There are many types and sizes of batteries that the prudent designer could select for most renewable energy systems. It is important to keep in mind that the battery bank is typically the only maintenance item in a PV or wind power system. Because of this fact, care should be taken when selecting the type and brand of battery to use in the system. The four principle questions to be addressed would be:

• Price - How much do I want to spend for higher quality and longer life batteries?
• Maintenance - What degree of freedom from maintenance do I require?
• Temperature - What are the temperature extremes likely to be encountered by the battery?
• Capacity - What is the likely "days of no-sun" capacity at the site or what is the total amount of capacity, in ampere-hours (AH) needed from the battery to provide enough energy to sustain the average daily load for consecutive cloudy days for a given location?

To better understand these critical questions, let's examine each one carefully.

1. The Price. For most industrial solar (PV) applications like those encountered in the oil and gas, and telecommunications industries, the cost of using quality batteries is so minor a point when compared to the value of the instruments they power that to offer an inferior, low cost battery is rarely justified. But for the family of five existing in primitive housing without much money somewhere in the developing world and with an almost desperate need for low cost solar (PV) systems, the cost of a basic deep cycle battery is of much concern to them. The main consideration relating to the "first cost" factor of a solar (PV) battery storage system is to remember that most well designed battery banks will have a large reserve capacity. They are designed not to have their capacity reduced to a low state of charge and thus do not wear out as quickly. This design factor, while it increases the first cost of a battery bank is most often a far sighted and wise investment. This is because a good deep cycle battery will provide reliable service for many more years if not deeply discharged on a daily basis.
2. The Maintenance. There are basically two types of batteries - ones that require absolutely no maintenance and others where careful attention has to be paid to periodically adding clean water to the cells. In most oil and gas industry applica- tions, the batteries may be for instrumentation along a long deserted pipeline where it is difficult and expensive to maintain the water levels. In these cases, a sealed gel cell is the logical choice because it is the least cost long term choice. In some telecommunication sites and places where people will be available to offer maintenance, a less costly deep cycle WET battery makes better economic sense. With the use of hydro-caps, the time interval between required maintenance is greatly extended due to their water saving characteristics.
3. The Temperature. All batteries are adversely affected by high and low temperatures. Even the highest quality nickel cadmium or Nicad battery has a point at which it's ability to perform its internal chemical reactions can no longer take place. At -60degC most batteries cannot perform without special provisions. In hot climates the same is true. Typical solutions to these problems are accomplished by providing additional insulation or simply increasing the capacity to compensate for the loss. Other more drastic measures such as supplying heated blankets for cold weather site batteries and partially filling the battery box with coolant for warm weather sites are sometimes required to decrease the loss due to temperature extremes.
4. The Capacity. As mentioned before, the capacity is important as far as long term costs are concerned. But even more importantly, it is often critical to some types of solar battery systems to know the numbers of consecutive cloudy days (often referred to as "no-sun" days) that will be encountered at the site. This means you need to know with some degree of certainty when the rainy season or winter months occur. Then when you design the capacity of the battery system, you would multiply the average daily electrical load in ampere-hours, (AH) by the number of consecutive cloudy days, plus a service factor, typically of 30%. This will give you the total ampere-hour capacity of the battery bank for the worst case insolation of the year.

   In this scenario, we treat a cloudy day as though it were totally void of light. In actuality, the current derived from a solar (PV) module is directly proportionally to the intensity of the sun, so that even on a cloudy, rainy day, some unknown and incalculable amount of charging of the battery takes place.

   Therefore to figure battery capacity is as follows: Average daily electrical load (AHPD) x number of consecutive cloudy days x 1.2 = battery capacity (AH)

SWPV STOCK THE MAJOR PROVEN BRANDS OF WET AND SEALED GEL CELL BATTERIES INCLUDING OUR OWN PRIVATE LABELED PHOTON ACCUMULATOR GEL CELL BATTERY SERIES. WE ARE EXPERIENCED IN SOLAR BATTERY SYSTEMS, LARGE AND SMALL. WE CAN GREATLY ASSIST YOU IN THE DESIGN OF YOUR BATTERY BANK FOR SPECIFIC LOCATIONS AND DUTIES.