Line interactive systems send part of the available mains power through the inverter -- enough to keep the battery charged. The battery can then support the main supply during a sag, or take over completely during a blackout. Sometimes manufacturers can be a bit loose with the terminology and label these as online, so take care. The load cannot be completely isolated from variations to the mains power without switching to the battery. Again a filter is required.
With online models (also called double-conversion), mains power constantly runs through the inverter, which supplies the battery. All power entering the UPS is rectified and reconstructed as a purified waveform (without need of further filtering) before reaching the load (Figure 1c).
Since all power is running through the inverter and rectifier at all times these components need to be very robust, thus increasing the price of this type of device. Online UPSs provide the most complete protection to networked computer systems.
Communications, Management
Power failures and fluctuations can occur at any time and you may not be there to shutdown your equipment should this be necessary. Fortunately UPSs normally come with hardware and software that automates shutdown prior to the battery being fully drained. Three basic interface methods are available: Serial port, USB, and Web based (SNMP).
Remember that your UPS need to support not only the servers, but also the associated administration workstations and other essential devices (switches, routers, external drives) that need to maintain power in the event of a mains failure. Particularly in the case on an SNMP device, switch protection is vital. What's the point of your server trying to communicate with the NMS if the network itself is compromised?
Batteries AND Runtime
Typically a UPS uses sealed lead acid (SLA) batteries. These are basically like a car battery that is sealed to prevent leakage. They don't need to be topped up, but, like any battery, they can wear out in time and thus require replacement. In the event that the batteries need to be changed it is normally possible to do a hot-swap; that is, the batteries can be changed over with out powering down the system first (and thus interrupting the hardware you are trying to prevent from being interrupted).
Advanced battery management (ABM) ensures that battery life is as long as possible. UPS batteries are charged to about 90 percent (sometimes less) rapidly and then allowed to charge fully at with a "trickle charge". The battery is then allowed to rest a while before being subjected to any further charging which might be required due to usage.
Typically during a power outage, batteries will last for five minutes when the UPS is fully loaded, increasing to around fifteen minutes for a half-loaded unit. It may well be that this is totally inadequate for your purposes. What happens if servers are in the middle of a lengthy process (such as a CAD/CAM rendering), which cannot be interrupted, or perhaps an urgent job that must be completed now or sooner? Fear not because external battery packs can normally be added to these heavy duty UPSs. As an example, both Powerware and Liebert claim that their machines can run for two hours with a 75 percent load when backed up with four external batteries.
