Backup Power Options for Data Storage and Servers
By Ilyas Ayub, Director of Application Engineering, Inventus Power
On average, a typical American will experience one power outage per year that lasts about three hours. This duration translates into an uptime of 99.97%, which many systems can never achieve. Although our electrical grid is reliable, there is still a 0.03% chance that our power will be lost. That chance of losing power leads us to have backup batteries in our alarm clocks so we don’t oversleep and miss critical business meetings, backup batteries in our smoke alarms so our houses don’t burn down, and backup batteries in our sump-pump systems so our basements don’t flood. The need for backup batteries for data storage and servers is also critical.
As we increasingly rely on digital devices, more and more information is loaded onto servers all around the world. These servers have volatile memory (non-permanent) that they use to perform calculations and do transactions. Use of this type of memory is faster and cheaper than trying to access and store data from permanent memory (hard drives). When the processing is done, groups of data from volatile memory are transferred onto hard drives in batches.
Now, what happens when a server experiences a power outage without backup power? It will abruptly shut down and all the data that was in volatile memory will be lost, since it wasn’t in permanent memory. You may ask, “Don’t server farms have backup generators that are meant to maintain power?” Most do and can provide backup power when an outage occurs, but the generators take time (minutes) to detect an outage and turn on. By that time the data may already be lost.
No data left behind
Most backup solutions in data storage systems have one of two goals:
- Ensure that enough energy is available to continue running the system until backup generators are operational
- Ensure that enough energy is available for any data in volatile memory to be transferred into non-volatile memory, then gracefully shut down the system. In the server industry, the quick transfer of data from cache to permanent memory is sometimes called a fire-hose dump. Figure 1 details the flow
Several solutions provide backup power for servers, and we will discuss three of them before looking at the performance characteristics of each one as relates to data storage.
Backup power options for data storage
Recent advanced in flash memory have allowed supercapacitors to act as a backup power source. The write time to flash memory is much faster than to hard drives, allowing low-energy options with high current capability to act as a backup option. That is where supercapacitors come into play. These devices have very high capacitance that can create a huge amount of discharge current. Supercapacitors are serving in data servers where the uptime requirement is very small (seconds), but a high amount of power is required.
For many years, lead-acid batteries have been the cornerstone technology for backup applications. They have a very cost-effective unit price relative to other technologies and offer good performance. Many UPS (uninterruptable power supply) units contain lead-acid batteries; they appear in backup servers and computer systems.
Li-ion batteries are becoming more popular as a backup solution for many servers, especially those that require a lot of energy and power to back up the system.
Which one is right for my server?
Each of the above options has advantages and disadvantages. The list below examines various characteristics and details why they are important for data storage.
Energy density is how much (watts-hours) you can get per unit volume and/or weight. The higher the energy density, the smaller (in size and weight) the battery that supports a given load. Lithium-ion batteries have the highest energy density (five times that of lead-acid), whereas supercapacitors have the lowest. Supercapacitors aren’t meant for applications that require a lot of energy. Servers that need a long uptime on batteries (especially those that have hard drives instead of flash memory) should consider lithium-ion solutions.
Power density relates to how much power can be exacted per unit volume and/or weight. Servers consume lots of power; a 1U server can draw more than 1kW. Supercapacitors offer the highest power density at the expense of energy density. If the requirement is a huge power draw for a very short time (seconds), supercapacitors may work well. Lead-acid batteries have the lowest power density.
Cycle life is the number of complete charge and discharge cycles an energy system can perform before it reaches 80% of its rated capacity. This characteristic is less important in backup solutions given that power outages are rare. Batteries are typically designed for 10 cycles a year. Supercapacitors have the biggest cycle life (over 100,00 cycles). Lead-acid batteries have the lowest (fewer than 500 cycles).
Calendar life is the number of years that the batteries have a recoverable capacity greater than 80%. It’s a critical factor, as it determines how often batteries must be replaced over the life of the server. Often, the batteries in server rooms experience high temperatures, which age them more quickly than room temperature. Supercapacitors have the best calendar life and can last for more than 10 years at elevated temperatures. Lithium-ion batteries can last up to 10 years with a lower charge voltage. Lead-acid batteries have the worst performance and may need replacement every two to four years.
Recharge time is the time required to charge from empty to full. It’s a critical feature in a backup solution, especially if back-to-back power outages are possible. The batteries must be fully charged as quickly as possible after a power event so that they are ready for another cycle. Most solutions are designed to handle back-to-back events because recharging them can take several hours. Supercapacitors can be fully charged in seconds, whereas lead-acid batteries can take six hours or more. Lithium-ion batteries can be charged in about three hours using standard charge rates.
Environmental concerns are leading to stricter worldwide environmental regulations that force many data-storage companies to use greener alternative components for their equipment. Supercapacitors are the friendliest to the environment, as they contain no hazardous materials. Lead-acid batteries are the least friendly because they contain lead.
Safety is also important. Lithium-ion may be the least safe option, but any safety concerns can be mitigated using redundant electronics protection, good thermal and mechanical designs, good cell selection and robust manufacturing processes. Supercapacitors and lead-acid batteries are safe options that don’t need additional circuitry to increase their safety.
In unit cost per watt-hour (Wh), lead-acid is the most cost-effective solution. Supercapacitors are the most expensive, but they are now meant to compete with high-energy batteries.
In total cost of ownership (TCO) per Wh, lead-acid batteries tend to be more expensive, even though they are initially much cheaper per unit than lithium-ion batteries. If you factor in the replacement cost of lead-acid batteries owing to their relatively short calendar life, high maintenance cost, large footprint and high transportation cost owing to their size, they are not the overall lowest-cost technology. Table 1 summarizes the differences between the three backup options.
Until utility companies guarantee 100% power availability, the need for backup power solutions to prevent data loss will remain. The best type of backup solution will depend on servers’ load requirements. If short duration (seconds) is all that is required, supercapacitors are a great choice and are becoming popular owing to their high power density, short recharge time, ‘green’ properties, and long calendar and cycle life. When longer backup time is needed, lithium-ion batteries are becoming more popular than their lead-acid counterparts.
Inventus Power will be exhibiting in booth 2228