Ups & Battery

5/22/2018 Ups & Battery

1/33


2/33

Battery

A batteryis a combination of two or more electrochemicalGalvanic cells which store chemical energy which can beconverted into electric potential energy i.e. electricity.


3/33

A battery is a device that converts chemical energy directlyto electrical energy.

It consists of one or more voltaic cells. Each voltaic cell consists of two half cells connected in

series by a conductive electrolyte. One half-cell is the negative electrode (cathode) and the

other is the positive electrode (anode).

In the redox reaction that powers the battery, reductionoccurs in the cathode, while oxidation occurs in the anode. The electrodes do not touch each other but are electrically

connected by the electrolyte, which can be either solid orliquid.

A voltaic cell for demonstration purposes.

In this example the two half-cells arelinked by a salt bridge separator thatpermits the transfer of ions, but not watermolecules.

Howbatteries work:
http://en.wikipedia.org/wiki/Cathodehttp://en.wikipedia.org/wiki/Image:ElectrochemCell.pnghttp://en.wikipedia.org/wiki/Cathode


4/33

Cont..

Each half cell has an electromotive force (emf), determinedby its ability to drive electric current from the interior to theexterior of the cell.

The nett emf of the battery is the difference between theemf,s of its half-cells, as first recognized by Volta a reknown

scientist. Thus, if the electrodes have emf,s and then the nett emf

is, in other words, the nett emf is the difference betweenthe reduction potentials of the half-reactions.

In many cells, the materials are enclosed in a container with

a separator, which is porous to the electrolyte and preventsthe electrodes from coming into contact.


5/33

Conti

The electrical potential difference across the terminals of abattery is known as terminal voltageand is measured in volts.

The terminal voltage of a battery that is neither charging nordischarging is called the open-circuit voltage and equals theemf of the battery.

Because of internal resistance, the terminal voltage of adischarging battery is smaller in magnitude than the open-circuit voltage and the terminal voltage of a battery that is incharging action exceeds the open-circuit voltage.

An ideal battery has negligible internal resistance, so it wouldmaintain a constant terminal voltage until exhausted and thendropping to zero.

If such a battery maintains 1.5 volts and stores a charge ofone Coulomb then it would perform 1.5 Joule of work.

In practical batteries, the internal resistance will increase as itis discharged, and the open circuit voltage will also decreaseas the cell is discharged.


6/33

Conti

If the voltage and resistance are plotted against time, theresulting graphs will typically not be a straight line, and theshape of the curve will vary with the chemical and the internalarrangement employed.

The voltage developed across a cell terminals depends on thechemicals used in it and their respective concentrations.

For example, alkaline and carbon-zinc cells both measureapproximately 1.5 volts, due to the energy release of theassociated chemical reactions.

Because of the high electrochemical potential changes in thereactions of lithium compounds, lithium cells can provide asmuch as 3 volts or more.


7/33

Battery capacity and discharging

The more electrolyte and electrode material is there in thecell, the greater the capacity of the cell.

Thus a small cell has less capacity than a larger cell, giventhe same chemistry (e.g. alkaline cells), though they developthe same open-circuit voltage.

Because of the chemical reactions within the cells, thecapacity of a battery depends on the discharge conditionssuch as the magnitude of the current, the duration of thecurrent, the allowable terminal voltage of the battery,temperature and other factors.

The available capacity of a battery depends upon the rate atwhich it is discharged. If a battery is discharged relatively ata high rate, the available capacity will be lower thanexpected.


8/33

The battery capacity that a battery manufacturers print on abattery, is the product of 20 hours multiplied by themaximum constant current that a new battery can supply for20 hours at 68 F (20 C), down to a predetermined terminal

voltage per cell. A battery rated at 100 Ah will deliver 5 A over 20 hour period

at room temperature.

However, if it is discharged at 50 A, it will run out of chargebefore the 2 hours as theoretically expected.

For this reason, a battery capacity rating is always related toan expected discharge duration.

Where

Qis the battery capacity (typically given in mAh). Iis the current drawn from battery (mA).

tis the amount of time (in hours) that a battery cansustain


9/33

The relationship between current, discharge time, and capacityfor a lead acid battery is expressed by Peukert's law.

Theoretically, a battery should provide the same amount of

energy regardless of the discharge rate, but in real, batteriesinternal energy losses causes the efficiency of a battery at adifferent discharge rates.

When discharging at low rate, the battery's energy is deliveredmore efficiently than at higher discharge rates.

In general, the higher the ampere-hour rating, the longer thebattery will last for a particular load.

Installing batteries with different Ah ratings will not affect theoperation of a device rated for a specific voltage unless theload limits of the battery are exceeded.

Theoretically, a battery would operate at its Ah rating, butrealistically, high-drain loads like digital cameras can result inlower actual energy, most notably for alkaline batteries.

For example, a battery rated at 2000 mAh may not sustain acurrent of 1 A for the full two hours.


10/33

VRLA BATTERY

VRLAstands for valve regulated lead acidand is the designationfor low maintenance lead-acid batteries, also called recombinantbatteries. VRLA batteries are commonly further classified as:Absorbent glass mat batteryGel battery
http://en.wikipedia.org/wiki/Lead-acid_batteryhttp://en.wikipedia.org/wiki/Lead-acid_batteryhttp://en.wikipedia.org/wiki/Lead-acid_batteryhttp://en.wikipedia.org/wiki/Lead-acid_battery


11/33


12/33

VRLA (valve regulated lead acid)BATTERY

Manufactured in multi-cell blocks (called modules).

The cases are often made of ABS plastic material that do notpermit visual inspection of plates/ electrolyte levels.

The hydrogen & oxygen are not expelled but recombined.

Cells are sealed & require no water addition and specificgravity readings.

These cells are typically lead calcium pasted-plate type cellswith the electrolyte retained in gel fiberglass mats.

These cells are not flooded & dont effectively dissipate heat.

Maintaining the cells as close as possible to 77F is imperative.

Dont allow sunlight or other heat sources to rise the temp. ofindividual cells.


13/33

Conti

These batteries are normally used for emergency lighting,telecommunication & other uninterrupted power supply service.

VRLA modules/ cells are typically shipped fully charged & dontrequire initial charge.


14/33

Guidelines for maintenance of batteries

The points to be observed during the inspections are

summarized as below-

General conditions of battery room and cells

Specific gravity of electrolyte in the cells

Charging current

Cell voltage

Condition of the plates and extent of deposits

Inter-cell connectors and main battery terminals


15/33

Specific gravity

Specific gravity is related to electrolyte temperature.

Should be maintained at about 1.210 at 27 degree C & when itdrops to 1.150 the cell may be considered as discharged.

Temperature correction hydrometer readings of specific gravityshall be made as follows-

a). For each 1 degree C above 27 degree C add 0.007 to theobserved readings

b). For 1 degree C below 27 degree C deduct 0.007 from theobserved readings.


16/33

Pilot cells

One of the cells in each row of the battery set should beselected and kept as the pilot cell.

Readings should be taken on these cells at a sufficientfrequency to indicate its state of discharge and charge andserve as a guide to the condition of other cells.

The height of electrolyte should invariably be kept at a fixedpoint above top of plates by adding distilled water everyfortnight, if necessary.


17/33

Trickle charging

A long life for battery is achievable if the battery is keptfloating on a battery charger so that the terminal voltage ofeach cell is maintained close to 2.15 v

This can be achieved if the battery is kept on a very low rateof charge, say 1 mA per Ah capacity of the battery.

The exact rate of charge should be fixed having regard tonormal and intermittent rates of discharge over a period of 24hours, so that the battery is always kept in fully chargedcondition and never overcharged or over-discharged.


18/33

Cell voltage The voltage of cell at the end of charging is not fixed value

but will vary depending on the age of battery.

No cells should ever be discharged below the point where thecell voltage reaches 1.85 V as measured when the cell isdischarged at normal 10 hour rate.

Sulphated plates, lug corrosion, partial short circuit due tocracked separators & other defects of cell causes a noticeable

drop in terminal voltage with current flowing in the cell.

This drop varies with amount of current flowing and in orderto get voltages that can be compared from month to month,the voltages should be taken with same current flowing in thecell.

The cell testing voltmeters in use should be periodicallychecked and recalibrated, if necessary. When not in use theyshould be kept in safe place.


19/33

Condition of plates & deposits Examine carefully the physical condition of the plates such as

cracks, distortions, accumulation of whitish deposits etc.

In healthy cells, deposit is brown in color but excessiveshedding of active material on positive plates indicates overcharging of battery. If this is noticed, reduce the charging rateimmediately.

If all cells shows whitish deposits immediate action should be

taken to give a boost charge at an appropriate rate & then toincrease the trickle charging rate sufficiently to keep thebattery in healthy condition at all time.

Weak cell should be immediately examined for any possibleshort circuit or metallic contact between positive & negative

plates. The short circuit should be removed and the cell should then

be given special additional charging by cutting it out andputting it back again when a healthy condition is regained,after it is attended to.


20/33

Inter-cell connections The inter-cell connectors of the battery should be examined to

ensure that they are clean & tight, making perfect contact with

cell lugs and there is no corrosion taking place. Light vaiseline should be applied to prevent corrosion.

Inspection of copper inter-row connectors should also be madefor any signs of copper sulphate corrosion which should becleaned up.

Acid proof paint or enamel should be applied to all exposedcopper work in the battery room & any flaking of paint workbe given prompt attention.


21/33

Maintenance New Battery Shift Monthly 3-Month 6-Month Annual

Visual

Inspection

General

Inspection

Battery FloatVoltage

Panel MeterFloat

Voltage

Battery FloatVoltage with

Digital Voltmeter

Cell Float

Voltage

All Cells with

Digital

Voltmeter

Pilot Cells with

Digital Voltmeter

All Cells with

Digital

Voltmeter

TemperatureReadings All Cells All Cells

Connection

Resistance

All

Connections

25 Percent

of All

Connections

All

Connections

Internal

Resistance

All Cells All Cells

Battery Testing Acceptance

Capacity

Testing

Capacity Test

6 months if

1-year test


22/33

UPS System


23/33

UPS (Uninterrupted power supply)

Also known as a continuous power supply

(CPS) or a battery backup is a device whichmaintains a continuous supply of electric power tothe connected equipment by supplying power from aseparate source when utility power is not available.

There are 3 distinct UPS systems:-

(a) off-line

(b) Line-interactive(c) On-line


24/33

Common power problemsThere are various common power problems that UPS units are

used to correct. They are as follows:

Power failure

Voltage sag

Voltage spike

Under-voltage

Over-voltage

Line noise

Frequency variation

Switching transient

Harmonic distortion


25/33

(a) Off-line UPS

Offline / standby UPS. Typical protection time: 0 - 20 minutes. Capacityexpansion: Usually not available


26/33

Off-line UPS:-

Remains idle until a power failure occurs, and thenswitches from utility power to its own power source, almostinstantaneously.

An on-line uses are double conversion method ofaccepting AC input, rectifying to DC for passing throughbattery, then inverting back to AC for powering theprotected equipment.

when the incoming utility voltage falls below apredetermined level the UPS turns on its internal DC-ACinverter circuitry, which is powered from an internal storagebattery. The standby system then mechanically switchesthe connected equipment on to its DC-AC inverter out ofpower. The switchover time can be as long as 25 ms.Depending on the amount of time it takes the standby UPSto detect the lost utility voltage.


27/33

(b) Line-interactive

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


28/33

Line-interactive type:

A line-interactive UPS maintains the inverter in line &

redirects the batterys DC current path from the normalcharging mode to supplying current when power is lost.

The line-interactive UPS is similar in operation to astandby UPS, but with the addition of a multi-tap variable-voltage autotransformer. This is a special type of electrical

transformer that can add or subtract powered coils of wire,thereby increasing or decreasing the magnetic field & the output voltage of the transformer.

This type of UPS is able to tolerate continuous undervoltage brownouts & over voltage surges without consuming

the limited reverse battery power.


29/33

(c)Double conversion / on-line UPS


30/33

Double conversion / online:

The On-line UPS is ideal for environments where electrical

isolation is necessary for equipment that is very sensitive topower fluctuations.

It is called a Double-Conversion UPSdue to the rectifierdirectly driving the inverter, even when powered from normalAC current.

In an On-line UPS, the batteries are always connected tothe inverter, so that no power transfer switches are necessary.When power loss occurs, the rectifier simply drops out of thecircuit & the batteries keep the power steady & unchanged.When power is restored, the rectifier resumes carrying most of

the load & begins charging batteries, through the chargingcurrent may be limited to prevent the high-power rectifierfrom overheating the batteries & boiling off the electrolyte.

The main advantage to the on-line UPS is its ability toprovide an electrical firewall b/w the incoming utility power &

sensitive electronic equipment.


31/33

UPS System Maintenance Periodic maintenance is extremely important to UPS

systems, as its reliability is crucial for the reliability

of the system it is supporting.

The small periodic time investment to checkbatteries, connections, fans, relays and sensors iswell spent compared to damage or lost data and

broken hardware after a power disturbance revealsa dead UPS system.

Records indicates that a properly maintained batterybackup system will last an average of five years.

Some units may fail because of lack of batterymaintenance, blocked air filters and fan failures.

Poor battery connections, dirt and dust within theunit also cause of premature failure and unexpected

added expenses.


32/33

The single most expensive scheduled item on a UPS(other than initial purchase) is battery replacement.

UPSs can be equipped with maintenance-freecapacitors to extend service life.


33/33

Documents

Ups & Battery