Solar Batteries,Solar GEL Battery,Solar AGM Battery

Batteries are one the most complex and involved topic in the discussion of components for PV Systems. Including battery banks in PV systems increases the complexity and cost of the system as well as maintenance requirements and choosing the right battery system is a critical part of the overall design of the system.

Batteries store DC energy in a chemical form that can be converted back to electricity to be used as required. Most PV system's power output varies throughout the day with no power being generated at night; a battery storage system can provide a relatively constant source of power storing energy when the PV system produces power and making it available as required. A battery is charging when energy is being put in and discharging when energy is being taken out. A cycle is considered one charge-discharge sequence which occurs over a period of one day in PV systems and most battery life is specified in cycles.

There are 2 major categories of batteries based on function: Starting and Ignition batteries and Deep Discharge batteries. Starting and Ignition batteries are designed for shallow cycle service and are damaged by deep discharges and are typically designed to provide a surge of power for a short period of time in automobile starting. Deep discharge batteries are typically used in electric vehicles and equipment and are designed for high discharge rates. Most Stand-Alone and Grid-Interactive systems need deep discharge batteries for optimal performance.

There are 2 major categories based on type: Lead-Acid Batteries and Alkaline batteries with lead acid being either liquid vented or Sealed (Valve regulated Lead Acid) and alkaline batteries being Nickel-Cadmium or Nickel Iron.

Lead Acid Batteries are the most commonly used battery types in residential PV Systems. Liquid lead acid batteries are like automobile batteries in construction with positive and negative plates made of lead and lead alloy placed in an electrolyte of sulfuric acid and water.

Sealed or Valve regulated lead acid batteries are maintenance free as there is no need to add water or access the electrolyte and are typically available as Gel Cell batteries or Absorbed Glass Mat (AGM) batteries. The main advantage of these batteries is that they are spill proof and produce less amount of hydrogen gas. They also do not require periodic maintenance or equalization. Gel Cell batteries cost more when compared to liquid lead-acid batteries and are susceptible to damage from overcharging especially in hot climates and have a shorter life expectancy than other battery types.Alkaline batteries such as Nickel-Cadmium and Nickel-Iron batteries have plates made of nickel and cadmium or nickel and iron in a potassium hydroxide electrolyte. They are expensive but are not affected by temperature and are recommended for commercial or industrial applications in locations where extreme cold temperatures exist.

Batteries are rated by capacity in Amp-Hours at a specified temperature, discharge rate and cut-off voltage. An Amp-Hour is the transfer of one amp over one hour. Many factors affect battery capacity including rate of discharge, depth of discharge, temperature are and recharging characteristics. When designing a battery bank for a PV system, you should specify a slightly larger battery capacity that is needed as batteries lose their capacity as they age but if the battery bank is oversized, it can remain in a state of partial charge shortening the battery life, reduced capacity and increases sulfation.

The rate at which a battery is discharged affects is capacity and the quicker it is discharged the lower its capacity will be. For example a six volt battery may have a 180 AH capacity if discharged over 24 hours and a 192AH capacity if discharged over 72 hours. A common specification is the battery capacity in relation to the discharge rate. A battery capacity when discharged over 20 hours is specified as C/20. Similar consideration should be taken when charging batteries and should not be charged at more than the C/10 rate. Gel Cell batteries should never be charged at higher than a C/20 rate - full capacity over 20 hours of charging.

Depth of discharge (DOD) refers to how much capacity will be removed from the battery and the battery life is directly related to how deep the battery is cycled. For example if a battery is discharged to 50% per cycle it will last twice as long as if it is cycled to 80%.

In general a battery must never be completely discharged even if some Nickel-Cadmium batteries are capable of withstanding a complete discharge. The most practical number to use when designing a system is a 50% DOD for best storage, life and cost of the system.

As discussed above, a battery life is specified in number of cycles to a specific depth of discharge. Batteries loose capacity over time and are considered to be at the end of their life when they loose 20% of their original capacity. Batteries are also highly affected by temperature. Manufacturers generally rate batteries at 25 degrees centigrade or 77 degrees F but their capacity decreases and life increases at lower temperatures and capacity increases and life decreases at higher temperatures. Very cold temperatures can also freeze the electrolyte when the battery is at a low state of charge and potentially break the battery.

When designing for temperature affects on the battery you can compensate for the effects by using a multiplier and there are standards you must follow, NEC and others, when designing the system to take into account these effects. Another key aspect of designing battery systems if you are using Lead-Acid batteries is to keep them well ventilated as they produce hydrogen gas which is explosive and corrosive.

Typical Battery specifications are given below: