The Photovoltaic batteries are an essential element for self-consumption and solar installations, especially off-grid ones. Below we will review some of their most important characteristics.

Capacity of a solar battery

The solar battery  capacity is the amount of electricity a battery can supply under a working condition (Ah). The capacity of a battery depends on three factors:

• Duration and type of discharge intensity.

• Temperature.

• Final shear stress.

 The useful capacity of the battery is the amount of energy it is capable of delivering during a number of days of autonomy without being recharged.

On the other hand, the nominal capacity of the battery is determined by the depth of discharge reached during the days of autonomy. The value of C100 is usually given for this capacity.

Depth of discharge (DOD)

The amount of energy used from a battery is usually expressed as a percentage of the total battery capacity and is known as the depth of discharge.

This is the battery capacity in % of the nominal capacity during the charging and discharging processes. In order to achieve a long battery life, manufacturers recommend not to go below 40 or 50% of its nominal capacity in lead-acid batteries.

Solar battery self-discharge

Self-discharge is the loss of capacity and density experienced by an element in open circuit without being connected to a load system. The load provided in flotation services must be greater than this effect. The density loss shall not exceed 0.0005 kg/l per day.

Battery self-discharge should not be confused with electrolyte stratification. The acid sample taken to measure the density is from the upper part of the element, and therefore, in case of stratification, this would be the one with the lowest density.

Battery self-discharge occurs due to internal leakage and can be avoided by periodic recharging. For storage it is better to have the battery separated from the electrolyte.

When a battery is completely discharged, deep discharge occurs. Irreparable damage to the battery may occur, some batteries cannot be recharged.

It must be taken into account that the natural self-discharge of a battery is increased by high temperatures, stratification of the electrolyte, impurities in the materials or internal short circuits. Therefore, a good maintenance is vital to extend the life of the photovoltaic battery.

Overcharging of a solar battery

Overcharging a battery is a process in which current is forced to flow in a fully charged battery. The battery can be damaged if it is overcharged for an excessive amount of time.

Only the gas generation process is useful in flooded batteries to achieve homogenization of the electrolyte. The small gas bubbles produced during equalization charging (the name given to this overcharge process) remove the electrolyte, which tends to have a higher density in the lower part of the element.

The higher the charging rate and the higher the percentage of overcharge, the shorter the life achieved by the battery.

Although a fully charged battery is capable of delivering the full ampere-hours indicated in its capacity, it is not advisable to subject it continuously to deep charges and discharges, as this reduces its useful life. The main drawback of lead-acid batteries is the danger of sulfation of the plates when the battery is subjected to deep discharges.

Efficiency of a solar battery

The efficiency of a battery in energy (Wh) or ampere-hours (Ah) are important factors used during the design of off-grid photovoltaic installations.

Battery efficiency specifications are not standardized and each manufacturer has its own definition and calculation method. Some manufacturers use average efficiency but others specify only maximum efficiency. The average energy efficiency is defined as a percentage value by dividing the energy discharged by the energy charged during the analysis period.