Friday, May 17, 2013

Recharging & Caring For a SLA (Sealed Lead Acid, Pb) Battery

The charging method for SLA batteries is the opposite concept from NiCad & NiMH batteries.  IT CAN'T HURT TO RECHARGE A PARTIALLY DISCHARGED SLA BATTERY. Lead Acid batteries by nature need to be constantly charged. If a lead acid battery becomes fully discharged, it is imperative that it be given a full charge immediately. This will prevent internal "sulfation" from occurring which is a condition that will permanently destroy your battery. Gel or sealed lead acid batteries are basically the same chemistry as a wet (flooded cell) battery. The batteries' electrolyte is in a gelatin form and is absorbed into the plates and the battery is sealed with epoxies. These batteries may be used in any position and the batteries are exceptionally leak resistant. Battery uses are for back up power supply, emergency lights, and alarm panels. These batteries are made up of 2 volts per cell. So the most common SLA batteries are 2, 4, 6, and 12 volt. 
Do Not store an SLA battery below freezing temperatures. Once a pack has been frozen, it will be permanently damaged and its service life will be drastically reduced. A previously frozen battery will only be able to deliver a limited number of cycles. 

Always keep lead acid batteries fully charged. If your battery serves as a backup energy source, be sure your equipment is always plugged in. So that a constant trickle charge is on. Store lead acid batteries in a cool, dry location. Never store your batteries on ground, metal or cement surfaces. If stored for more than 90 days, give your batteries a full charge.  Warmer temperatures adversely affect shelf life and the need for recharging.  Charge SLA batteries after each use, no matter how short the usage. Remember it is good to do a Top-Off charge for SLA batteries. Do not over stock your SLA batteries. It is better to buy smaller quantities that will be put into service within 90 days. 
Four major factors that affect battery life (both sealed and flooded cells): 
Ambient temperature Battery chemistry Cycling Maintenance and service Ambient Temperature The rated output capacity of a battery is based on an ambient temperature of 25C (77F). It is important to realize that any variation from this operating temperature can alter the performance of the battery, and shorten its expected life. High temperature reduces the battery life greatly. A rule of thumb when determining battery life in relation to temperature is: that for every 8.3 degreesC (15F) above the average annual temperature (that being 25 degreesC (77F), then the life of the battery will be reduced by 50 percent. 

Battery Capacity:
Battery capacity is determined by the battery's ability to convert chemical energy into electrical current at a specified rate for a specified amount of time. 

No battery will last forever-- even one that experiences minimal use. This is because sealed lead acid batteries are Electro-chemical devices whose ability to store and deliver power slowly decreases over time. Even if you follow all the guidelines for proper storage temperature and maintenance, you still must replace them after a certain period of time. 

Cycling During a utility power failure (severe brownout or blackout conditions), back up equipment operates on battery power. Once utility power is restored, the battery is recharged for future use. This entire "loop" is considered a discharge cycle. 

At installation time, the battery is at 100 percent of rated capacity. Each discharge and subsequent recharge reduces the relative capacity of the battery by a small percentage. The length of the discharge cycle will determine the reduction in battery capacity. 

The "loaf of bread" analogy is most often used to illustrate the relationship between cycling and battery life. A loaf of bread can either be cut into many thin slices or a few thicker slices. Similarly, a lead battery can provide power over a large number of short cycles, or fewer cycles of longer duration. 

Maintenance and Service The final factor to consider is the maintenance and service of the batteries and the back-up equipment. The gradual decrease in battery life can be monitored and evaluated through voltage checks, load testing and retorquing connections, as part of a periodic maintenance program. Without regular maintenance and service checks, your back-up battery may experience:  Heat-generating resistance at the terminals Improper loading Reduced protection Premature failure With proper maintenance and service, the end of battery life can be estimated and replacements scheduled without any interruption or loss of backup power. 

To charge a SLA battery, a DC voltage higher that the open circuit of 2.15 volts per cell is applied to the terminals of the battery. Any of the conventional charging techniques may be used, but to obtain maximum service life and capacity, along with acceptable recharge time, constant voltage-current limited charging is recommended. 
During constant voltage or taper charging, the battery's current acceptance decrease as voltage and state of charge increase. The battery is fully charged once the current stabilizes at a low level for a few hours. 

Cycle Applications: Limit initial current to 0.20C (C is the nominal A.H. capacity of the battery). Charge until battery voltage (under charge) reaches 2.45 per cell at 68 degrees F (20 deg.C). Hold at 2.45 volts per cell until current drops to approximately 0.01C ampere. Battery is fully charged under these conditions, and charge to "float" voltage. 
"Float" of Stand-By Service: Hold battery across constant voltage source of 2.25 to 2.30 volts per cell continuously. When held at this voltage, the battery will seek its own current level and maintain itself in a fully charged condition. 
Continuous over or undercharging is the single worst enemy of a lead acid battery. Caution should be exercised to insure that the charger is disconnected after cycle charging, or that the float voltage is set correctly. Because there is a chance of off-gassing hydrogen and oxygen if the battery is overcharged, it is important to provide adequate air circulation. ( Never charge or discharge a battery in a sealed enclosure.) Batteries should not be stored in a discharged state (or in a hot place). If a battery has been discharged for some time it may not readily take a charge. To overcome this, leave the charger connected and the battery for an extended time. The battery may eventually begin to accept charge. Due to the self-discharge characteristics of this type of battery, it is imperative that they be charged after 3-6 months of storage, otherwise permanent loss of capacity might occur as a result of sulfation. To prolong shelf life without charging, store batteries at 50 degrees F (10 deg. C) or less. 

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