Despite the range in battery types and applications, the characteristics particularly important in PV applications are the maintenance requirements of the battery and the ability to deep charge a battery while maintaining a long lifetime. To promote long cycle life with deep discharge, deep cycle batteries may be either of the open-flooded type with an excess of electrolytic solution and thick plates, or of the immobilized electrolytic type. Sealed gelled batteries may be rated as deep cycle batteries, but they will usually withstand fewer cycles and lower discharges than the specially designed flooded plate or AGM batteries. Shallow-cycle batteries typically use thinner plates made from lead calcium alloys and do not typically have a depth of discharge above 25%.
The stringent requirements for batteries used in photovoltaic systems have prompted several manufacturers to make batteries specifically designed for PV or other remote power systems. The batteries most commonly used in stand-alone photovoltaic systems are either deep-cycle lead acid types, or shallower cycle maintenance-free batteries. Deep-cycle batteries may be open flooded batteries (which are not maintenance-free) or captive electrolyte AGM batteries which are maintenance-free (but which do require care in regulator selection). Special shallow-cycle maintenance-free batteries that withstand infrequent discharging may also be used in PV applications, and provided that the battery bank is appropriately designed, never require a DOD of more than 25%. A long-life battery in an appropriately designed PV system with correct maintenance can last up to 15 years, but the use of batteries which are not designed for long service life, or conditions in a PV system, or are part of a poor system design can lead to a battery bank which fails after only a few years.
Several other types of specific purpose batteries are available and these are described below.
Starting, lighting ignition batteries (SLI).These batteries are used in automotive applications and have high discharge and charge rates. Most often they use electrode plates strengthened with either lead antimony in a flooded configuration, or lead calcium in a sealed configuration. These batteries have a good life under shallow-cycle conditions, but have very poor lifetime under deep cycling. SLI batteries should not be used in a PV system since their characteristics are not optimized for use in a renewable energy system because lifetime in a PV system is so low.
Traction or motive power batteries. Traction or motive batteries are used to provide electric power for small transport vehicles such as golf carts. Compared to SLI batteries, they are designed to have a greater ability to be deep-cycled while still maintaining a long lifetime. Although this feature makes them more suited to a PV system than one which uses SLI batteries, motive power batteries should not be used in any PV systems since their self discharge rate is very high due to the use of lead antimony electrodes. A high self discharge rate will effectively cause high power losses from the battery and make the overall PV system inefficient unless the batteries experience large DOD on a daily basis. The ability of these batteries to withstand deep cycling is also far below that of a true deep-cycle battery. Therefore, these batteries are not suited to PV systems.
RV or marine batteries. These batteries are typically a compromise between SLI batteries, traction batteries and true deep-cycle batteries. Although they are not recommended, both motive and marine batteries are used in some small PV systems. The lifetime of such batteries will be restricted to a few years at best, so that the economics of battery replacement mean that such batteries are typically not a long-term cost effective option.
Stationary batteries. Stationary batteries are often used for emergency power or uninterruptable power supply applications. They are shallow-cycle batteries intended to remain close to fully charged for the majority of their lifetime with only occasional deep discharges. They may be used in PV systems if the battery bank is sized so that it never falls below a DOD of between 10% and 25%.
Deep-cycle Batteries. Deep-cycle batteries should be able to maintain a cycle life of several thousand cycles under high DOD (80% or more). Wide differences in cycle performance may be experienced with two types of deep cycle batteries and therefore the cycle life and DOD of various deep-cycle batteries should be compared.