and are interconnected internally. For example, to get the typical battery voltage of 12 V,
six cells are connected in series. As the name suggests, lead-acid batteries use an acidic
electrolyte, namely diluted sulphuric acid H 2 SO 4 . Two plates of opposite polarity are
inserted in the electrolyte solution, which act as the electrodes. The electrodes contain
grid-shaped lead carrier and porous active material. This porous active material has a
sponge-like structure, which provides sufficient surface area for the electrochemical
reaction. The active mass in the negative electrode is lead (Pb), while in the positive
electrode lead dioxide (PbO 2 ) is used. In the figure, the chemistry of charging and
discharging the battery is also shown.
When the battery is discharged, electrons flow from the negative to the positive
electrode through an external circuit, causing a chemical reaction between the plates and
the electrolyte. This forward reaction also depletes the electrolyte, affecting its state of
charge (SoC). When a source with a voltage higher than the actual battery voltage is
connected to the battery, the reverse reaction is enabled. Then, the flow of electrons is
reversed and the battery is recharged. In PV systems, this source is nothing but the PV
module or array. In grid-connected systems, the inverter operating as an AC-DC converter
can be used to charge the battery.