Primary Design Factors for Lead Acid Batteries
By Steve Bell, Technical Support Specialist
In a previous Tech Tip, the relationship between proper charging and the life/performance of lead-acid batteries was presented. In addition to charging considerations, there are also many system design factors that impact battery health. Site conditions, ambient temperature ranges, available solar resources, and daily load sizes all affect the proper sizing/design of the battery bank which leads to good system reliability and acceptable service life. Most battery manufacturers have data and tables for addressing many of these design factors that pertain to your off-grid system.
Selecting the appropriate battery for the application is a good starting point. There are two general classifications for lead-acid batteries: shallow cycle and deep cycle. Shallow cycle batteries are the typical automotive starting batteries and some marine batteries. Shallow cycle batteries are built to provide short bursts of current, like starting an engine. Their maximum daily discharge should be limited to about 15% Depth of Discharge (DOD) and their maximum allowable DOD is about 50%. They are not really designed for deep-cycle service. Many RV/Marine batteries are a slightly heavier-duty version of the starting battery. A true deep-cycle battery is built for a daily discharge of 50% before recharging. A deep-cycle battery can be discharged up to 80% without damaging the battery, but regularly discharging this deeply will shorten the cycle-life of the battery.
A battery bank needs to be properly sized so that its available capacity is adequate to reliably power the system loads, even during periods of poor solar availability. There are many factors that effect the available capacity of a battery bank, including battery temperature, depth of discharge (DOD) limits, and typical discharge rates.
The temperature of the battery affects its amp-hour capacity and the expected life of the battery. As the battery temperature increases above the ‘standard’ 25°C (77°F), its amp-hour capacity increases and its expected life decreases. For every 18°F above 77°F the expected battery life is decreased by about 50%. At colder temperatures the amp-hour capacity decreases and the expected life increases. At 32°F the battery capacity will be decreased by about 20-25%. At 0°F about 50% of the capacity remains.
Temperature also affects how deeply the battery can be discharged without a risk of freezing the battery. As a battery is discharged, the freezing point of the electrolyte decreases. A fully charged battery should not freeze until about -94°F (-70°C). A battery that is at about 50% DOD will freeze around -13°F (-25°C), and at a 75% DOD it will freeze at about 9°F (-13°C). The system should be designed so that it never discharges the battery bank below its expected coldest operating temperature. If the battery freezes, it will likely be permanently damaged.
The available amp-hour capacity of a battery also varies depending on how rapidly the battery is discharged. Battery specifications show the amp-hour rating at specific discharge rates; typically, a 20-hour rate and a 100-hour rate. The discharge rate indicates how many total amp-hours are available if the battery is discharged at a steady rate over the specified time period. The slower the battery’s discharge rate, the more total amp-hours are available.
Rate of discharge also affects how quickly the battery voltage changes. The heavier the discharge rate, the more quickly the battery voltage is depressed. Heavy power draws, such as large inverter loads, can quickly depress the battery voltage during short, high-power loads. This drop in battery voltage can trigger Low Voltage Disconnects (LVD) or Float cancellation. Appropriate battery capacity and adequate cable sizing can diminish these concerns.
Accounting for your site conditions, load requirements, and your off-grid system design will help you determine the maximum acceptable DOD for the battery bank. Adhering to this maximum DOD of the rated capacity of the battery bank will provide a good indication of the available Ah capacity from the batteries. If the battery bank is sized adequately and there is enough solar charging available, then good battery life and reliable system performance should be expected.