The battery 'hosts' a certain reversible chemical reaction that stores electrical energy that can later be retrieved when needed. Electrical energy is transformed into chemical energy when the battery is being charged, and the reverse happens when the battery is discharged.
A battery is formed by a set of elements or cells arranged in series. Leadacid batteries consist of two submerged lead electrodes in an electrolytic solution of water and sulfuric acid. A potential difference of about 2 volts takes place between the electrodes, depending on the instantaneous value of the charge state of the battery.
The most common batteries in photovoltaic solar applications have a nominal voltage of 12 or 24 volts. A 12 V battery therefore contains 6 cells in series. The battery serves two important purposes in a photovoltaic system: to provide electrical energy to the system when energy is not supplied by the array of solar panels, and to store excess energy generated by the panels whenever that energy exceeds the load.
The battery experiences a cyclical process of charging and discharging, depending on the presence or absence of sunlight. During the hours that there is sun, the array of panels produces electrical energy. The energy that is not consumed immediately it is used to charge the battery. During the hours of absence of sun, any demand of electrical energy is supplied by the battery, thereby discharging it.
These cycles of charge and discharge occur whenever the energy produced by the panels does not match the energy required to support the load. When there is sufficient sun and the load is light, the batteries will charge. Obviously, the batteries will discharge at night whenever any amount of power is required. The batteries will also discharge when the irradiance is insufficient to cover the requirements of the load (due to the natural variation of climatological conditions, clouds, dust, etc.)
If the battery does not store enough energy to meet the demand during periods without sun, the system will be exhausted and will be unavailable for consumption. On the other hand, the oversizing the system (by adding far too many panels and batteries) is expensive and inefficient. When designing a stand-alone system we need to reach a compromise between the cost of components and the availability of power from the system. One way to do this is to estimate the required number of days of autonomy.
In the case of a telecommunications system, the number of days of autonomy depends on its critical function within your network design. If the equipment is going to serve as repeater and is part of the backbone of your network, you will likely want to design your photovoltaic system with an autonomy of up to 5-7 days. On the other hand, if the solar system is responsible for a providing energy to client equipment you can probably reduce number of days of autonomy to two or three. In areas with low irradiance, this value may need to be increased even more. In any case, you will always have to find the proper balance between cost and reliability.
Types of batteries
Many different battery technologies exist, and are intended for use in a variety of different applications. The most suitable type for photovoltaic applications is the stationary battery, designed to have a fixed location and for scenarios where the power consumption is more or less irregular. 'Stationary' batteries can accommodate deep discharge cycles, but they are not designed to produce high currents in brief periods of time.
Stationary batteries can use an electrolyte that is alkaline (such as Nickel- Cadmium) or acidic (such as Lead-Acid). Stationary batteries based on Nickel-Cadmium are recommended for their high reliability and resistance whenever possible. Unfortunately, they tend to be much more expensive and difficult to obtain than sealed lead-acid batteries.
In many cases when it is difficult to find local, good and cheap stationary batteries (importing batteries is not cheap), you will be forced to use batteries targeted to the automobile market.
Using car batteries
Automobile batteries are not well suited for photovoltaic applications as they are designed to provide a substantial current for just few seconds (when starting then engine) rather than sustaining a low current for long period of time. This design characteristic of car batteries (also called traction batteries) results in an shortened effective life when used in photovoltaic systems.
Traction batteries can be used in small applications where low cost is the most important consideration, or when other batteries are not available. Traction batteries are designed for vehicles and electric wheelbarrows. They are cheaper than stationary batteries and can serve in a photovoltaic installation, although they require very frequent maintenance. These batteries should never be deeply discharged, because doing so will greatly reduce their ability to hold a charge. A truck battery should not discharged by more than 70% of its total capacity. This means that you can only use a maximum of 30% of a lead-acid battery's nominal capacity before it must be recharged.
You can extend the life of a lead-acid battery by using distilled water. By using a densimeter or hydrometer, you can measure the density of the battery's electrolyte. A typical battery has specific gravity of 1.28. Adding distilled water and lowering the density to 1.2 can help reduce the anode's corrosion, at a cost of reducing the overall capacity of the battery. If you adjust the density of battery electrolyte, you must use distilled water, as tap water or well water will permanently damage the battery.