A lead-acid battery consists mainly of a lead dioxide anode, also known as the positive electrode or pole, a lead cathode, also known as the negative electrode or pole, and an electrolyte of sulfuric acid diluted in water.

Its operation is based on a chemical reaction of oxidation and/or reduction that occurs between the anode and the cathode. This makes it possible to convert chemical energy into electrical energy. As it is possible to reverse the chemical reaction, it is possible to have both a discharge process of the  solar battery and a charging process.

During the operation of the  photovoltaic battery different chemical reactions occur depending on whether it is charging or discharging:

• During charging, lead oxide is formed at the anode, pure lead at the cathode and sulfuric acid is released into the electrolyte.

• During discharge, lead sulfate forms on both electrodes and the electrolyte absorbs sulfuric acid.

Characteristics of lead-acid and nickel-acid batteries

While most nickel batteries (NiCd, NiMH) are being replaced by lithium batteries due to cost and pollution factors, they are still present in many installations.

We can distinguish two types of lead-acid batteries, which, although identical in their internal chemistry, have a difference in the electrolyte:

• Flooded battery ( or liquid electrolyte battery). It contains a liquid solution of acid and water, and distilled water can be added to replenish the electrolytes when necessary.

• Sealed battery or VRLA battery. The electrolyte is captive and has a special venting valve. It is not possible to add electrolyte, so they are known as maintenance-free batteriesThere are gel batteries where the electrolyte is gelled and AGM (Absorbed Glass Mat) batteries that use saturated absorbent glass mats instead of gelled or liquid electrolyte.

There are three clear differences between liquid electrolyte and VRLA batteries:

• Upright orientation to avoid electrolyte leakage.

• Ventilated environment to remove gases generated during charge/discharge cycles.

• Routine maintenance of electrolyte levels.

Maintenance and precautions to be followed with electrochemical batteries

In order to extend the service life of electrochemical batteries, a series of maintenance operations can be carried out, such as:

• Top up with water at the end of several charging cycles (every 3 or 6 months depending on operating conditions).

• Keep the battery clean and dry.

• Do not discharge more than 80% of rated capacity.

• Do not overload. The maximum load factor is 1.2.

• Maintain the electrolyte temperature below 55°C.

The main precautions to be followed in a photovoltaic installation that includes electrochemical batteries, as well as other types, are included in the manuals supplied by the manufacturers, which must be read. The main points to be followed with any type of electrochemical battery can be summarized as follows:

• Danger of corrosion due to contact with sulfuric acid.

• In case of short circuits, the current can be very high, with risk of battery explosion.

• Explosive gases may be generated during operation (location in ventilated areas).

• Read the manufacturer's instructions in the technical manuals.