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 | | A small free-standing UPS | An uninterruptible power supply (UPS), also known as an uninterruptible power source or a battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available. There are two distinct types of UPS: off-line and line-interactive (also called on-line). Image File history File links Download high resolution version (921x1245, 240 KB) Summary UPS - Front View Licensing File links The following pages link to this file: Uninterruptible power supply ...
Image File history File links Download high resolution version (913x1621, 362 KB) Summary UPS - Rear View Licensing File links The following pages link to this file: Uninterruptible power supply ...
For delivered electrical power, see Electrical power industry. ...
An off-line UPS remains idle until a power failure occurs, and then switches from utility power to its own power source, almost instantaneously. An on-line UPS continuously powers the protected load from its reserves (usually lead-acid batteries), while simultaneously replenishing the reserves from the AC power.
The on-line type of UPS, in addition to providing protection against complete failure of the utility supply, provides protection against all common power problems, and for this reason it is also known as a power conditioner and a line conditioner. A power conditioner (also known as a line conditioner) is an electronic device, commonly taking the form of a powerstrip, that smooths out the peaks and dips of common household AC power. ...
While not limited to safeguarding any particular type of equipment, a UPS is typically used to protect computers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units come in sizes ranging from units which will back up a single computer without monitor (around 200 VA) to units which will power entire data centers or buildings (several megawatts). Larger UPS units typically work in conjunction with generators. This article is about the machine. ...
Copy of the original phone of Alexander Graham Bell at the Musée des Arts et Métiers in Paris Telecommunication is the assisted transmission of signals over a distance for the purpose of communication. ...
In the United States the term (Volt-amps) in Electrical terms, means watts. ...
Historically, UPSs were expensive and were most likely to be used on expensive computer systems and in areas where the power supply is interrupted frequently. As prices have fallen, UPS units have become an essential piece of equipment for data centers and business computers, and are also used for personal computers, entertainment systems and more.
In certain countries, where the electrical grid is under strain, providers struggle to ensure supply during peak demand (such as summer, when air-conditioning usage increases). To prevent unplanned blackouts, electrical utilities will sometimes use a process called rolling blackouts or load shedding, which involves cutting the power to large groups of customers for short periods of time. Several major blackouts occurred in 2003, most notably the 2003 North America blackout in the north-eastern US and eastern Canada and the 2003 Italy blackout, both of which affected over 50 million people, and brought attention to the need for UPS power backup units. Rolling blackout refers to an intentionally-engineered electrical power outage, caused by insufficient available resources to meet prevailing demand for electricity. ...
The 2003 North America blackout was a massive power outage that occurred throughout parts of the northeastern United States and eastern Canada on Thursday, August 14, 2003. ...
The 2003 Italy blackout was a serious power outage that affected all of Italy, except the island of Sardinia for 9 hours and part of Switzerland near Geneva for 3 hours on 28 September 2003. ...
A UPS should not be confused with a standby generator, which does not provide protection from a momentary power interruption and may result in an interruption when it is switched into service, whether manually or automatically. Such generators are typically placed upstream of the UPS to provide cover for lengthy outages. Integrated systems that have UPS and standby-generator components are often referred to as emergency power systems. Auxiliary power: Electric power that is provided by an alternate source and that serves as backup for the primary power source at the station main bus or prescribed sub-bus. ...
A backup generator for a large apartment building Emergency power systems are a type of system, which may include lighting, generators and other apparatus, to provide backup resources in a crisis or when regular systems fail. ...
Common power problems There are various common power problems that UPS units are used to correct. They are as follows (with a typical example of damage that might be caused): - Power failure — Total loss of utility power: Causes electrical equipment to stop working.
- Voltage sag — Transient (short term) under-voltage: Causes flickering of lights.
- Voltage spike — Transient (short term) over-voltage ie spike or peak: Causes wear or acute damage to electronic equipment.
- Under-voltage (brownout) — Low line voltage for an extended period of time: Causes overheating in motors.
- Over-voltage — Increased voltage for an extended period of time: Causes light bulbs to fail.
- Line noise — Distortions superimposed on the power waveform: Causes electro magnetic interference.
- Frequency variation — Deviation from the nominal frequency (50 or 60 Hz): Causes motors to increase or decrease speed and line-driven clocks and timing devices to gain or lose time.
- Switching transient — Instantaneous undervoltage (notch) in the range of nanoseconds: May cause erratic behavior in some equipment, memory loss, data error, data loss and component stress.
- Harmonic distortion — Multiples of power frequency superimposed on the power waveform: Causes excess heating in wiring and fuses.
UPS units are divided into categories based on which of the above problems they address. Some manufacturers categorize their supplies as a level 3, 5, or 9, if they address the first 3, 5, or 9 power problems respectively. A power outage is the loss of the electricity supply to an area. ...
A power outage is the loss of the electricity supply to an area. ...
Voltage spikes are fast, short duration surges in the electric potential in a given circuit. ...
Brownout may refer to: A dimming of the vision caused by loss of blood pressure or hypoxia. ...
When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. ...
In science, and especially in physics and telecommunication, noise is fluctuations in and the addition of external factors to the stream of target information (signal) being received at a detector. ...
The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental. ...
UPS designs The general categories of modern UPS systems are on-line or off-line, the latter often referred to as standby. An on-line UPS always powers the load from its own internal energy supply, which is continuously charged by the input power. In a standby ("off-line") system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails. Most UPS below 1 kVA are of the standby variety which are cheaper, though inferior to on-line systems which have no delay between a power failure and backup power being supplied.
A true 'uninterruptible' system is a double-conversion system. In a double-conversion system alternating current (AC) comes from the power grid, goes to the battery (direct current or DC), then is converted back to AC power. Most systems sold for the general market are of the "standby" type where the output power is drawn from the battery if the AC power fails or weakens.
For large power units, Dynamic Uninterruptible Power Supply are sometimes used. A synchronous motor/alternator is connected on the mains via a choke. Energy is stored in a flywheel. When the mains power fails, an Eddy-current regulation maintains the power on the load. DUPS are sometimes combined or integrated with a diesel-genset.
Fuel cell UPS have been developed in recent years using hydrogen and a fuel cell as a power source, potentially providing long run times in a small space. A fuel cell replaces the batteries used in other UPS designs. A fuel cell is an electrochemical device similar to a battery, but differing from the latter in that it is designed for continuous replenishment of the reactants consumed; i. ...
Rotary Rotary uninterruptible power supply equipment use a motor-generator system to create a perfect sine wave output. These units can be configured as (1) a motor driving a mechanically connected generator, (2) a combined synchronous/synchronous motor/generator wound in alternating slots of the field and stator, or (3) a Hybrid Rotary UPS utilizing a Rectifier and Inverter as found in traditional double conversion UPS with the addition of a motor being driven by the inverter and coupled to a generator.[1] In case #3 the motor generator can be synchronous/synchronous or induction/synchronous. The motor side of the unit in case #2 and #3 can be driven directly by an AC power source (typically when in inverter bypass), a 6-step double-conversion motor drive, or a 6 pulse inverter. Case #1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external, electrically coupled gensets to start and be brought online. Case #2 and #3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source. Sometimes, in case #1, a diesel engine can be run up to speed and then mechanically coupled to the generator, or the flywheel itself can be used to start the diesel engine (which is mechanically coupled as required to the flywheel and generator). Radio station motor-generator, converting from low to the high voltage power supply. ...
Rotary UPS equipment can provide up to 17x fault clearing capabilities (peak current to blow a fuse) without going to bypass. These units provide superior current inrush handling for inductive loads such as motor startup or compressor loads as well as medical MRI and cath lab equipment.
The life cycle of these units is usually far greater than that of their static siblings, up to 30 years or more.
Standby (offline) The Off-line Standby Power Supply (SBS) offers the bare bones power protection of basic surge protection and battery backup. Through this type of SBS a user's equipment is connected directly to incoming utility power with the same voltage transient clamping devices used in a common surge protected plug strip connected across the power line. When the incoming utility voltage falls below a predetermined level the SBS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The SBS then mechanically switches the connected equipment on to its DC-AC inverter output. The switch over time is stated by most manufacturers as being less than 4 milliseconds, but typically can be as long as 25 milliseconds depending on the amount of time it takes the SBS to detect the lost utility voltage.
Users selecting this type of an SBS must be aware that their computer equipment, as well as most electronic equipment is designed for use in the United States. As such it was designed to operate from a 120 volt, 60 Hertz (Hz), sinewave utility source. Most Off-line SBS products on the market today only provide a sinewave output when operating normally from the utility line. When they switch to their internal DC-AC inverter they may only provide a square wave, modified square wave or quasi-sinewave, not a pure sinewave. In many cases equipment may appear to operate normally on these waveforms, but over time may be damaged by them. When only minimal protection is needed, it is always best to select an SBS or UPS that states it has an inverter with a true sinewave output. Most off-line SBS units will not be capable of accepting additional battery packs for extended battery operation. To keep the cost down and prevent overheating, their inverters are designed to only operate as long as the internal battery capacity allows. Units of all three design types typically provide from 5 to 15 minutes of battery back-up time when loaded to their full output capacity. Slightly longer backup times can be achieved by overrating the SBS or UPS size.
Line-interactive The Line-interactive SBS offers the same bare bones surge protection and battery back-up as the offline, except it has the added feature of minimal voltage regulation while the SBS is operating from the utility source. This SBS design came about due to the off-line SBSs inability to provide an acceptable output voltage to the connected equipment during “brown-out” conditions. A “brown-out” happens when the utility voltage remains excessively low for a sustained period. Under these conditions the off-line SBS would go to battery operation and if the brown-out was sustained long enough, the SBS battery would become fully discharged, turn the power off to the connected equipment and not be able to be turned back on until the utility voltage returned to normal. To prevent this from happening a voltage regulating transformer was added, hence the term line-interactive was born. This feature really does help as low voltage utility conditions are common. The down side for this design, most of the units available have to switch to battery momentarily when making transformer voltage adjustments and this can be a bit annoying in a quiet home office on a bad power day.
Again when selecting a Line-interactive SBS it is always best to select a model with a true sinewave output. Several manufacturers have models available that will accept extended battery packs to provide additional battery runtime. This type of SBS typically costs more than the off-line type, but is worth the additional cost.
Dual conversion online The On-line UPS provides the highest level of power protection for the serious home office user. It does typically cost more, but like all electronic equipment today the cost is coming down as the technology advances. The true advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and sensitive electronic equipment. While the off-line and line-interactive designs leaves equipment connected directly to the utility power with minimal surge protection, the On-line UPS provides an electronic layer of insulation from power quality problems. This is accomplished inside the UPS in several tiers of circuits.
First the incoming AC utility voltage is passed through surge protected rectifier stage where it is converter to a Direct Current (DC) and is heavily filtered by large capacitors. This tier removes line noise, high voltage transients, harmonic distortion and all 50/60 Hertz frequency related problems. The capacitors also act as an energy storage reservoir giving the UPS the ability to “ride-through” momentary power interruptions. The battery is also connected to this tier and takes over as the energy source in the event of a utility loss. This makes the transition between utility and battery power seamless, without an interruption.
Larger UPS are expensive but are often better value. Fewer larger UPS tend to be more reliable than many smaller units (that don't contain bypass circuits). These units may be marketed as power conditioners. In data centers, multiple sets of UPS units may run in parallel providing dual sources of conditioned power to static switches that then send power to server loads. In such a system, a complete UPS failure can occur without the loads connected to the switches being affected. A power conditioner (also known as a line conditioner) is an electronic device, commonly taking the form of a powerstrip, that smooths out the peaks and dips of common household AC power. ...
Dual conversion UPS are more expensive and not quite as efficient as line interactive, standby or delta conversion units. Efficiencies reach 94%, but are not influenced by deviations at input and only marginally by non-sinusoidal load currents.
Ferro-resonant Ferro-resonant units operate in the same way as a standby UPS unit with the exception that a ferro-resonant transformer is used to filter the output. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time. Many ferro-resonant UPSs are 90-93% efficient and offer excellent isolation.
While this used to be the dominant type of UPS, they are no longer used for common applications. Power factor correcting equipment found in newer computer systems interacts with static ferro-resonant transformers, causing potentially damaging oscillations, and the transformer itself can create distortions which yield power less acceptable than poor quality line AC. These units are still used in some industrial settings, but have mostly disappeared from use with general computer equipment. Many ferro-resonant UPSs utilizing controlled ferro technology may not interact with power-factor-correcting equipment. The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power, and is a number between 0 and 1. ...
DC systems Many systems used in telecommunications use DC power (often 48 V). Rather than converting AC to DC to charge batteries, then DC to AC and then convert it back to DC again, some equipment accepts 48 V DC power directly. By simply converting AC power to DC power and adding batteries to the DC side, one or more conversion steps can be eliminated. There has been much experimentation with DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. Because there is more current to transfer the same amount of energy at the lower DC voltage, larger conductors are needed, and more energy is lost as heat. Eliminating a conversion step may seem more reliable, but the ability of online double conversion AC systems to entirely remove themselves from operation and transfer to bypass mode during certain UPS failures and maintenance allows for the connected servers to continue to function on unconditioned AC power while the UPS is repaired. DC-based power systems do not have this luxury, as it requires that all equipment has special DC power inputs that cannot utilize AC voltages in the event of a main DC rectifier or power distribution failure. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers. Higher voltage DC (380 volts) is finding use in some data center applications.[2]
Outdoor UPS When a UPS system is placed outdoors, it should have some specific features that guarantee that it can tolerate weather with a 'minimal to none' effect on performance. Factors such as temperature, humidity, rain, and snow among others should have been considered by the manufacturer when designing an outdoor UPS system. Operating temperature ranges for outdoor UPS systems could be around -40ºC to +55ºC. Humidity is the amount of water vapor in air. ...
Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). ...
An outdoor UPS system is normally made of several components designed for this particular task: Image File history File links No higher resolution available. ...
Image File history File links No higher resolution available. ...
- Outdoor enclosure: provides protection against the elements to all the components placed within. Quality outdoor enclosures are powder coat finished for corrosion resistance and long life. Outdoor enclosures are normally NEMA 3R compliant This article is about the standards association NEMA. For other uses, see Nema. ...
- Power Module: is the UPS itself. The boards of this power module should be conformal coated to avoid humidity damage. This UPS unit is normally based on Line Interactive or Double Conversion topology. Some manufacturers prefer Line Interactive because it provides a better Mean Time Between Failures (MTBF), and that is a critical part of an outdoor UPS system. Mean time between failures (MTBF) is the mean (average) time between failures of a system, the reciprocal of the failure rate in the special case when the failure rate is constant. ...
- Batteries: The batteries used in outdoor UPS systems must operate in a wide temperature range, usually from -40°C to +60°C. Batteries normally used in outdoor UPS systems are Gel Cell Batteries. The outdoor UPS's Power Module should provide a temperature compensated battery charging mechanism to optimize the life of the batteries.
A proper outdoor UPS system requires that all its components are designed for this environment. As seen from the features of the components above, an outdoor UPS system is not an indoor UPS inside an outdoor enclosure.
Outdoor UPS systems can be pole, ground (pedestal), or host mounted. Outdoor environment could mean extreme cold, in which case the outdoor UPS system should include a battery heater mat, or extreme heat, in which case the outdoor UPS system should include a fan system or an air conditioning system.
Typical applications Outdoor UPS systems are ideal for protection of WiFi/GSM/CDMA/satellite base stations, wireless communications/perimeter surveillance and security/gate control systems, LED traffic light/roadway display systems and remote terminal units (RTUs). Official Wi-Fi logo Wi-Fi (pronounced wye-fye, IPA: ) is a wireless technology brand owned by the Wi-Fi Alliance intended to improve the interoperability of wireless local area network products based on the IEEE 802. ...
Global System for Mobile communications (GSM: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. ...
Code division multiple access (CDMA) is the current name for the cellular technology originally known as IS-95. ...
“LED†redirects here. ...
A RTU, or Remote Terminal Unit is a device which interfaces objects in the physical world to a DCS or SCADA system by transmitting telemetry data to the system and/or altering the state of connected objects based on control messages received from the system. ...
Internal UPS Internal UPS are a group of uninterruptible power supplies (UPS) designed to be placed inside computer chassis. There are two types of Internal UPS. First type is miniaturized regular UPS that are made small enough to fit into a 5.25” CD-ROM slot bay of a regular computer chassis. The other type is re-engineered switching power supplies that utilize dual power sources of AC and/or DC as power inputs and have an AC-DC built-in switching management control units.
The first type often requires extra connection wires between the internal UPS and computer's power supply. Some internal UPS of this group output high voltage (110 V - 220 V) direct current (DC) and some output nine-step table wave AC. Neither design is safe or energy efficient. As of 2006, there are only couple companies still selling this type of internal UPS in Australia, Asia and some part of Europe The top cover has been removed to show the internals of a computer Power supply Unit. ...
The second group of internal UPS replaces the regular switching power supplies. There are three main design mechanisms:
- Optic-coupling that imitates AC during AC outages. This mechanism was first introduced by American Advanced Power of USA and Magnum Power of UK in 1997, as well as Apollo Power of Taiwan in 1998. This design provides a low-cost solution but its efficiency is low and it has a very low overall wattage limit (<300 W).
- An analog-circuitry-controlled AC-DC switching mechanism. This design also provides a low-cost solution. However, because of the bulky component circuit board, little space is available for increasing wattage output. Plus, the final products are very sensitive to factors such as local heat and causing frequent operational errors. Nevertheless, because of its low cost, it is still popular in China. Most Asian internal UPS manufacturers belong to this category.
- A CPU controlled AC-DC switching mechanism. This design was first introduced by American Advanced Power Inc. of USA and Amsdell of Canada. It provides error-free switching control and a complicated communication protocol between the power supply and computer.
Using a UPS
Choosing a UPS The first question to ask when choosing a UPS system should be: is this unit going to be placed inside a controlled environment? If the answer to this question is yes, choose an indoor UPS. If the answer is no, choose an outdoor UPS. If an outdoor UPS is placed in a controlled environment, it is probably a waste of money (exceptions to this include powering small loads during an extended period of time, where outdoor UPS systems are sometimes the only available option). If an indoor UPS is placed in a non-controlled environment, the useful life of this system will be considerably shortened, threatening the integrity and backup of the equipment the UPS is protecting.
Besides choosing a UPS design, there are 2 key ratings to be aware of when choosing a UPS unit. The first is the load rating, expressed as both volt amps (VA) and watts (W). Both the ratings represent the maximum amount of load that the UPS can support and the connected load typically should not exceed 80% of either. Special considerations must be made when connecting certain equipment such as printers or any type of motorized load. In the United States the term (Volt-amps) in Electrical terms, means watts. ...
For other uses, see Watt (disambiguation). ...
The second factor in deciding which unit to purchase is the amount of runtime the unit will be able to provide when the power fails. This number will vary with the load amount that is plugged into the UPS. For example, a unit may run a single computer for 30 minutes, but with 2 computers it will generally last less than half that time. Larger units typically can provide more runtime for the same load than smaller units, however that is not always the case. Some UPS units are designed to provide extended runtime or have the ability to have external battery packs connected.
Another consideration is the anticipated usage. If the UPS is only intended to provide enough power to gracefully shut down the computers, serial or USB ports on the UPS and support software are essential. If the purpose of the UPS is to provide power until a standby generator kicks in (typically under a minute), the UPS input capabilities should be matched to the generator outputs. Specifically, most standby generators made for home use (15 kW or less) and most portable generators lack microprocessor voltage-and-frequency control and may not create a smooth sine wave. This can result in voltage and frequency fluctuating by 5% or more. While most UPS systems handle voltage fluctuations gracefully, most do not handle frequency fluctuations well. A UPS with a wide "frequency window" is essential in such cases. However, this can double the cost of the unit. Only a double conversion UPS can deliver a stable output frequency when powered by an unstable input frequency. This article is about machines that produce electricity. ...
If the UPS needs to be quiet when running from battery, or will power AC motors (as found in air conditioners and fans) or other equipment requiring a clean sine wave (such as high-end computer power supplies), a UPS that outputs a smooth sine wave is needed. For some other uses, a block or quasi-sine wave waveform[3] is acceptable. UPS systems with square wave, or "simulated", "approximated" or "stepped" sine wave output do not give smooth sine waves. In fact, their output voltage contains a lot of harmonic distortion. This is why this type of output exists only in small power ratings standby UPS, where electrical noise and excess heat in wiring is generally not an issue. The total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental. ...
Another consideration should be based on the type of load or connected equipment the UPS will support. If the UPS is connected to "mission critical" equipment or sensitive electronics (like lasers), a rotary solution will be more suitable with 100% line to load isolation. This would not only protect the equipment from a power outage, but will also protect the connected equipment from any anomaly that comes from the utility feed. For alternative meanings see laser (disambiguation). ...
Features to look for:
- Output frequency regulation within 0.5% (prevents connected equipment from over heating)
- Electromagnetic interference (EMI) AC noise suppression (noise filtering).
- Reasonable cost for replacement batteries.
- If energy efficiency is important avoid "Standby On-Line Hybrid", "Standby-Ferro", and "Double Conversion On-Line" UPS systems.
- If the UPS outputs a sine wave, a high quality unit will feature a voltage regulating transformer.
- If the UPS outputs a square wave, a high quality unit will use Pulse-width modulation (PWM)
Electromagnetic interference (or EMI, also called radio frequency interference or RFI) is a (usually undesirable) disturbance caused in a radio receiver or other electrical circuit by electromagnetic radiation emitted from an external source. ...
An example of PWM: the supply voltage (blue) modulated as a series of pulses results in a sine-like flux density waveform (red) in a magnetic circuit of electromagnetic actuator. ...
Replacing batteries In order to provide the desired protection, UPS units must be properly maintained. Sealed lead/acid batteries have a useful lifetime of 3–5 years. In determining when to replace batteries, it is important to remember that the batteries can be completely bad after 3–5 years and lose their ability to hold a charge gradually over that time.[4] If a UPS started with 1 hour of runtime for the connected load, after 1 year, it may only provide 45 minutes of backup time. Battery failure can also be caused by temperature. If the application requires the battery to operate properly at temperatures exceeding 25 °C, using gel batteries will allow the UPS to work in temperatures between -40 to +70 °C. A gel battery is a lead-acid battery with gelified electrolyte. ...
Disposing of UPS batteries -
Many UPS units contain sealed lead-acid batteries and electronics which can be detrimental to the environment. In the United States, it is illegal to dispose of lead-acid batteries in a landfill, and they must be properly recycled. Sealed lead-acid batteries are recycled in the same manner as car batteries, so any auto shop that accepts used car batteries for recycling will also accept sealed lead acid batteries. This article or section does not cite any references or sources. ...
A valve-regulated, sometimes called sealed, lead acid battery Lead-acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. ...
The international recycling symbol. ...
See also Wikimedia Commons has media related to: Image File history File links Commons-logo. ...
Emergency power systems are a type of device, which may include lighting, generators and other apparatus, to provide backup resources in a crisis or when regular systems fail. ...
A power conditioner (also known as a line conditioner) is an electronic device, commonly taking the form of a powerstrip, that smooths out the peaks and dips of common household AC power. ...
External links
Notes - ^ Hybrid Rotary UPS white paper
- ^ http://hightech.lbl.gov/documents/DATA_CENTERS/DCDemoFinalReportJan17-07.pdf
- ^ http://www.kerchner.com/electrical/sinewave.htm
- ^ Battery Care white paper
References - Part of this article was originally taken from a public-domain entry in Federal Standard 1037C
- Part of this article was copied with permission from Uninterruptible Solutions
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