What is battery capacity and what does it depend on

If you look at a modern battery, you will almost always find information about what voltage the element is designed for and what its capacity is.

With voltage, everything is clear anyway, but understanding how much the capacity of this or that battery will last is not always clear. In this article I will tell you what capacity is and what it depends on.

What is battery capacity

Let's take a conventional rechargeable battery for consideration and see what it says:

As you can see, this element is designed for 1200 mAh. If you take a car battery, then the capacity there is incomparably greater and equal, for example:

As you can see, in both cases, "Ampere * hour" is used as the unit of measurement and the prefix "hour" is used in all cases for a reason.

If we use a simple explanation, the capacity of the battery, measured in "Ampere * hours", shows how long a given battery is able to supply the load at a certain current.

Let's take an example with a so-called ideal battery. You have a 12 volt battery fully charged with a capacity of 6 ampere * hour. So, if a load that consumes 0.6 A * h is connected to it, then the battery will be able to provide energy for 10 hours.

Until such a state of the battery comes, in which the subsequent charge will be simply dangerous for it (for its operating parameters). And if the load is 1 Ampere, then the discharge will come after 6 hours of operation (in theory, of course).

The indicators of a real battery have significant differences from the ideal, and each of them has a limit on the upper limit of the discharge current.

After all, the greater the current flowing during the discharge, the less the linearity of the discharge characteristic, therefore, the battery will discharge much faster than the estimated time showed.

In addition, the voltage to which the battery can be discharged or charged is also regulated and displayed without fail in the technical documentation for the product.

So if you take the most common lithium-ion battery, rated at 3.7 V,

then its maximum discharge voltage is 2.75 V. And the voltage to which you can charge the battery is 4.25 V.

If these boundaries are violated, then with a strong discharge below 2.75 V, the battery will lose part of its original capacity, and if overcharged above 4.25 V, the battery may explode.

Considering a 12 V lead acid battery, the lower voltage limit is 9.6 V and the upper limit is 13 V.

As you've probably already noticed, when we talk about capacitance, we don't mention voltage in any way. But if the hours are converted into seconds and then a simple mathematical calculation is made, then we get the following expressions:

It turns out that the capacity of the battery does not depend on the voltage at the terminals. But as soon as we put our battery on charge, the first thing we do is focus on the voltage that is shown at the terminals.

Moreover, if the battery is charged to the rated voltage, then we count on the entire capacity stored in it. If the measurement showed that the battery is discharged, then the capacity does not matter anymore.

Also, the real capacity of the battery has a direct dependence on the discharge current.

As you can see from the table above, a 10 hour discharge and a 10 minute discharge will show an approximately two-fold difference in capacity.

The mathematical dependence on the discharge current and discharge time was revealed by the scientist Peikret, who later introduced the "Peikert coefficient".

So, for example, this coefficient for lead-acid batteries is 1.25 and the statement is true: the higher the discharge current, the shorter the discharge time.

It is quite easy to calculate the real capacity of the battery: we charge the battery to the maximum allowed voltage (according to the passport data), and then we discharge it with a constant current close to 10 hour characteristic to the lower voltage (also specified in passport).

After that, we simply multiply the discharge current and the time (in hours) during which the battery was completely discharged. This way you will find the real capacity of the battery.

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