BATTERY CLASSIFICATION

Because many types of batteries exist in the market, it is necessary to establish a way to classify them as to the particular application and/or general characteristics.

First of all, electrical storage batteries are classified as either primary or secondary. Primary batteries are designed to be used only once and discarded afterwards. Secondary batteries, on the contrary, have been designed to support repetitive cycles of charge and discharge. Therefore, all batteries for PV applications have to be secondary batteries.

The second useful classification refers to either shallow or deep cycle batteries. Perhaps the best example of shallow batteries is the automobile battery, also known as SLI or Starting-Lighting-Ignition battery. These batteries may not be discharged to more than about 10 percent of its capacity. These have many thin plates per cell delivering hundreds of amperes for a few seconds. Deep cycle batteries also refer as Traction batteries, are designed to support discharge up to about 80 percent of its capacity and provide storage electricity for long periods. These batteries have fewer thick plates delivering few amperes for hundreds of hours. These batteries are used in electrical vehicles and equipment such as golf carts, forklifts, marine, RVs, and hybrid automobiles. Because of these qualities, they are also use in PV applications. Another type of batteries are the Stationary batteries, which provide occasional deep-discharge at limited cycles commonly used in  uninterruptible power supplies for computers and other critical loads, they last long period of time in standby. For PV systems only deep cycle batteries are recommended.

The third classification refers to the characteristic of the electrolyte in the battery. According to this classification batteries are made either wet or flooded and sealed. Both types of batteries in this classification are suitable for PV applications.

The electrolyte in wet or flooded batteries is in liquid form. Perhaps the best known of this type of batteries is the lead-acid battery for which the electrolyte is a solution of sulfuric acid in water. In nickel-cadmium wet or flooded batteries the electrolyte is a solution of potassium hydroxide in water.

Sealed deep cycle batteries are also referred as VRLA or valve-regulated-lead-acid batteries. This is because pressure relief valves are supplied to the batteries to vent or prevent the formation of excess build up gas pressure. There are two best known technologies to manufacture sealed deep cycle or VRLA batteries, which refer to the characteristics of the electrolyte in the batteries, there are GEL and AGM.

As the name indicates in GEL type batteries the electrolyte is absorbed into other substance: silicon dioxide; forming a jellied type electrolyte. In AGM or absorbed-glass-mat batteries the electrolyte is also absorbed into other substance: a fiber glass type material.

The following scheme summarizes the type of batteries best suited for PV use.

Figure. PV batteries classification.

Wet or flooded deep cycle batteries require periodical care and maintenance, mostly to check electrolyte density, distilled water most be added to replenish that lost through gassing. Lead-acid batteries are widely available in many sizes, at a relatively low cost. Nickel-cadmium batteries are more common use in a few critical low-temperature applications, at relatively high initial cost. Wet or flooded batteries could be equipped with catalytic recombination caps (CRC) to reduce electrolyte loss by recombining the gasses and returning them as water.

The main advantage of Sealed batteries as compared to Wet or Flooded batteries is that they are spill proof which makes them specially convenient for transportation and handling. The other important advantage is that they do not require periodic maintenance. The disadvantage is the additional cost as they tend to be more expensive than wet batteries because of the more involved production process. However, as market preference for sealed batteries increase, pricing should be coming down, thus becoming less important when these prices are weighted against the inherent advantages, especially with respect to AGM batteries.