\( 90\% \) of the energy produced by the photovoltaic panels during the day that was not consumed by the consumers is stored in batteries \( 90\% \). The stored energy may be consumed at night or on days with bad weather conditions. The cost of off-grid energy storage comes to about \( 30\% \) of the cost of the entire PV system. An electrical schematic of an off-grid photovoltaic system with energy storage, i.e., a set of batteries, is shown in Fig. 1. When charging, electrical energy is converted to chemical energy, and in the process of discharging (energy extraction), chemical energy is converted to electrical energy. The advantages of batteries are:

1. the ability to carry electrical energy storage,
2. the ability to deliver a large amount of energy in a short time,
3. reliability,
4. delivery of energy in the form of direct current.

The energy generated in photovoltaic panels during the day that was not consumed by the receiver is stored in batteries. The stored energy can be used at night or on days with bad weather conditions. Batteries in photovoltaic systems are often charged/discharged, so they must meet more requirements than batteries used in cars, for example. The most commonly used batteries are classic lead-acid (PbA) batteries manufactured specifically for photovoltaic systems, which however, have the disadvantage that they break down when regularly discharged below \( 50\% \). Other types of batteries used include nickel-cadmium (NiCd) or nickel-metal-hydride (NiMH) batteries, where deep discharge is required, as well as lithium-ion batteries.
Hermetic batteries often contain a gel electrolyte and thus do not require maintenance. The life of typical batteries connected to photovoltaic systems is three to five years, depending on charge/discharge cycles, temperature, and other parameters. The more often a battery is charged/discharged, the shorter its life.

Depending on the type, batteries can be charged in different ways, such as at a constant current, at a constant voltage, etc. The charging method is determined by the manufacturer.
Batteries developed specifically for photovoltaic systems are designed to have the following properties [1]:

1. cyclic resilience,
2. reliability under cyclic discharge conditions,
3. increased performance at slow discharge rates,
4. low self-discharge,
5. high energy (kWh - kilowatt-hours) and capacity (Ah-amp-hours),
6. ability to operate over a wide temperature range,
7. profitability,
8. long life,
9. low maintenance.

Technical requirements for off-grid, or on-grid, photovoltaic system performance and energy are presented by Rydh and Sandén [2]. To get enough energy in winter for storage, more photovoltaic panels are required than the average energy consumption. They are needed to capture enough energy during the few sunny hours in winter and store it in batteries. To harness the energy from the batteries and from the photovoltaic panels, a suitable electronic circuit is needed, a schematic of which is shown in Fig. 1.

A schematic of how the electrical system of an off-grid photovoltaic system works is shown in Fig. 1. The photovoltaic panel converts solar energy into electrical energy, which is directed through a switch and a meter to a charge controller, then to energy storage and subsequently to direct current (DC) consumers. From the energy storage, energy flows through the meter and the switch to the inverter, which converts the direct current (DC) into alternating current (AC). The AC current is then passed on to the consumers (AC).
Energy stores in the form of lead batteries consist of a chamber that contains a liquid electrolyte, \( H_{2}SO_{4} \) in which the electrodes, the positive electrode \( PbO_{2} \) and the negative electrode Pb, are immersed. The following charge and discharge processes occur here - see Fig. 2.

A fully charged battery is one in which the positive electrode is coated with lead peroxide and the negative electrode is pure lead. At this point, all of the acid is in the electrolyte, meaning the specific gravity of the electrolyte is at its highest. During discharge, some of the acid combines with the electrode plate to form the compound lead sulfate, which is formed on both electrodes. During this process, the specific gravity of the electrolyte can be measured and will decrease, giving an indication of the battery's state of discharge. As the battery is charged, the sulfuric acid is forced out of the battery plates into the electrolyte. At this point, the specific gravity of the electrolyte will increase until the battery is fully charged.
Once the battery is fully charged, the energy delivered to the battery causes electrolysis of water, during which hydrogen and oxygen are released. This is the main reason for adding water to battery cells.

Popular battery manufacturers include Tesla, Mercedes Benz, MA, LG Chem, and Enphase [3]. Tesla batteries have a high efficiency of \( 90\% \) with a 10-year warranty. Mercedes Benz makes 2.5 kWh batteries that can be combined in an assembly of up to 8 batteries.
Tesla's PowerWall 2 product includes a battery (made by Panasonic) along with an inverter. It features an efficiency of \( 90\% \) and the possibility of discharging up to \( 100\% \). SMA manufactures inverters that can be used in off-grid and on-grid systems as well as integrated with batteries from Mercedes-Benz, LG Chem, Sony and others. LG Chem has developed a series of batteries with efficiencies as high as \( 95\% \). The Enphase brand produces small batteries with efficiencies up to \( 96\% \) and discharge capabilities up to \( 95\% \). The table (on Fig. 3 ) shows a comparison of battery parameters from different manufacturers. The most important of these are capacity and guaranteed life.
The table compares the parameters of batteries from different manufacturers.

The capacity of the battery determines how much energy can be stored, obviously the higher the value the better. The next parameter is the technology in which the battery is manufactured. The service life of the battery determines after how long (how many years) the ability to store energy will decrease to what initial value. The next lines give the weight, dimensions, possibility of expansion and use as a UPS.