Battery operation is generally the least understood area of a solar PV system and often makes it the weakest link of the system because battery life depends upon its discharge behavior. There are two common parameters for depth of discharge (DOD) in PV systems: the maximum allowable DOD and the average daily DOD.
Battery Life: Life of a solar PV system battery depends upon a number of design and operational factors. As long as a deep cycle battery is not overcharged, over-discharged or operated at excessive temperatures, the battery life is proportionate to its average state of charge. A typical flooded lead-acid battery that is maintained above 90 percent state of charge will provide two to three times more full charge/discharge cycles than a battery allowed to reach 50 percent state of charge before recharging. This suggests limiting the maximum allowable and average daily DOD to prolong battery life, though exact quantification of battery life is difficult due to the number of variables involved.
Temperature Effects: Higher operating temperatures accelerate corrosion of the positive plate grids, resulting in greater gassing and electrolyte loss. Lower operating temperatures generally increase battery life but reduce the battery capacity. Battery manufacturers often claim that battery life decreases by a factor of two for every 10˚C increase in average operating temperature. When there are wide temperature variations from the ambient, batteries are located in an insulated or temperature-regulated enclosure to minimize battery to minimize temperature effects.
Average Daily DOD: This is the daily average discharge level compared with the full battery capacity. If the load depends upon season, the daily level of discharge will also change. If the load is constant but ambient temperature reduces the daily DOD will increase due to reduced battery capacity at lower temperatures. The average daily DOD is inversely related to amp-hour capacity; meaning that systems designed for longer autonomy periods (more capacity) have a lower average daily DOD. But if the system is only marginally sized daily depletion can be quite high, say 50 percent or so. This has a direct bearing on battery life; keeping the daily DOD low prolongs battery life.
Maximum allowable DOD: This is the maximum percentage of full-rated capacity that can be withdrawn from a battery. For deep cycle batteries the maximum allowable depth of discharge is 80 percent; allowing discharge beyond this limit is not prudent. It must be fixed at the time of PV system design because it decides amp-hour capacity of the battery or battery-bank.
A system design with a lower allowable DOD will result in a shorter autonomy period or needs larger capacities for convenient autonomy. In standalone PV systems, the low voltage load disconnect (LVD) set point of the battery charge controller dictates the allowable DOD limit at a given discharge rate. This limit is generally triggered by factors such as seasonal deficit in sunshine, low temperatures and unexpected excessive load.
Temperature becomes a major factor in cold climates as lower temperature reduces battery capacity. In a fully charged lead-acid battery, the electrolyte is roughly 35% by weight and the freezing point is quite low (below -50˚C). As a lead-acid battery is discharged, the electrolyte becomes diluted and the freezing point approaches 0˚C. If the internal battery temperature drops below the freezing point of the acid solution, the electrolyte can freeze and expand, causing irreversible damage to the battery.
Self Discharge Rate: Even without charging or discharging, a battery undergoes a reduction in its state of charge, due to internal mechanisms and losses within the battery. Higher temperatures also cause higher discharge rates. It is an important issue with standalone solar street lights in hot climates where the battery is mounted at the pole. Different battery types have different self discharge rates depending upon factors such as nature of the active materials and grid alloying elements used in the design.