Thanks to their use of sensitive electronics, which are based in large part on digital controls, manufacturing plants, telecommunications facilities, transportation hubs, and financial networks can be brought to a grinding halt by a simple voltage variation on the power distribution system that lasts less than a second. Production lines in new manufacturing plants operate through digital switches. Transportation companies rely on digital devices for scheduling and ticketing processes. Telecommunications facilities rely on digital equipment to provide customers with the best service. And financial institutions use binary code to carry out numerous transactions. Even a sag in voltage, which lasts just a few cycles, can shut down an entire plant and/or operation.
This is where the need for stored energy in uninterruptible standby systems comes into play. And the classical stored energy system, dating back to the '60s, is a string of individual batteries. But don't think that battery technology has stood still during this time. In recent years, manufacturers have developed new battery types and new methods have evolved to provide stored energy for standby functions, offering a wider selection of choices for coping with the problem.
Getting started. The need for stored energy in a standby system can be predicated upon at least two parameters: time duration of delivery (term) and power (energy). It's possible to further define the requirements as short-, medium-, and long-term.
Short-term — This duration typically involves standby systems without available transfer means to engine-generator sets, alternate utility feeders, or other sources. These systems can be as large as 1,000kW and offer up to five minutes of stored energy capability, based on the time required to start engine-generator sets and make the transfer.
Medium-term — This duration typically involves stand-alone standby systems without available transfer means to engine-generator sets or alternate utility feeders. These systems range from 100W to 500kW and offer five minutes to 30 minutes of stored energy capability, based on the estimated time of a utility outage.
Long-term — This duration involves standby systems that operate as part of a utility system and provide, in addition to standby function, other functions such as peak shaving, voltage and frequency stabilization, and reactive-power supply. These systems can be rated as high as 20MW and are capable of delivering energy for up to eight hours.
Types of batteries. The specific energy and energy density of the batteries used for standby service is shown in the Figure above. The batteries employed for standby service can be described as follows:
Flooded, lead acid — Lead acid batteries are the most popular choice in the United States, although nickel cadmium (Ni-Cad) is appropriate for some specialty applications. The lead acid battery has been used in UPS systems and equipment since the '60s and as a backup power supply for communications equipment prior to 1983, primarily because of its low initial cost and good performance characteristics.
One drawback of these batteries is you must periodically add water to them to make sure they meet specific gravity measurements. They can also discharge inflammable gas, requiring you to maintain special facilities for safety reasons. To allow venting, the gas space in flooded cells is open to outside air but separated from it through a vent that incorporates a flash arresting device.
Lead acid batteries also have a maximum storage time of six months at temperatures between 65°F and 85°F. At the end of this time, you must either install the battery or give it a freshening charge.
Note in the Figure on page 28 that the flooded lead acid battery has the lowest specific energy and energy density compared to other battery types.
Valve-regulated lead-acid (VRLA) — This type of battery has seen tremendous growth in standby usage over the last two decades. Note its approximately two-to-one advantage over the flooded type battery in specific energy and energy density as depicted in the Figure on page 28. In the VRLA cell, the vent for the gas space incorporates a pressure relief valve to minimize the gas loss and to prevent direct contact between the headspace and outside air.
Some people consider VRLA batteries as maintenance free, but that's not necessarily true. Table 1 shows a brief comparison of the considerations for flooded and VRLA batteries. The installation issues are similar for both battery types.
Standard VRLA battery warranties range from five to 20 years depending upon construction and manufacturer-based requirements for proper charging and maintenance, as well as keeping the battery in a 77°F environment. When placed in an outdoor environment, the batteries must be heated to prevent freezing. At 20°F battery capacity is reduced.
Lithium-ion (Li-ion) — Telecom systems need small power sources to back-up the supply to mobile-phone relay stations, cable TV terminals, and other facilities. Because these power systems need to be small and lightweight, they need to use batteries with an energy density higher than that of VRLA batteries.
The quest for a lighter battery that uses metallic lithium as its anode was driven by the fact that lithium is the lightest and the most electropositive of metals. The specific energy of lithium metal (1727Ah/lb) compares very favorably with that of materials traditionally used in stationary batteries, such as lead (118Ah/lb) and cadmium (218Ah/lb). The Figure also indicates the comparative positioning of the Li-ion battery. It's about one-fourth the weight and one-half the size of traditional lead-acid batteries. Table 2 above shows a comparison of Li-ion and VRLA batteries, rated 36V, 160Ah.
According to information provided by Isidor Buchmann, founder and CEO of Cadex Electronics, from the Web site BatteryUniversity.com, aging of Li-ion is an issue that's often ignored. Depending on the state-of-charge and storage temperature, lithium-based batteries have a typical lifetime of two to three years (longer if partially charged and kept cool). However, the clock starts ticking as soon as the battery comes off the manufacturing line. The capacity loss manifests itself in increased internal resistance caused by oxidation. Eventually, the cell resistance will reach a point where the pack can no longer deliver the stored energy, although the battery may still contain ample charge.
Buchmann also says that some Li-ion batteries fail due to excessively low discharge. If discharged below 2.5V per cell, the internal safety circuit opens, the battery appears dead, and a charge with the original charger is no longer possible. However, some battery analyzers feature a boost function that reactivates the protection circuit of a failed battery and enables a recharge. Keep in mind, though, that if the cell voltage has fallen below 1.5V/cell and has remained in that state for a few days, a recharge should be avoided because of safety concerns.
A low-cost Li-ion battery installation for cable TV and telecom power back-up features battery modules constructed from small Li-ion cells using a proprietary technology, with a rating of 80Ah and nominal voltages of 11V and 14V. The modules can be combined in series and parallel to produce 24V, 36V, 48V, and 96V battery packs, with capacities from 25Ah to 300Ah for back up.
Nickel-metal hydride (NiMH) — The explosion of telecom services throughout this country has made it necessary to replace metallic cables with optical fiber cables. This change is supported by an increased use of batteries to back up the power sources on these networks. Optical fiber networks require a power source at each terminal because electric power can't be supplied over the optical fiber path, as is done with metallic cables. Optical network terminals not only convert optical signals, but supply electric power as well. Therefore, batteries are required to back up the utility power feed, and they must be ready to supply power for several hours.
The NiMH battery is assembled from six cells, is rated 2000/mAh, weighs 0.55 lb, operates between 50°F to 131°F, and has a projected life of seven and a half years at 86°F. This battery relies on a pulse charging method to prevent overcharging.
You must cycle new NiMH batteries three to five times before they reach peak performance. You can do this by simply using the batteries and recharging them or using a battery conditioner.
At room temperature, NiMH batteries will self-discharge in 30 to 60 days without usage, depending on environmental condition. In other words, if you leave the batteries on the shelf for more than 30 to 60 days, you should recharge them before use. In fact, it's quite normal for these batteries to be fully depleted of power after long-term storage.
The number of times you can recharge NiMH batteries will depend on certain operating parameters, including drain rate and battery care. In general, you can expect NiMH batteries to last up to 500 recharges. In absolute best conditions, however, NiMH batteries can last up to 1,000 recharges.
Generally speaking, NiMH batteries don't suffer from “memory effect,” so they don't require conditioning. But to ensure top performance, you should condition Ni-MH batteries once for every ten charges.
Reliability of electric power supply for all types of industrial, commercial, and institutional customers that use computer and electronic loads leads to the use of batteries and inverters to transition from normal to back-up power supplies. Selecting the right battery for the specific application is important. Using the same battery in two different applications may give you excellent performance in one instance and a complete failure in another.
Kusko is vice president of Exponent, Inc. (formerly Exponent Failure Analysis Associates), Menlo Park, Calif.
Sidebar: Battery Functions Differ Depending on Application
It's common knowledge that batteries consist of one or more cells electrically interconnected to achieve a required voltage and stored energy level. What isn't common knowledge are the two types of battery operation: cycling and float (Figure).
Cycling operation — This describes the function of batteries in standby systems, such as in a UPS, where the battery charge is drawn down to supply the inverter and the AC load when the utility power fails. In UPS systems rated 100kVA and higher, these batteries are typically rated 460VDC. The batteries are recharged when utility power returns or engine generators are started.
Float operation — This, on the other hand, describes the function of batteries in telephone central offices, for example, where batteries maintain a relatively constant voltage of 48VDC.
Many other applications for batteries require operations in either the float or cycling mode.