UPSs for military scenarios weather heat, provide long lifetimesStory
April 26, 2010
Uninterruptible Power System (UPS) topologies and battery chemistries are impacting battery runtime and lifetime in modern military applications.
There’s still truth to the classic remark attributed to Napoleon Bonaparte: “An army marches on its stomach.” Today’s military runs on electric power (and also still on its stomach). But while food can be brought in from outside sources, AC power must be obtained either from local utilities or from closeby generators. And compared to power supplied by utilities throughout most of the United States and many other countries, the local or generator power provided in many military locations is subject to many problems. Among these issues are 1) brief or prolonged “brownouts” – sags in voltage levels, which can also be caused by excessive demand by other devices; or 2) outages – periods of seconds or minutes with no power at all.
If not prevented or mitigated, these AC power sags and outages can interfere with application or device operations. Other possible ramifications include scrambled data and damaged hardware, which in turn increase the potential for mission failure, personnel injury, or even death. To the rescue: Uninterruptible Power Systems (UPSs), which can be used to mitigate these power sags and outages1. Meanwhile, a thorough understanding of UPS topology choices and battery chemistries is imperative for proper selection of power alternatives that can offer the longest lifetime possible in today’s heat-intensive military environments.
UPSs versus “the old way”
UPS performance requires that the battery retain its ability to hold a charge (capacity), deliver this power as needed, and recharge. One key indicator of battery life and health is the number of discharge cycles the battery is rated for. A typical number is 260 cycles; thus, if batteries are being used in “standby service” – meaning the batteries are rarely cycled or discharged – they should last the full rated life.
Several factors can degrade battery performance and lifetime, particularly: how frequently and deeply a battery is discharged or “cycled,” and also temperature. (These circumstances are also likely in many industrial and field-service scenarios.)
Given that these UPSs are often in remote field locations, re-provisioning is often expensive and inconvenient – and might be difficult on short notice. Reprovisioning is also an added expense – on top of the cost of more-frequent fresh batteries and returning, recycling, or disposing of the old ones. Of even greater concern is that an overstressed UPS battery that has not been replaced in time might fail at a critical time, with potentially serious consequences.
However, the good news is that various methods exist to help mitigate excessive battery drain and environmental heat, including:
- Selecting an appropriate UPS topology
- Selecting a more heat-tolerant battery chemistry
UPS topology impacts battery lifetime
UPSs come in three topologies: Off-Line, Line-Interactive, and On-Line, each of which has its own unique effect on battery lifetime.
The Off-Line UPS, during utility mode operation using the externally provided power, essentially feeds the utility power directly to its output. Since undervoltage can be as bad for operations as zero power, an Off-Line UPS is designed to transfer to battery mode not only when utility power fails, but also when it drops below a specified level (typically, for a 120 VAC line, 102-105 VAC). This type of drop might occur during a brownout or a sag, for example.
In a location with a sustained low-utility-voltage condition, an Off-Line UPS can continuously transfer into and out of battery mode, resulting in battery overcycling and significantly shortening battery life. Additionally, in some cases the batteries might never have a chance to recharge, rendering the UPS useless.
The Line-Interactive UPS incorporates an internal tap-switching transformer circuit, often referred to as an Automatic Voltage Regulation (AVR) circuit. This circuit monitors the utility voltage and, when appropriate, automatically switches transformer taps to increase or lower the UPS output voltage in gross steps, to sustain the target voltage while avoiding going to battery power as much as possible. Depending on the brand and model, the output voltage is maintained between ±12 percent (105.6-134.4 VAC) down to ±5 percent (114-126 VAC) without having to switch to the battery.
While this is an improvement over the off-line design, some line-interactive designs must transfer to battery mode during the tap transfers; thus, in locations with an unstable utility voltage, the batteries are again subjected to overcycling.
The On-Line UPS is also known as “dual conversion” UPS. On-Line UPS converts the utility power to a filtered DC current, which is then electronically regulated and supplies a continuous duty, sine-wave inverter circuit. This inverter, in turn, regenerates new, tightly regulated AC power to its output – typically ±2 percent voltage regulation – hence the “dual conversion.”
The On-Line UPS can avoid drawing on the battery, yielding an ever-wider utility voltage range than the Line-Interactive UPS: typically -25 percent to +15 percent or 90-138 VAC. Thus, using an On-Line UPS greatly reduces the number of battery cycles versus the other UPS topologies in locations where the utility power level fluctuates frequently.
Disparate battery chemistries tolerate discharge, heat differently
Heat, whether due to buildup in a small or enclosed space or because of ambient weather, also significantly impacts battery performance. Insulated enclosures and other design approaches can shield UPS batteries from some heat buildup. Another approach is to use a battery with better heat tolerance.
For well over two decades Valve Regulated Sealed Lead-Acid (VRLA) batteries (which are related to, but not the same as, those found in automobiles, RVs, and boats) have typically been used to provide the backup power for most UPS products. These batteries have the high energy storage capacity needed for UPS applications and are available at a low cost.
The typical usable lifetime of a VRLA battery under normal operating conditions – such as being operated in a 25 °C (77 °F) temperature environment – is three to five years; thereafter, VRLA battery capacity and/or ability to hold a charge might decline below accepted thresholds.
Unfortunately, typical VRLA batteries do not tolerate high temperatures. As the battery gets warmer, its lifetime and storage capacity degrade, as does the maximum current it can output. So instead of getting 15 minutes of backup time during a power outage, the user might only get a few minutes. Also, in a sustained 50 °C (122 °F) environment, the same battery will have a service life of less than one year (see Figure 1).
Figure 1: Typical measurements for 3- to 5-year-rated VRLA batteries
Within the past few years, improved VRLA batteries with longer life are becoming available from companies including CSB Battery Co., Ltd. (www.csb-battery.com). These new VRLA batteries have up to a remarkable 10-12 year rating at room temperature, and somewhat better heat tolerance levels have become available – with pricing competitive to the traditional five-year VRLA batteries. Additionally, these new VRLA batteries have somewhat better operational life at higher temperatures – up to four years at 50° C. This makes these new VRLA products better matches for UPSs in heat- and power-challenging environments.
In 2009, Falcon Electric introduced the first UPS with long-life, high-temperature batteries. The SSG Series UPS is rated to 55 °C, and the internal batteries are rated to operate for four years at 50 °C. These new batteries, housed inside an enclosure that is inserted into the UPS, are shielded from ambient heat and reduce TCO by significantly extending the traditional battery replacement cycle.
However, like traditional VRLA batteries, these new VRLA batteries’ life will be shortened if operated above 25 °C (77 °F), or if the proper charge level is not maintained2 (Figure 2).
Figure 2: Typical measurements for 10- to 12-year-rated VRLA batteries
UPSs solve modern military power issues
When looking to purchase a COTS UPS for a specific application, a high-temperature On-Line UPS design is a good place to start. Through its robust design, it represents the best reliability model even if used in a normal ambient temperature application. And the On-Line topology maximizes battery performance in the face of highly unreliable utility power levels and availability. There’s no downside to this choice: Particularly where the success of military missions and safety of personnel are at stake, it’s the right choice.
Ron Seredian, Falcon Electric Inc.’s Vice President of Marketing, has more than two decades of experience in the power conversion industry, both in the U.S. and in Europe. He has written many articles on the topic of power protection and holds a Bachelor of Science degree in Marketing from California State University, Northridge.
Falcon Electric, Inc. 800-842-6940 www.falconups.com
1 Generators or other on-site technologies are needed to address longer utility outages or locations with no utility power available. And UPSs are still advised in concert with a generator.
2 The short battery life at elevated temperatures can be mitigated to some degree by implementing good thermal insulation practices during battery-pack design.