Just as we need backup power for our sump pump systems to ensure our basements do not flood during a power outage, data servers need backup power to ensure they do not lose data during any loss of power. From generators to battery packs for an entire data center to individual server backup batteries, there are many ways that servers handle backup. This article will focus on backup solutions that are within the server itself to ensure data is not lost during an outage.

 

Different options for backup power

The main goal of the backup solution is to ensure there is enough energy available to continue running the system until all of the data that is in volatile memory can be transferred into permanent memory.

There are several solutions that are available to provide backup power for servers and we will discuss three different ones and then look at the performance characteristics of each option as it relates to a data storage application.

  • Option 1 - Supercapacitors: Recent advancements in flash memory have allowed the use of supercapacitors to act as a backup power source. The write time to flash memory is much faster than hard drives, which allows low energy options with high current capability to act as a backup option. That is where supercapacitors come into play. Supercapacitors have very high capacitance that can create a huge amount of discharge current. Supercapacitors are being used in data servers where the uptime requirement is very small (seconds) but still needs a high amount of power.

  • Option 2 - Lead acid batteries: Pb-acid batteries have been the cornerstone technology for backup applications for many years. They offer a very cost-effective unit price over other technologies and offer good performance. Many uninterruptable power supplies (UPSs) are still built with lead acid batteries and are used to backup data farms, servers, and computer systems.

  • Option 3 - Lithium-ion batteries: Li-ion batteries are becoming more popular as a backup solution for many servers especially for applications that require a significant amount of energy and power to back up the system.

Each of these options have their own advantages and disadvantages. Next we will discuss which factors are most important for a data storage application.

  • Energy density: This is how much energy (watt-hours) you can get with respect to volume and/or weight. The higher the energy density, the smaller (size and weight) the battery can be to support a given load. Lithium-ion batteries have the highest energy density (5X of lead acid), while supercapacitors have the lowest. Supercapacitors are not meant for applications that require a lot of energy. Servers that need a long uptime on batteries (especially the ones that have hard drives vs. flash memory) should consider looking at Li-ion solutions.

  • Power density: This relates to how much power can be withdrawn with respect to volume and/or weight. Servers consume a lot 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), then supercapacitors could work well. Lead acid has the least amount of power density.

  • Cycle life: 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 is not as important in backup solutions as power outages are rare. The typical number of cycles that the batteries are designed to is 10 cycles a year. Supercapacitors have the biggest cycle life (over 100,000 cycles) whereas lead acid batteries have the lowest (less than 500 cycles).

  • Calendar life: This is the number of years the batteries will have recoverable capacity above 80%. This is a critical factor as this determines how often batteries need to be replaced over the life of a server. Oftentimes, the batteries in server rooms experience very high temperatures which ages the battery much faster than when it is at room temperature. Supercapacitors have the best calendar life and can last for more than 10 years at elevated temperatures. Lithium-ion solutions can last up to 10 years with a lower charge voltage. Lead acid batteries have the worst performance and may need to be replaced every 2 to 4 years.

  • Recharge time: This is the time required to charge from empty to full. This is a critical feature in a backup solution especially if there are back-to-back power outages. The batteries need to be fully charged as quickly as possible after a power event so that it is ready for another cycle. Most battery backup solutions are designed to be able to do back-to-back events due to the fact that it can take several hours to recharge them. Supercapacitors can be fully charged within seconds, while lead acid can take over six hours. Lithium-ion batteries can be charged in about three hours using standard charge rates.

  • Safety concerns: Lithium-ion solutions are subject to safety concerns, but they can be mitigated with redundant electronics protection, good thermal and mechanical designs, proper cell selection, and a robust manufacturing process. Supercapacitors and lead-acid solutions are very safe solutions that do not require additional circuitry to increase their safety factor.

  • Unit cost per Wh: When it comes to initial purchase price per watt-hour, lead acid batteries are the most cost-effective solution. Supercapacitors are the most expensive, but they are not meant to compete with high energy batteries.

  • Total cost of ownership per Wh: Although lead acid batteries have a much cheaper initial unit price over lithium-ion batteries, they actually tend to be more expensive when we look at the total cost of ownership over the life of the server. The total cost of lead acid batteries accumulates when we factor in the cost of replacements due to shorter calendar life, higher maintenance costs, larger footprint costs, and higher transportation costs due to size.

The need for backup power for data server applications will always be present. Determining which one is the right backup solution for you will depend on several factors discussed above. Lithium-ion batteries are becoming a more popular option for data servers since they offer high power and energy density, but they may not be a perfect choice. One must factor in the increased safety risks and cost. Supercapacitors or lead acid batteries could also be a potential option.