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 good rule of thumb when determining battery life in relation to temperature is that for every 8.3C (15F) average annual temperature above 25C (77F), the life of the battery is reduced by 50 percent.

UPS Battery Capacity

UPS 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.

UPS Battery Chemistry

No UPS battery will last forever-- even one that experiences minimal use. This is because UPS batteries are electro-chemical devices whose ability to store and deliver power slowly decreases over time. So, 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), the UPS 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 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 UPS 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 UPS. 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 UPS 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.

What is the "end of useful life"?

The IEEE defines "end of useful life" for a UPS battery as being the point when it can no longer supply 80 percent of its rated capacity in ampere-hours. (Because the relationship between amp-hours and load protection time is not linear, a 20% reduction in capacity results in a much greater reduction in protection time. For example, a UPS battery that supports a full load for 15 minutes when new, will support the same load for only about 8 minutes when it reaches its defined "end of life"). When your battery reaches 80 percent of its rated capacity, the aging process accelerates and the battery should be replaced.

Which factors contribute most to end-of-life for UPS batteries?

In North America, Western Europe and similar countries, positive grid corrosion has been the most common end-of-life factor for UPS batteries. This is a result of the normal aging process due to UPS battery chemistry (regardless of battery cycling) and involves the gradual breakdown of the inner wires of the positive grid within the battery.

In other areas, cycling is often the major contributing factor, due to very unpredictable utility power.

How do I make sure that my UPS batteries are maintained and serviced properly?

With proper maintenance, battery life can be predicted and replacements scheduled without interrupting your operations. These are IEEE and OEM recommendations for general maintenance:

• Comprehensive maintenance programs with a monthly inspection, and more rigorous quarterly and annual checks.
• Retorquing all connections, as required
• Load testing
• Cleaning the battery area, as required

Yes, this information applies to all UPS batteries. It is also important to understand the fact that battery warranty does not necessarily equal battery life and the estimated range for battery life is five to eight years.

Are Maintenance Free batteries maintenance free?

Though sealed batteries are sometimes called "maintenance free," they still require scheduled maintenance and service. The term "maintenance free" refers to the fact that they do not require fluid. Preventive maintenance is the key to maximizing your UPS battery service life.

What about the disposal of batteries?

It is imperative that your service technicians adhere to EPA guidelines for the disposal of all UPS batteries. Remember, it is the owner's responsibility (and liability) to make sure these guidelines are followed.

Never toss the UPS batteries in the trash!
Recycle them.

To charge an SLA battery, a DC voltage higher than 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 hermetically sealed enclosure.)

Batteries should not be stored in a discharged state or in a hot location. If a battery has been discharged for some time it may not readily take a charge.

Due to the self-discharge characteristics of this type of battery, it is imperative that they be charged after 3-4 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.