefore, approximately estimate the potential of the ROM for a specific electrical consumer.

The capacity values ​​in mAh for the CARKU E-Power-3 ROM, when correctly calculated for 5V and 12V, are not as impressive as for a nominal voltage of 3.7V, but this does not detract from the high consumer performance of this little one. The compact and lightweight E-Power-3 allows, for example, to fully charge an iPhone4 3 times or a classic Nokia 106 6 times, as well as confidently start 4-liter gasoline engines in summer and 1.6-liter gasoline engines in winter, which is confirmed by real tests and numerous videos in Youtube.

Some into the forest, some for firewood

In ROM descriptions and passports, first of all, it is necessary to indicate the capacity in Wh. Additionally, you can indicate the nominal capacity of the ROM in mAh, paying tribute to the historically popular dimension, easily recognized by the mass consumer and widely used for power banks (external batteries), batteries for mobile phones, tablets, etc.

All CARKU ROMs have an absolute capacity in Wh and a nominal relative capacity in mAh. Some manufacturers incorrectly indicate the ROM capacity only in mAh, reflecting a secondary capacity characteristic and completely forgetting about the most important one.

There are also situations where some sites indicate inflated specifications in mAh. For example, the absolute capacity of the CARKU E-Power-Elite ROM is 44.4 Wh, which means its nominal capacity is 12000 mAh (44.4 Wh / 3.7 V = 12 Ah). Therefore, there cannot be a CARKU E-Power-Elite ROM with an absolute capacity of 44.4 Wh and at the same time with a nominal capacity of 14000 mAh or 15000 mAh, as some sales companies indicate.

It is also worth keeping in mind that the vast majority of portable starter-chargers currently presented on the Russian market have an actual capacity much less than the declared one. For example, 5000 mAh instead of 8000 mAh, 8000 mAh instead of 14000 mAh, etc. The difference between the declared and actual capacity sometimes reaches 2 or more times. This is a very common situation, because it is very difficult for the consumer to check the actual capacity, much less measure it. In turn, the actual capacity of the CARKU ROM fully corresponds to the declared one. This is confirmed, for example, by an independent review of the Russian ROM market and, in which the CARKU ROM demonstrates a greater number of launches than analogues with a larger capacity.

Why is it so important to pay attention to ROM capacity? Because the duration of autonomous operation of electrical consumers powered from the ROM directly depends on it. The capacity of the ROM is especially important in the winter when starting a vehicle engine, since the larger the capacity, the more attempts there will be to start the engine and their duration, and, consequently, the likelihood of a successful start. In addition, the battery is the main element of the ROM, so the cost of the ROM directly depends on its capacity. So keep this in mind when choosing a ROM for yourself.

I think the topic may be of interest to many, because... Almost everyone is facing this now.

We are talking about battery capacity and its designation.
Historically, battery capacity is most often indicated in mAh (mAh) or Ah (Ah). In some cases, this can lead to serious misconceptions. For example, it may happen that a person sees two batteries, say 800 mAh and 2400 mAh. And most likely he will decide that the second one stores three times more energy. But this may not be the case. It may well turn out that the “800 mAh” battery will store much more energy. And now I’m not talking about the cunning Chinese who write whatever they like on the label, but about physics.

Let's figure out what a battery capacity of say 4000 mAh means. Quite simply, this means that the battery can supply 4000 mA of current for one hour. Or 1000 mA for four hours. Or 2000 mA for two hours and so on. But the current consumed by the device/supplied by the battery is only one characteristic; there is another one - voltage. With the same current, the voltage can be different. Remembering the school physics course, you can calculate that, for example, with a current of 1 A and a voltage of 10 V, the load consumes 10 W. And with the same current of 1 A and voltage of 3 V, the load consumes only 3 W. Therefore, voltage is the most important characteristic and it is impossible to talk about the amount of energy that a battery can store, knowing only about the current.

The most correct characteristic of battery capacity is W*h (Wh, Wh). Let's say a battery capacity of 10 Wh will tell us that it can power a 10 W load for one hour. At the same time, what current and voltage there is is no longer important to us. Capacity in Wh is very easy to calculate - just multiply the capacity in Ah and the rated voltage of the battery in volts.

Why did the mAh designation still stick?
The fact is that the voltages on batteries are not random, but depend on the type of element. Nowadays these are most often lithium cells. The nominal voltage on one lithium element is 3.7V. As long as we are talking about the same type of battery and the same number of consecutive cells in the battery, we can “legally” compare mAh capacity. But as soon as one battery has one cell, and the second has two connected in series (7.4V), it is no longer possible to compare capacities in mAh, because with the same mAh, the second will have twice as much energy.

When should you bother?
When you are not sure that the batteries are of the same type, with the same number of cells. For example, phones always use lithium batteries in the amount of one cell (maybe there are exceptions, but I haven’t seen them). This means they can be easily compared in mAh. You can also safely compare batteries intended for one device, because it is extremely rare that a device supports batteries with different numbers of consecutive cells. But you can’t compare batteries of different devices and types like that. Let's say laptops have batteries with two serial cells (7.4V) and three (11.1V).

Also, sometimes people are surprised that a regular AA battery says 2700 mAh, while a phone with approximately the same capacity has only 800 mAh. This is exactly the case when it is wrong to compare mAh, because
The capacity of an AA battery is 1.2V*2.7Ah=3.24Wh, while the capacity of a lithium battery is 3.7V*0.8Ah=2.96Wh, that is, they are almost the same, and do not differ three times at all.

Conclusion: you can only talk about the battery capacity in mAh if you also specify the type of battery (chemistry and number of consecutive cells) or its voltage. In other cases, comparing capacity by this parameter is absolutely meaningless.

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

A little background

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

Like many geeks, I have been itching to buy this type of device for a long time. I was considering the options of buying a box for 18650 batteries, as well as a ready-made device (which most likely contains the same 18650 batteries, just like laptop batteries). As a result, the need arose to have a charged phone at work in the absence of an outlet, and a portable battery DF TRIO-02 was purchased.

To be honest, I didn’t have much time to choose and read reviews. I just quickly combed through one well-known online store (the one that is part of a group of companies along with a bank and a jewelry store) and selected it according to the following criteria:

• required capacity
• price quality
• appearance (yes, yes, you need to strive not only for ergonomics, but also enjoy aesthetically)

Briefly about this very device

Pros:

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

Minuses:

1. Stainless glossy body

Arithmetic for calculating capacity

For ease of calculation, we introduce the following assumptions:

1. We take the efficiency of the voltage converter as 100%
2. we accept all indicated capacities as real values
3. We assume constant values ​​of current and voltage during charging
4. The phone charges from ideal 0% to 100% (without taking into account the residual charge provided by manufacturers, etc.)

To eliminate the inaccuracy, let's take a look at Wikipedia:

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

Nowadays, batteries are increasingly indicated with energy capacity - 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, a non-systemic unit is used - watt-hour. 1 Wh = 3600 J.

On the packaging we have a proud inscription: “13000 mAh”. This is our charging capacity.
Looking carefully at the sticker on the back 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 operating voltage of the device is also important. To the fullest extent, “capacity” characterizes stored energy.

People often confuse the concepts stored charge And stored energy calling it "capacity". If great 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 Wh = 1 V 1 Ah.

Now, having understood the concepts, let's move on to our example: 48.1 Wh of battery is 13 Ah (13000 mAh) multiplied by 3.7 V. So far everything fits. 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 of the stored energy and the output voltage.
48.1 Wh / 5 V = 9.62 Ah (9620 mAh).

Analyzing

Now you can easily calculate “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's against 5.5

Drawing conclusions

The calculated battery capacity of 9620 mAh was 26% less than the 13000 mAh we see on the box. And 26% less than what a user inexperienced in calculations would expect. Although, in fact, the manufacturer did not deceive us at all. It's just a marketing ploy.

Useful articles and sources.