Closer Look at the Capacity of SLA Batteries and Solar Energy Systems|
Article - February 15, 2016 By LarsonElectronics.com
A Closer Look at the Capacity of SLA Batteries and Solar Energy Systems
Sealed lead acid (SLA) batteries have a reputation for being durable when used to support portable electronic devices. They require minimal maintenance and offer leak-proof capabilities. But when used with solar energy systems, the units present issues that can hinder the potential and performance of the machines.
For example, general purpose SLA batteries should not be deep cycled, as continuously discharging below 50-70 percent of its rated capacity could damage the units. In application, this would mean that large solar energy systems that rely on SLAs would need a handful of batteries to avoid unnecessary strain. This is not sustainable for supporting remote operations because such components take up space and transporting a series of batteries is no easy task, especially to areas with damaged or unreliable infrastructure.
What does this mean for individuals who want to utilize off-grid solar PV systems? Keep reading to learn about how deep cycling affects general purpose SLA batteries and cutting-edge solutions some solar manufacturers are applying to remedy its limitations.
Pitfalls of SLA Batteries
Most general purpose SLA batteries in the market today offer roughly 200-300 cycles. In order to reach the higher end of this estimated range, you must carefully maintain the units. This includes adhering to best practices during usage, charging and storing. The first thing you should know is that almost all batteries self-discharge. For SLAs, the rate this occurs depends on several factors with focus on storage and operating temperatures. When exposed to temperatures above 80 degrees Fahrenheit, the cells discharge at a rate of around four percent every seven days. Under ideal conditions (65-70 degrees Fahrenheit), SLAs discharge at a rate of three percent per month (or between 36 to 40 percent per year). It is advisable to monitor such activities by checking on the unit every 12 weeks.
Fully discharging an SLA battery (or below the recommended threshold of 50 percent), will create problems inside the cell. When discharged deeply, the polarity of weak components inside the unit is affected, which contributes to permanent damage. Furthermore, low cell voltage may cause lead sulfate created at discharge to linger on the plates. As a result, the compound crystalizes and causes a loss in capacity. It is important to consider that constantly using a unit that is partially charged, as well as failing to store SLAs in a charged state may also cause unwanted formation of the compound. Lead sulfate build up is greatly minimized when equalizing charges are consistently performed every 10 discharge cycles or every four weeks.
Overcharging SLA Batteries
Overcharging SLAs can be just as damaging as draining them deeply. When this happens, the unit can swell in size due to heat, resulting in abnormal bloat. After completing a full charge, excessive current makes its way into the battery. This occurrence forces water to decompose in the electrolyte. As the unit increases in temperature, it will continue to bring in more current. Hence, to stop this vicious cycle, it is vital to remove the battery from the connected device and assess the damage. In addition to causing bloat, overcharging may promote corrosion on the plate of the unit.
For deep cycling requirements, it might be worth trying deep cycle SLA or gel SLA batteries. Both options perform better in demanding systems that often discharge deeply. Such units are designed to be discharged as much as 80 percent and typically have thicker plates. Although it is possible to discharge deep cycle batteries at 20 percent charge, it is still recommended to only discharge the units at 50 percent. This practice will help maintain the lifespan and performance of the battery. For solar PV systems, industrial deep cycle batteries (also known as “traction” batteries, commonly found in forklifts and golf carts) may also be used.