While the majority of power infrastructure operates on AC (alternating current) power, certain applications and industries are better suited for DC (direct current), especially those that require either a long duration discharge or low amperage output over an extended period of time. This is due to the fact that with DC power, the current consistently flows in one direction. Conversely, with AC power, the electric current periodically reverses direction. DC power is widely used in applications such as telecom, automotive, aircraft and other low-voltage, low-current applications.
In a DC power system, the uninterruptible power system (UPS) takes in primary power — usually utility AC — and outputs DC voltage while providing backup power from the integrated batteries in the event of an extended power outage. Although DC units may vary depending on the type of application they are designed for, most systems consist of five main components:
1. Rectifier / Charger — An electrical device that converts alternating current, the rectifier has two main purposes. Its primary job is to provide DC power to the supported loads, with a secondary role of charging and maintaining the batteries to ensure the system will perform in the event of a power failure. When selecting a rectifier, consider whether the system that is being backed up requires redundancy, efficiency and/or scalability.
2. Controller – The brains of a DC power system, the controller provides logic to the system, commanding the various components and providing insight into the UPS’s status and functionality. Some DC systems may have a separate controller. To ensure compatibility, the controller must be able to communicate with the network; for instance, tie into a building automation system or connect via SNMP or another communication protocol.
3. Batteries — Depending on the application, a DC system may use VRLA, lithium-ion, NICAD or wet cell batteries, with almost all batteries running in a series due to the amount of power needed. Selecting a battery type will depend on a number of factors, including whether a long duration discharge is needed, the environment of the facility (such as high heat or humidity), life cycle cost and any footprint limitations at the site.
4. Distribution system — Many DC applications have specific requirements around distribution, with one of the most common considerations being future load requirements. The distribution is usually integrated into the rectifier enclosure, but it can also be external.
5. Enclosure — There are a variety of enclosure designs available for DC systems, including rack-mount and shelf-mount. In some installations, batteries are included within the enclosure, while other times they are deployed in a separate rack.
When choosing the optimal DC power system for your environment, it is important to take into account all of these considerations. Since DC UPSs can differ depending on the application they are intended for, it is critical to understand the critical system components to ensure your backup solution will perform as expected.