How to calculate battery capacity

Solar panel batteries, or photovoltaic solar panels, are one of the most expensive products that are usually included in solar installation kits. An investment of this type requires a large outlay, so it is essential to take care of them to prevent them from being damaged as a result of overloading or incorrect installation —which progressively reduces their useful life—. Therefore, calculating the capacity of a battery is essential for them to perform their function safely and to guarantee their duration over time.

Related: Meaning of battery capacity in C100, C20 or C5

How to calculate battery capacity

The capacity of a battery is nothing more than the maximum amount of energy that can be harnessed under certain specific conditions. That is, depending on the consumption used by the electronic devices that are connected to it to work. Likewise, it is important to know that batteries have a percentage of discharge, which means that they lose their maximum charge capacity over time, due to heat and deterioration due to use.

Something that must be taken into account when buying a battery for solar panels. Below, we discuss how to calculate battery capacity—or, what amounts to the same thing, how to calculate battery autonomy—and discharge time, as well as some issues related to proper battery charging and fault recognition. damaged batteries.

How to calculate the autonomy of a battery

To calculate the discharge time of a battery, or how to calculate the capacity of a battery, it is essential that we learn to detect —in the first place— indications that will express the information we need to carry out the process. These are usually printed or stuck on a label on the battery. They are:

1. discharge time: The time code appears in the form of a “C” accompanied by a number. In general, these numbers usually follow the following series: C10, C20, C100, C120…
2. discharge current: This usually appears in the form of ampere-hour “Ah” and is accompanied by the figure that indicates the amount of electrical charge.

The combination of these is very useful to us for an essential reason. With them we can know the number of hours —C— at which the battery will be able to deliver the specific discharge current —Ah—. Let’s take an example. If we have a battery with C10 and Ah50, it would mean that it could be discharged at a rate of 10 hours with a current of Ah 50.

How to calculate the charge capacity of a battery depending on the installation

Now that we have clarified how to calculate the discharge time of a battery, or how to calculate the autonomy of a battery, we can now go on to analyze the formula to carry out the specific calculation with which to know what charge capacity the battery should have. in a given facility. To know the capacity of a battery we must follow the following steps:

1. Know the capacity in ampere-hours of the device that we are going to install: To do this, it is essential to also look at the indications of the device -as we have done previously with the indications of the battery-. Suppose we have an irrigation pump that works under the following conditions: 160mh for 24 hours. Well, in that case, to calculate its capacity in amp-hours and compare it, later with the battery, it will be necessary to apply this formula: C = X · T. In this case “X” is equivalent to amps and “T” to time on time. In the example mentioned, the result would be equivalent to C = 0.16 · 24. That is, C = 3.84 Ah.
2. Compare it with the battery: We will have to choose a battery with a capacity greater than 3.84 Ah. Keep in mind that it is not advisable for the batteries to be completely discharged if they are being used in a cycle -as in the case of solar panel batteries-, therefore it is recommended that the battery does not draw more than 50 % of its charge, approximately. To do this we will have to divide the figure obtained previously —capacity in ampere-hours of the device— by 0.5. The battery charge capacity should be 7.68 Ah or higher.

How to know if a battery has gone bad

There are several ways to detect if a battery has gone bad. In the case of solar batteries, the first thing you should analyze is whether the voltage provided by the battery is the same as that indicated in the product specifications. To use a device to measure tension. Another way to know the status of the battery, in case the previous measurement has not presented us with any anomaly, is subject the battery to a discharge effort.

To do this, what we must do is connect all the possible consumptions in the solar installation —such as the refrigerator, the lighting, the water pump…— and start them up. Next, we will have to look again at the voltage of each of the vessels. If any of them varies, it is possible that the battery is giving problems. Be careful, it is normal for the voltage to be less than 2 V in each case, however, in general, they will all have the same figure —between 1.90 V or 1.85 V—, the problem is if there is one that is in the margins superiors —in which case, we will find ourselves before an indicator of problems in the battery—.