The Lithium-Ion UPS
In the context of a data center, it is hard to be hyperbolic when describing an event as a “disaster.” An event as commonplace as a spring thunderstorm can be enough to wreak havoc: flooding, fallen trees, lightning strikes, and power outages are all possible. One ever-increasing threat to the data center environment is grid stability. In the last 30 years, grid-level power outages in the United States have increased nearly 300%, with over 3,600 blackouts costing American businesses $150 billion dollars in 2014. Aging infrastructure, increased power demand, and more erratic weather systems will cause those numbers to continue to rise. Hardening your data center against power loss is critical to maintaining up-time.
BRIDGING THE GAP
When auditing the disaster recovery plan of a facility, the ability to survive a loss of power is crucial. Most facilities with critical equipment will be equipped with an emergency generator sized to run critical equipment for the desired time, usually at least an hour. However, gensets — particularly those large enough to back up an entire facility — can take 60, even 90 seconds to come online. The power supplies on the front ends of a typical server are only designed to have 12 milliseconds of ride-through time. Uninterruptible power supplies (UPS), which are often provisioned solely for back-up power, are increasingly being deployed as ride-through devices. Both line interactive and double conversion UPS equipment can provide AC via the battery well under the 12 milliseconds of ride-through on the power supply of a server. When sized correctly, a UPS can be the perfect device to cover both short-term power dips and blips, as well as longer outages that require a genset.
TRUSTING YOUR POWER LOSS PLAN
Given the ever-increasing number of outages, it is important to not only deploy the equipment required to survive an outage, but to ensure that it is working properly. Disaster preparedness advisors typically recommend testing power back-up equipment on a monthly basis and running at expected full load on a quarterly basis. For a UPS, this means running a battery test. A battery test can be scheduled through the standard interface of most UPSs. The test entails discharging the battery and comparing actual capacity to expected or nameplate capacity.
One caveat to the recommendation to regularly test a UPS battery is that the very test designed to provide state of health will actually degrade battery state of health in lead-acid systems. UPSs designed with new lithium-ion battery technology have up to 300% the cycle life of lead-acid batteries at deep discharge. In addition, lithium-ion batteries have integrated battery monitoring, meaning battery voltage and capacity can be measured and reported in real-time without requiring a full discharge-charge cycle. This translates to the UPS battery remaining at or near full charge all of the time, thus reducing risk associated with an outage overlapping with a back-up system test.
A BETTER BATTERY FOR THE JOB
Lithium-ion batteries offer other advantages in the event of an outage. Lithium-ion has three times the energy density (by both weight and size) of lead acid and nearly identical cycle life whether utilizing the UPS battery at 30% depth-of-discharge or 80% depth-of-discharge. These two characteristics combine in very meaningful ways for a data center manager looking to mitigate power outage risk.
First, a lithium-ion battery for a given power and duration requirement can be much smaller than a lead acid battery deployed in the same application. Not only does the lithium-ion battery pack more energy into a smaller package, its tolerance for high depth-of-discharge means it has more usable capacity.
A smaller UPS battery can be re-charged quickly and thus be primed and ready for repeat outages or power pulses. A more compact battery can also be deployed with or immediately adjacent to critical equipment. Rather than relying on large power runs from a centralized battery room, back-up power can be located directly in the rack. This lessens the risk associated with power transmission and reduces sheer square footage of a facility that must be monitored and neutralized.
By extension of cycle-life independence from depth of discharge, lithium-ion batteries also demonstrate a low degree of capacity fade. This translates directly into a longer life for the UPS battery.
LITHIUM-ION IN PRACTICE
California serves as a prime location for many data centers and other critical infrastructure. Pacific Gas and Electric, the primary utility for the state of California, shows 83 power outages on its live service interruption website as this article is being written. Eaton’s annual Blackout Tracker reports 537 major outages experienced in 2014 — equivalent to an outage every 16 hours — with an average duration of 49 minutes. It is not a matter of if a data center will lose power; it is a matter of when.
In order for a UPS to be a reliable, long-term asset in the battle against blackouts, it must have a high cycle life and low capacity fade. Consider an application in which 5 kW of equipment must be backed up for 90 seconds in outage-prone California. A comparison of this hypothetical 125 watt-hour battery can be made using commonly available battery data. Assuming 80% depth-of-discharge, a typical absorbent glass mat (AGM) lead acid battery would only last 0.9 years while a lithium-ion pack would last 3.7 years, or 300% longer. To prolong the life of the lead-acid battery, a less aggressive depth of discharge can be used, but will require the battery to be over-sized by 400% from a capacity standpoint. When translated into the physical realm, the results are more stark, as seen in Table 1.
Of course, when sizing a UPS, margin for both size and run-time should be used to ensure proper functionality, but the underlying principle that lithium-ion batteries offer reduced risk and increased efficiency in the data center still holds.
When preparing or even updating a data center disaster recovery plan, careful consideration of back-up power should be given. Having a UPS to provide power during short outages or while a genset is brought online will maintain uptime, even during non-ideal circumstances. Lithium-ion UPS batteries provide a longer lasting, more reliable solution that is compact enough to be deployed right where the equipment requiring back-up-is located, greatly reducing the overall risk from power outage at a facility.
Emilie Stone is the general manager of Methode Active Energy Solutions located in Boulder, CO. She brings nearly a decade of experience in automotive design and manufacturing to data center equipment, helping engineer reliability and robustness.