How to build a homemade solar panel - The battery

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.