UPS (uninterruptible power supply)

UPS

An uninterruptible power supply, alsouninterruptible power source, UPS orbattery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typicallymains power, fails. A UPS differs from an auxiliary or emergency power system orstandby generator in that it will provide near-instantaneous protection from input power interruptions, by supplying energy stored in batteries or a flywheel. The on-battery runtime of most uninterruptible power sources is relatively short (only a few minutes) but sufficient to start a standby power source or properly shut down the protected equipment.

A UPS is typically used to protectcomputers, data centers,telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units range in size from units designed to protect a single computer without a video monitor (around 200 VA rating) to large units powering entire data centers or buildings. The world's largest UPS, the 46-megawatt, Battery Electric Storage System (BESS), in Fairbanks, AK, powers the entire city and nearby rural communities during outages.^[1]

Contents   [hide]  1 Common power problems 2 Technologies 2.1 Offline / standby 2.2 Line-interactive 2.3 Online / double-conversion 3 Other designs 3.1 Hybrid topology / double conversion on demand 3.2 Ferro-resonant 3.3 DC power 3.4 Rotary 4 Applications 4.1 N+1 4.2 Multiple redundancy 4.3 Outdoor use 4.4 Internal systems 5 Machine standards 5.1 Measuring efficiency 5.2 Warranty 6 Difficulties faced with generator use 6.1 Frequency variations 6.2 Power factor 7 Communication 8 Batteries 8.1 Common battery characteristics and load testing 8.2 Testing of strings of batteries/cells 8.3 Series-parallel battery interactions 8.4 Series new/old battery interactions 9 See also 10 Notes 11 References 12 External links

Common power problems

The primary role of any UPS is to provide short-term power when the input power source fails. However, most UPS units are also capable in varying degrees of correcting common utility power problems:

1. Voltage spike or sustained Overvoltage
2. Momentary or sustained reduction in input voltage.
3. Noise, defined as a high frequency transient or oscillation, usually injected into the line by nearby equipment.
4. Instability of the mains frequency.
5. Harmonic distortion: defined as a departure from the ideal sinusoidalwaveform expected on the line.

UPS units are divided into categories based on which of the above problems they address^{[dubious – discuss]}, and some manufacturers categorize their products in accordance with the number of power-related problems they address.^[2]

Technologies

The general categories of modern UPS systems are on-line, line-interactive orstandby.^[3] An on-line UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to 120 V/230 V AC for powering the protected equipment. A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost. 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 line-interactive or standby variety which are usually less expensive.

For large power units, dynamic uninterruptible power supplies 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 as long as the flywheel's energy is not exhausted. DUPS are sometimes combined or integrated with a diesel generator that is turned on after a brief delay, forming a diesel rotary uninterruptible power supply (DRUPS).

A fuel cell UPS has been developed in recent years using hydrogen and a fuel cell as a power source, potentially providing long run times in a small space.

Offline / standby

Offline / standby UPS. Typical protection time: 0–20 minutes. Capacity expansion: Usually not available

The offline / standby UPS (SPS) offers only the most basic features, providing surge protection and battery backup. The protected equipment is normally connected directly to incoming utility power. When the incoming voltage falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The SPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switchover time can be as long as 25 milliseconds depending on the amount of time it takes the standby UPS to detect the lost utility voltage. The UPS will be designed to power certain equipment, such as a personal computer, without any objectionable dip or brownout to that device.

Line-interactive

Line-interactive UPS. Typical protection time: 5–30 minutes. Capacity expansion: Several hours

The line-interactive UPS is similar in operation to a standby UPS, but with the addition of a multi-tap variable-voltage autotransformer. This is a special type oftransformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer. This is also known as a Buck–boost transformer.

This type of UPS is able to tolerate continuous undervoltage brownouts and overvoltage surges without consuming the limited reserve battery power. It instead compensates by automatically selecting different power taps on the autotransformer. Depending on the design, changing the autotransformer tap can cause a very brief output power disruption,^[4] which may cause UPSs equipped with a power-loss alarm to "chirp" for a moment.

This has become popular even in the cheapest UPSs because it takes advantage of components already included. The main 50/60 Hz transformer used to convert between line voltage and battery voltage needs to provide two slightly different turns ratios: One to convert the battery output voltage (typically a multiple of 12 V) to line voltage, and a second one to convert the line voltage to a slightly higher battery charging voltage (such as a multiple of 14 V). The difference between the two voltages is because charging a battery requires a delta voltage (up to 13-14 volts for charging a 12 volt battery). Furthermore, it is easier to do the switching on the line-voltage side of the transformer because of the lower currents on that side.

To gain the buck/boost feature, all that is required is two separate switches so that the AC input can be connected to one of the two primary taps, while the load is connected to the other, thus using the main transformer's primary windings as an autotransformer. The battery can still be charged while "bucking" an overvoltage, but while "boosting" an undervoltage, the transformer output is too low to charge the batteries.

Autotransformers can be engineered to cover a wide range of varying input voltages, but this requires more taps and increases complexity, and expense of the UPS. It is common for the autotransformer to cover a range only from about 90 V to 140 V for 120 V power, and then switch to battery if the voltage goes much higher or lower than that range.

In low-voltage conditions the UPS will use more current than normal so it may need a higher current circuit than a normal device. For example to power a 1000-watt device at 120 volts, the UPS will draw 8.33 amperes. If a brownout occurs and the voltage drops to 100 volts, the UPS will draw 10 amperes to compensate. This also works in reverse, so that in an overvoltage condition, the UPS will need less current.

Online / double-conversion

The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. Although once previously reserved for very large installations of 10 kW or more, advances in technology have now permitted it to be available as a common consumer device, supplying 500 watts or less. The initial cost of the online UPS may be slightly higher, but its total cost of ownership is generally lower due to longer battery life. The online UPS may be necessary when the power environment is "noisy", when utility power sags, outages and other anomalies are frequent, when protection of sensitive IT equipment loads is required, or when operation from an extended-run backup generator is necessary.

The basic technology of the online UPS is the same as in a standby or line-interactive UPS. However it typically costs much more, due to it having a much greater current AC-to-DC battery-charger/rectifier, and with the rectifier and inverter designed to run continuously with improved cooling systems. It is called a double-conversion UPS due to the rectifier directly driving the inverter, even when powered from normal AC current.

In an online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.

The main advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and sensitive electronic equipment.

Other designs

Hybrid topology / double conversion on demand

These hybrid designs do not have an official designation, although one name used by HP and Eaton is double conversion on demand.^[5] This style of UPS is targeted towards high efficiency applications while still maintaining the features and protection level offered by double conversion.

A hybrid (double conversion on demand) UPS operates as an off-line/standby UPS when power conditions are within a certain preset window. This allows the UPS to achieve very high efficiency ratings. When the power conditions fluctuate outside of the predefined windows, the UPS switches to online/double conversion operation.^[5]In double conversion mode the UPS can adjust for voltage variations without having to use battery power, can filter out line noise and control frequency. Examples of this hybrid/double conversion on demand UPS design are the HP R8000, HP R12000, HP RP12000/3 and the Eaton BladeUPS.

Ferro-resonant

Ferro-resonant units operate in the same way as a standby UPS unit; however, they are online 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 82–88% efficient (AC/DC-AC) and offer excellent isolation.

The transformer has three windings, one for ordinary mains power, the second for rectified battery power, and the third for output AC power to the load.

This once was the dominant type of UPS and is limited to around the 150 kVArange. These units are still mainly used in some industrial settings (oil and gas, petrochemical, chemical, utility, and heavy industry markets) due to the robust nature of the UPS. Many ferro-resonant UPSs utilizing controlled ferro technology may not interact with power-factor-correcting equipment.^{[further explanation needed]}

DC power

A UPS designed for powering DC equipment is very similar to an online UPS, except that it does not need an output inverter. Also, if the UPS's battery voltage is matched with the voltage the device needs, the device's power supply won't be needed either. Since one or more power conversion steps are eliminated, this increases efficiency and run time.

Many systems used in telecommunications use 48 VDC power, because it has less restrictive safety regulations, such as being installed in conduit and junction boxes. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers.

There has been much experimentation with 48 VDC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. However, to supply the same amount of power, the current must be greater than an equivalent 115 V or 230 V circuit; greater current requires larger conductors or more energy lost as heat.

Most PCs can be powered with 325 VDC. This is because most ATX switching power supplies convert the AC input voltage to approximately 325 VDC (230 × √2). On units with a voltage selector switch, the 115 V setting enables a voltage doubler that puts the top half of the AC wave in one capacitor, and the bottom half in the other capacitor. This mode uses half of the bridge rectifier and runs twice as much current through it. The 230 V setting simply rectifies the AC using the full bridge rectifier, and puts the it into both capacitors. These two capacitors are hardwired in series.^[6] These power supplies can almost always be safely run on 280-340 VDC long as the selector is in the 230 V position. They will not work at all with DC power in the 115 V position; with 162 VDC applied, nothing will happen because only one capacitor is being charged; if 325 V is applied, you will blow the fuse and a surge suppressor or capacitor. Power supplies with Active-PFC are usually Auto-ranging and have no voltage selector switch. They usually have one input capacitor, it is charged to 320-400 VDC by a boost-mode power supply that is part of the PFC circuit. It is uncertain how various Auto-ranging and Active-PFC power supplies will respond to having DC power applied when they are expecting AC 50-60 Hz power.^{[citation needed]} A laptop computer is a classic example of a PC with a DC UPS built in.

High voltage DC (380 V) is finding use in some data center applications, and allows for small power conductors, but is subject to the more complex electrical code rules for safe containment of high voltages.^[7]

Rotary

A rotary UPS uses the inertia of a high-mass spinning flywheel (flywheel energy storage) to provide short-term ride-through in the event of power loss. The flywheel also acts as a buffer against power spikes and sags, since such short-term power events are not able to appreciably affect the rotational speed of the high-mass flywheel. It is also one of the oldest designs, predating vacuum tubes and integrated circuits.

It can be considered to be on line since it spins continuously under normal conditions. However, unlike a battery-based UPS, flywheel-based UPS systems typically provide 10 to 20 seconds of protection before the flywheel has slowed and power output stops. It is traditionally used in conjunction with standby diesel generators, providing backup power only for the brief period of time the engine needs to start running and stabilize its output.

The rotary UPS is generally reserved for applications needing more than 10,000 watts of protection, to justify the expense and benefit from the advantages rotary UPS systems bring. A larger flywheel or multiple flywheels operating in parallel will increase the reserve running time or capacity.

Because the flywheels are a mechanical power source, it is not necessary to use an electric motor or generator as an intermediary between it and a diesel engine designed to provide emergency power. By using a transmission gearbox, the rotational inertia of the flywheel can be used to directly start up a diesel engine, and once running, the diesel engine can be used to directly spin the flywheel. Multiple flywheels can likewise be connected in parallel through mechanical countershafts, without the need for separate motors and generators for each flywheel.

They are normally designed to provide very high current output compared to a purely electronic UPS, and are better able to provide inrush current for inductive loads such as motor startup or compressor loads, as well as medical MRI and cath labequipment. It is also able to tolerate short-circuit conditions up to 17 times larger than an electronic UPS, permitting one device to blow a fuse and fail while other devices still continue to be powered from the rotary UPS.

Its life cycle is usually far greater than a purely electronic UPS, up to 30 years or more. But they do require periodic downtime for mechanical maintenance, such asball bearing replacement. In larger systems redundancy of the system ensures the availability of processes during this maintenance. Battery-based designs do not require downtime if the batteries can be hot-swapped, which is usually the case for larger units. Newer rotary units use technologies such as magnetic bearings and air-evacuated enclosures to increase standby efficiency and reduce maintenance to very low levels.

Typically, the high-mass flywheel is used in conjunction with a motor-generatorsystem. These units can be configured as:^[8]

1. A motor driving a mechanically connected generator,
2. A combined synchronous motor and generator wound in alternating slots of a single rotor and stator,
3. A hybrid rotary UPS, designed similar to an online UPS, except that it uses the flywheel in place of batteries. The rectifier drives a motor to spin the flywheel, while a generator uses the flywheel to power the inverter.

In case No. 3 the motor generator can be synchronous/synchronous or induction/synchronous. The motor side of the unit in case Nos. 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 No. 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 Nos. 2 and 3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source.

Applications

N+1

In large business environments where reliability is of great importance, a single huge UPS can also be a single point of failure that can disrupt many other systems. To provide greater reliability, multiple smaller UPS modules and batteries can be integrated together to provide redundant power protection equivalent to one very large UPS. "N+1" means that if the load can be supplied by N modules, the installation will contain N+1 modules. In this way, failure of one module will not impact system operation.^[9]

Multiple redundancy

Many computer servers offer the option of redundant power supplies, so that in the event of one power supply failing, one or more other power supplies are able to power the load. This is a critical point – each power supply must be able to power the entire server by itself.

Redundancy is further enhanced by plugging each power supply into a different circuit (i.e. to a different circuit breaker).

Redundant protection can be extended further yet by connecting each power supply to its own UPS. This provides double protection from both a power supply failure and a UPS failure, so that continued operation is assured. This configuration is also referred to as 1+1 or 2N redundancy. If the budget does not allow for two identical UPS units then it is common practice to plug one power supply into mains powerand the other into the UPS.^[10]

Outdoor use

When a UPS system is placed outdoors, it should have some specific features that guarantee that it can tolerate weather with no effect on performance. Factors such as temperature, humidity, rain, and snow among others should be considered by the manufacturer when designing an outdoor UPS system. Operating temperatureranges for outdoor UPS systems could be around −40 °C to +55 °C.

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.

Internal systems

UPS systems can be designed to be placed inside a computer chassis. There are two types of internal UPS. The first type is a miniaturized regular UPS that is made small enough to fit into a 5.25-inch CD-ROM slot bay of a regular computer chassis. The other type are re-engineered switching power supplies that utilize dual AC or DC power sources as inputs and have built-in switching control units.

Machine standards

Measuring efficiency

The way efficiency is measured varies massively in the UPS market, and there are a number of reasons for this. Many UPS manufacturers claim to have the highest level of efficiency, often using different sets of criteria in order to reach these figures. The industry norm can be argued to be anything between 93%-96% when a UPS is in full operational mode, and to reach these figures companies often put their UPS in an ideal scenario. Efficiency figures on site are often much closer to the 90% mark, due to varying power conditions. The perfect scenario will never happen in reality, due to ongoing voltage sags from the mains and the declining efficiency of UPS batteries.^{[citation needed]}

Warranty

This section includes a list of references, related reading or external links, but the sources of this section remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations. (May 2012)

Warranty on uninterruptible power supplies has varied over the past couple of years, often depending if a machine is single phase or three phase. Few companies compete on warranty, with the focus mainly on efficiency and maintenance contracts. The standard manufacturers warranty is anything between 1–2 years and can even be limited to certain aspects of the machine, often excluding the more expensive items such as battery replacement. Focusing on one market, companies supplying three phase however now offer lengthier warranties, with the norm closer to 2 years rather than the single year.

Difficulties faced with generator use

Frequency variations

The voltage and frequency of the power produced by a generator depends on the rotational speed. The speed is regulated by a device called a governor. Some governors are mechanical, and some are electronic. The job of the governor is to keep the voltage and frequency constant, while the load on the generator changes. However, the governor does not respond instantaneously to load changes. This may pose a problem when, for example, the startup surge of an elevator causes the frequency of the generator to drift temporarily as the governor is adjusting to the new load, thus affecting all other devices powered by the generator. Many radio transmission sites have backup diesel generators—in the case of amplitude modulation (AM) radio transmitters, the load presented by the transmitter changes continuously and instantaneously with the signal level. This leads to the scenario where the governor is constantly trying to correct the output voltage and frequency as the load changes.

It is possible for a UPS unit to be incompatible with a generator or a poor mains supply; in the event that its designers had written the microprocessor code to require exactly a 50.0 Hz or 60.0 Hz supply frequency in order to operate, the UPS could remain on battery power, being unwilling to reconnect to the fluctuating input.

The problem of input frequency should not be an issue on a Double Conversion / online UPS. A UPS of this topology should be able to adapt to any input frequency, using its own internal clock source to generate the required 50 or 60 Hz supply frequency.

Power factor

Main article: Power factor

A problem in the combination of a "double conversion" UPS and a generator is the voltage distortion created by the UPS. The input of a double conversion UPS is essentially a big rectifier. The current drawn by the UPS is non-sinusoidal. This can cause the voltage from the AC mains or a generator to also become non-sinusoidal. The voltage distortion then can cause problems in all electrical equipment connected to that power source, including the UPS itself. It will also cause more power to be lost in the wiring supplying power to the UPS due to the spikes in current flow. This level of "noise" is measured as a percentage of "Total Harmonic Distortion of the current" (THD(i)). Classic UPS rectifiers have a THD(i) level of around 25–30%. To reduce voltage distortion, this requires heavier mains wiring or generators more than twice as large as the UPS.

There are several solutions to reduce the THD(i) in a double conversion UPS:

Passive power factor correction:

Main article: Passive PFC

Classic solutions such as passive filters reduce THD(i) to 5–10% at full load. They are reliable, but big and only work at full load, and present their own problems when used in tandem with generators.

Active power factor correction:

Main article: Active PFC

An alternative solution is an active filter. Through the use of such a device, THD(i) can drop to 5% over the full power range. The newest technology in double conversion UPS units is a rectifier that doesn't use classic rectifier components (thyristors and diodes) but high frequency components. A double conversion UPS with an IGBT rectifier and inductor can have a THD(i) as small as 2%. This completely eliminates the need to oversize the generator (and transformers), without additional filters, investment cost, losses, or space.

Communication

This section requires expansion.(December 2009)

Power management (PM) requires

1. The UPS to report its status to the computer it powers via a communications link such as a serial port, Ethernet,GSM/GPRS or USB
2. A subsystem in the OS that processes the reports and generates notifications, generates PM events or commands an ordered shut down.^[11]Some UPS manufacturers publish their communication protocols, but other manufacturers (such as APC) use proprietary protocols.

The basic computer-to-UPS control methods are intended for one-to-one signalling from a single source to a single target. For example, a single UPS may connect to a single computer to provide status information about the UPS, and allow the computer to control the UPS. Similarly, the Universal Serial Bus protocol is also intended to connect a single computer to multiple peripheral devices.

In some situations it is useful for a single large UPS to be able to communicate with several protected devices. For traditional serial or USB control, a signal replicationdevice may be used, which for example allows one UPS to connect to five computers using serial or USB connections.^[12] However, the splitting is typically only one direction from UPS to the devices to provide status information. Return control signals may only be permitted from one of the protected systems to the UPS.^[13]

As Ethernet has increased in common use since the 1990s, control signals are now commonly sent between a single UPS and multiple computers using standard Ethernet data communication methods such as TCP/IP.^[14] The status and control information is typically encrypted so that for example an outside hacker can not gain control of the UPS and command it to shut down.^[15]

Distribution of UPS status and control data requires that all intermediary devices such as Ethernet switches or serial multiplexers be powered by one or more UPS systems, in order for the UPS alerts to reach the target systems during a power outage. To avoid the dependency on Ethernet infrastructure, the UPSs can be connected directly to main control server by using GSM/GPRS channel also. The SMS or GPRS data packets sent from UPSs trigger software to shutdown the PCs to reduce the load.

Batteries

The run-time for a battery-operated UPS depends on the type and size of batteries and rate of discharge, and the efficiency of the inverter. The total capacity of a lead–acid battery is a function of the rate at which it is discharged, which is described asPeukert's law.

Manufacturers supply run-time rating in minutes for packaged UPS systems. Larger systems (such as for data centers) require detailed calculation of the load, inverter efficiency, and battery characteristics to ensure the required endurance is attained.^[16]

Common battery characteristics and load testing

When a lead–acid battery is charged or discharged, this initially affects only the reacting chemicals, which are at the interface between the electrodes and the electrolyte. With time, the charge stored in the chemicals at the interface, often called "interface charge", spreads by diffusion of these chemicals throughout the volume of the active material.

If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a fast charge for only a few minutes, then during the short charging time it develops only a charge near the interface. The battery voltage may rise to be close to the charger voltage so that the charging current decreases significantly. After a few hours this interface charge will spread to the volume of the electrode and electrolyte, leading to an interface charge so low that it may be insufficient to start the car.^[17]

Due to the interface charge, brief UPS self-test functions lasting only a few seconds may not accurately reflect the true runtime capacity of a UPS, and instead an extended recalibration or rundown test that deeply discharges the battery is needed.^[18]

The deep discharge testing is itself damaging to batteries due to the chemicals in the discharged battery starting to crystallize into highly stable molecular shapes that will not re-dissolve when the battery is recharged, permanently reducing charge capacity. In lead acid batteries this is known as sulfation but also affects other types such as nickel cadmium batteries and lithium batteries.^[19] Therefore it is commonly recommended that rundown tests be performed infrequently, such as every six months to a year.^[20][21]

Testing of strings of batteries/cells

Multi-kilowatt commercial UPS systems with large and easily accessible battery banks are capable of isolating and testing individual cells within a battery string, which consists of either combined-cell battery units (such as 12-volt lead acid batteries) or individual chemical cells wired in series. Isolating a single cell and installing a jumper in place of it allows the one battery to be discharge-tested, while the rest of the battery string remains charged and available to provide protection.^[22]

It is also possible to measure the electrical characteristics of individual cells in a battery string, using intermediate sensor wires that are installed at every cell-to-cell junction, and monitored both individually and collectively. Battery strings may also be wired as series-parallel, for example two sets of 20 cells. In such a situation it is also necessary to monitor current flow between parallel strings, as current may circulate between the strings to balance out the effects of weak cells, dead cells with high resistance, or shorted cells. For example, stronger strings can discharge through weaker strings until voltage imbalances are equalized, and this must be factored into the individual inter-cell measurements within each string.^[23]

Series-parallel battery interactions

Battery strings wired in series-parallel can develop unusual failure modes due to interactions between the multiple parallel strings. Defective batteries in one string can adversely affect the operation and lifespan of good/new batteries in other strings. These issues also apply to other situations where series-parallel strings are used, not just in UPS systems but also in electric vehicle applications.^[24]

Consider a series-parallel battery arrangement with all good cells, and one becomes shorted or dead:

• The failed cell will reduce the maximum developed voltage for the entire series string it is within.
• Other series strings wired in parallel with the degraded string will now discharge through the degraded string until their voltage matches the voltage of the degraded string, potentially overcharging and leading to electrolyte boiling and outgassing from the remaining good cells in the degraded string. These parallel strings can now never be fully recharged, as the increased voltage will bleed off through the string containing the failed battery.
• Charging systems may attempt to gauge battery string capacity by measuring overall voltage. Due to the overall string voltage depletion due to the dead cells, the charging system may detect this as a state of discharge, and will continuously attempt to charge the series-parallel strings, which leads to continuous overcharging and damage to all the cells in the degraded series string containing the damaged battery.
• If lead-acid batteries are used, all cells in the formerly good parallel strings will begin to sulfate due to the inability for them to be fully recharged, resulting in the storage capacity of these cells being permanently damaged, even if the damaged cell in the one degraded string is eventually discovered and replaced with a new one.

The only way to prevent these subtle series-parallel string interactions is by not using parallel strings at all and using separate charge controllers and inverters for individual series strings.

Series new/old battery interactions

Even just a single string of batteries wired in series can have adverse interactions if new batteries are mixed with old batteries. Older batteries tend to have reduced storage capacity, and so will both discharge faster than new batteries and also charge to their maximum capacity more rapidly than new batteries.

As a mixed string of new and old batteries is depleted, the string voltage will drop, and when the old batteries are exhausted the new batteries still have charge available. The newer cells may continue to discharge through the rest of the string, but due to the low voltage this energy flow may not be useful, and may be wasted in the old cells as resistance heating.

For cells that are supposed to operate within a specific discharge window, new cells with more capacity may cause the old cells in the series string to continue to discharge beyond the safe bottom limit of the discharge window, damaging the old cells.

When recharged, the old cells recharge more rapidly, leading to a rapid rise of voltage to near the fully charged state, but before the new cells with more capacity have fully recharged. The charge controller detects the high voltage of a nearly fully charged string and reduces current flow. The new cells with more capacity now charge very slowly, so slowly that the chemicals may begin to crystallize before reaching the fully charged state, reducing new cell capacity over several charge/discharge cycles until their capacity more closely matches the old cells in the series string.

For such reasons, some industrial UPS management systems recommend periodic replacement of entire battery arrays potentially using hundreds of expensive batteries, due to these damaging interactions between new batteries and old batteries,within and across series and parallel strings.^[25]