Panels photovoltaic solar energy

What Are Solar Batteries?

Solar batteries aim to accumulate the electrical energy generated by photovoltaic solar panels so that it can be used at night or on cloudy days.

What are solar batteries?

The use of batteries also makes it possible to provide a higher current intensity than that which a functioning photovoltaic panel can offer. This would be the case if several electrical devices were used at the same time.

A battery consists of small 2V electric accumulators built into the same element. The batteries supply direct current at 6, 12, 24 or 48V. The accumulator is the cell that stores energy through an electrochemical process.

For example, when we talk about a 12V battery, we refer to a series set of 6 lead-acid cells of 2 volts each. Monoblock batteries are batteries made up of several 2V cells forming a single block.

Stationary batteries, also known as solar accumulators, are batteries made up of 2-volt elements connected in series until the desired working voltage for the solar installation is reached.

How Do Solar Batteries Work?

The batteries have the function of supplying electrical energy in the system at the moment when the photovoltaic panels do not generate the necessary electricity. For example during the night or in moments of low light.

When the photovoltaic panels can generate more electricity than that demanded by the electrical system, all the energy demanded is supplied by the panels and the surplus is used to charge the batteries.

The batteries transform the electrical energy received from the photovoltaic modules into chemical energy. This conversion is made from the reaction that occurs when two different materials, such as those on the positive and negative plates, are submerged in the electrolyte. The electrolyte is a solution of sulfuric acid and water.

Electricity flows from the battery as soon as there is a circuit connected between the positive and negative poles.

As the battery discharges, the composition of the lead in the plates becomes more similar. At this time, the acid density decreases and the voltage between terminals decreases. 

 The ability to undergo a constant charge and discharge process is known as the resistance to cycling of a battery.

Solar Battery Class

There are two types of batteries according to their cycle:

  • Low cycle batteries

  • Deep cycle batteries

Low Cycle Batteries

Low-cycle batteries are designed to supply a quantity of current for a short period of time and withstand small overloads without losing electrolytes, as in the case of automobile batteries.

However, these batteries do not support deep discharges. If they are repeatedly discharged below 20%, their useful life is shortened considerably. Therefore these batteries are not a good choice for photovoltaic solar systems.

Deep Cycle Batteries

They are designed to be repeatedly discharged up to 80% of their capacity. This feature makes them the best choice for solar energy systems.

Features to Take into Account of a Solar Battery

Most important features when choosing a battery or a solar kit:

  • Capacity. The capacity is the current intensity in amps (A) that can be obtained from a complete discharge of the electric accumulator when it has a state of full charge.

  • Charging efficiency. Charging efficiency is the ratio of the energy used to fill the accumulator to that actually stored. Therefore the closer to 100% the better.

  • Self-discharge. Self-discharge is the process of an electric accumulator that tends to discharge without being in use.

  • Discharge depth. The depth of discharge is the amount of energy obtained during a discharge while fully charged (%).

What Is the Lifespan of a Solar Battery?

The useful life of a battery for solar installations is usually around 10 years. However, if frequent deep discharges are carried out (> 50%), their useful life falls sharply. Therefore, it is advisable to install enough capacity so that 50% of the discharge is not exceeded.

Another very important factor is the temperature. If the temperature is maintained between 20 and 25ºC, the useful life will be around 10 years. In contrast, if the temperature is altered by 10ºC, the useful life can be reduced by half.

Battery Type

Batteries are classified according to the type of manufacturing technology as well as the electrolytes used.

The most used batteries in solar installations are lead-acid, due to the price ratio for available energy. Its efficiency is between 85-95%, while Ni-Cad is 65%.

Surely the best batteries would be lithium (mobile). However, the lithium battery is not economically viable for this application.

Lead-acid Batteries for Solar Applications

All lead-acid batteries fail prematurely when they are not fully recharged after each cycle.

If a lead-acid battery is left discharged (for days) at any time, it will cause a permanent loss of capacity.

Liquid Batteries - Liquid Electrolyte

They are the most used. There are two types of liquid batteries:

  1. Open, with lids that allow the change of water.

  2. Sealed, they are closed but with valves that allow the exit of possible gases during excessive loads.

Advantages of Liquid Batteries

  • The oldest

  • Its production allows low prices.

  • They are less problematic at overloads.


  • There is a danger of losing (aggressive) fluid.

  • They usually have a short useful life, between 400 charge and discharge cycles.

  • Very low temperatures can quickly destroy them.

There are other types of batteries. Where the electrolyte is not in a liquid state, it has been immobilized. 

In the case of gel batteries, the electrolyte has become a gel. In the case of the AGM (Absortion Glass Mat) battery, it has been separated by a fiberglass, with great absorption power, which acts like a sponge.

Both gel batteries and AGM batteries are maintenance free. You will never need to add water.

AGM Batteries - Absortion Glass Mat

They are the most modern batteries and the acid is fixed in glass fibers that absorb it.

Almost all AGM batteries are valve regulated: VRLA (valve regulated lead acid)

They have all the advantages of gel, in addition to the following:


  • Good shelf life.

  • More resistance to cold climates.

  • Your auto download is minimal.

  • Low internal resistance that allows high currents.

  • Deep cycle.


  • Higher price.

There is an increasing trend towards lead AGM batteries. They have their best life / price ratio. If handling is easier.

For someone who can ensure the necessary care, the liquid battery may be the best option. Especially considering the price.


Solar batteries are stores of electricity. They store the electricity generated by solar panels at times of low demand and supply it at times of greatest demand.

These elements are capable of transforming electrical energy into chemical energy at the moment of charge. At the moment of discharge they transform, again, chemical energy into electricity.


Published: May 13, 2015
Last review: May 30, 2020