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Extend UPS battery life, avoid catastrophic data center failure

Are you managing your UPS battery life expectations? Just because the warranty says it will last doesn't mean you should take their word for it.

UPS batteries are like an unreliable friend -- you think you can trust them, but then they let you down when you least expect it and sometimes when you need them most.

Your data center's backup power system could have several faults: a bad circuit breaker, poor power management or phase balance putting you on the verge of an overload, or even a design or installation problem. But even when everything else is done right, uninterruptible power supply (UPS) batteries are a weak link.

Batteries can seem just fine until put under sudden and unexpected stress. UPS batteries deliver power ride-through when the utility fails -- a sudden, heavy load. That's stressful for the batteries, and may cause them to fail with devastating consequences for the data center.

UPS battery options

The choice in UPS batteries hasn't changed much in decades: either flooded lead acid wet cells or valve regulated lead acid (VRLA) sealed cells.

Wet cells are still the large data center standard thanks to long-term reliability. They are essentially large, expensive jars of sulfuric acid that require regular monitoring and maintenance, as well as safety precautions such as a sealed, reinforced acid-ready room separated from the UPS.

VRLAs are the norm in most modern UPS systems for data centers. These batteries use a paste electrolyte in a sealed container, as well as a different charging system that makes them much safer and circumvents special facilities to house them. VRLAs take longer to recharge than wet cell batteries, and the other major tradeoff is the UPS battery life. While flooded lead acid batteries can be expected to last up to 25 years with proper maintenance, VRLA batteries often need replacement after only three to five.

Extend UPS battery life

Data center managers must take steps to maximize UPS battery life and prevent catastrophic failures.

Every UPS should be equipped with at least two battery strings (see Figure 1). If one string fails, the second should still maintain power -- although this will last for less than half the duration of two strings. UPS battery life is a non-linear function. Two strings in parallel will deliver more than twice the duration of either string alone. At least you won't be in for a sudden crash.

Data center UPS battery design.
All UPS systems should use two strings of batteries to prevent catastrophic failure when one battery stops working.

VRLA batteries tend to fail open: one failure takes out the whole string, without any indication of which battery cell is to blame (see Figure 2). Wet cells fail closed or shorted, which reduces the total voltage of the battery string but allows the UPS to continue delivering power.

VRLA battery failure.
VRLA batteries fail open, taking out the entire string of batteries with one failure.

There's never a guarantee of how something will fail, so use dual battery strings regardless of the type of battery technology used.

Plan a battery replacement budget. Under the best conditions, batteries eventually fail. When one cell fails, it is generally best to replace the whole string. A new cell will have different electrical characteristics than the older ones, which means more failures when mixed in a string. The exception to this rule is infantile failure: when a cell fails within the first six months of operation.

Also rely on battery monitoring. Many UPS systems include battery monitors, but third-party monitors may be more exact. Some UPS battery monitors measure internal cell resistance, while others impose a small voltage to test operation. One vendor claims that its battery monitor rebalances strings so you can mix new cells with old ones. Manufacturer statements notwithstanding, no monitor will accurately predict every battery failure before it happens.

The electrolyte in VRLA batteries dries out over time, but battery failures are significantly accelerated by overuse, caused by multiple discharges and recharges in locations with unstable utility power; overheating from operating in the wrong environment; and overcharging, which also causes overheating. All of these shorten UPS battery life and any can cause sealed cells to rupture and leak. Overcharging can also cause hydrogen gas emission.

The warranties on batteries are structured so that the value goes down rapidly in the early years. For example, if you have batteries advertised as "10 year," and they fail in only three, your warranty value could easily be far less than half.

Batteries operate best, and last longest, at temperatures below 77 degrees Fahrenheit (25 degrees Celsius). Many UPS systems now operate in the same room as servers rather than a separate battery room. Since ASHRAE expanded the recommended temperature envelope for data center IT equipment, inlet air can now exceed 77 degrees Fahrenheit, and hot exhaust air can be 100 degrees Fahrenheit (38 degrees Celsius) or more. Take this into account when locating a UPS in the white hall space. Tests have shown that raising the battery temperature 15 degrees Fahrenheit (8.3 degrees Celsius) can reduce the UPS battery life by 50%.

Batteries get hotter when they are charging or discharging, so restore cooling as quickly as possible after a power failure. In operations without backup generators, expect deep battery discharge, combined with increased temperature, to significantly reduce battery life.

There is really no reason to overcharge UPS batteries. Good chargers include temperature monitoring and compensation to reduce the charging current if the temperature rises.

Even with proper care, actual VRLA UPS battery life should not be expected to last to the theoretical max, particularly if conditions are not ideal. And a good battery monitor of any type is far better than no monitoring at all.

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