Users Guide to Off-Grid Energy Solutions

 Batteries and Electrical Storage Fact Sheet

  Solar cell modules generate electricity only when the sun is shining. They do not store energy. Electric charge generated during the day must be stored to be available at night. The most obvious answer to this problem is to use batteries, which chemically store electric charge.

 When designing a solar system, a solar designer needs to keep several points about batteries in mind. As mentioned elsewhere, the battery is the weak link of the solar lighting system so it needs to be selected carefully.

 Good batteries are expensive, but worth the investment if you can afford it.

• Batteries wear out. No matter the type, they will eventually wear out and need to be replaced. Find out how long your battery is expected to last from the supplier.
• Batteries need to be maintained. Keep them clean and, especially if your solar array is undersized, take them for boosting when they have been deeply discharged.
• Do not mix battery types in your battery bank

 The two most common types of secondary battery systems on the world market today are lead acid and nickel-cadmium batteries. Lead-acid batteries are far more readily available than nickel-cadmium batteries.

 There are a number of types of lead-acid batteries, which fall in two general categories: deep discharge and shallow discharge. Deep discharge batteries are preferred for solar electric systems because more energy can be taken out of deep discharge batteries than shallow discharge batteries than shallow discharge batteries without causing damage to the cells.

 The amount of energy that a battery can store is called its capacity. The capacity of a battery is measured in amp hours (Ah) . This indicates the amount of energy that can be drawn from the battery before it is completely discharged. A battery of 100 Ah should ideally give a current of 2 amps for 50 hours.

battery capacity: the total number of amp hours that can be removed from a fully-charged battery or cell at a specified discharge rate.

battery: a device that converts chemical energy contained in its active materials directly into electrical energy by means of an electrochemical reaction.

blocking diode: a solid-state electrical device planed in circuit between the module and the battery when the voltage of the battery is higher than that of the module (i.e. at night)

cell (battery): the smallest unit or section of a battery that can store electrical energy and is capable of providing a current to an external load.

cycle life: of a battery, the number of cycles it is expected to last before being reduced to 80% of its rated capacity

cycle: one discharge and charge period of a battery

deep discharge battery: a type of battery that is not damaged when a large portion of its energy capacity is repeatedly removed (i.e. motive batteries).

depth of discharge: a measure in percentage of the amount of energy removed from the battery during a cycle.

depth of discharge: a measure in percentage of the amount of energy removed from the battery during a cycle.

discharge: the removal of electric energy from a battery

dry cell battery: can be made of alkaline cells, consisting of common dry cell carbon or zinc, of types "AA" "AAA", "C", "D" (most common torch/flashlight battery), others used for radios, cassette players, various other small appliances. A dry cell battery contains electrolytes that are in the form of paste rather than liquid.

ni-cad battery: a nickel cadmium (ni-cad) battery. NiCad batteries have been used to power small electronic devices for a number of years. They are rechargeable, have higher energy densities than lead-acid batteries, and have higher energy densities that lead-acid batteries. Rarely used for PV applications for a variety of reasons including reduction in recharge voltage, the so-called "memory effect", among others. [for further reading see: http://www.pfa.org.uk/ni-cad.htm].

shallow discharge batteries: batteries designed to supply high power for a short duration (e.g. automotive batteries); taking too much energy out of these batteries before recharging them is likely to damage the plates inside.

storage: any system by which energy is stored. Storage generally includes batteries, although more advanced storage systems can include heat pumps or water reservoirs.

trickle charge: a low current charge. When the batteries are fully charged, some charge controllers reduce the energy from the module to the battery to a trickle charge so that the batteries are not overcharged, but so that they still get enough current to overcome self-discharge.

For more information, check out these sites: http://www.windsun.com/Batteries/batterie.htm ;

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