battery notes

8/12/2019 Battery Notes

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Different types of battery used for auxiliary power supply in substations and power plants

In industrial or substation applications mainly three types of batteries are used namely:

Vented / Flooded Lead Acid batteries

Sealedmaintenance free batteries/Valve Regulated Lead Acid

Nickel Cadmium (Ni-cd) batteries

For UPS applications batteries are the most popular and hence are widely used. Hence, in this

detailing, mainly emphasize has been put on these type of batteries.

Vented / Flooded Lead Acid Batteries
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Vented / Flooded Lead Acid Batteries

There are two types for vented or flooded lead acid batteries namely tubular and Plante. The

difference between the two is the construction. For tubular battery normal life is 8-10 years. The

Plante battery is both mechanically and electrically more durable. The normal life for Plante

batteries is 15-20 years. Because this type of battery generates corrosive fumes when charging

and because the sulfuric acid electrolyte does evaporate to some extent, these batteries must

be used in a special room, which is well ventilated to the outside and kept away from delicate

electronic equipments.

It needs separate room/racks with acid proof tiles for installation. Because of evaporation, it

needs regular maintenance to check specific gravity, to add water and acid. These batteries can

withstand high temperature, voltage, and deep discharge with minimum damage to itself. A

notice should be exhibited in the battery room prohibiting smoking and use of naked flames.

These batteries cannot be transported in charged condition and therefore need charging at site.

Typical initial charging of the battery will take about 55 to 90 hours. Nominal cell voltage is

2V/cell. The charger for this battery should be able to provide the first charge at 2.6 to 2.7 V/cell.

Tubular Type Lead acid battery

These types of batteries are typically used for UPS Systems of very high rated capacity,

typically engaged for plant application, wherein maintenance and space is not really an issue.


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Sealed Maintenance Free batteries (SMF)

VRLA/SMF type Lead acid battery

These are also known asValve Regulated Lead Acid (VRLA) batteries. These batteries are the

most popular for usage with UPS systems for computer or commercial application. Being

sealed, these batteries do not emit any fumes and hence can be very well installed next to

electronic equipment. These batteries also can be housed in a close enclosure if necessary.These batteries are also maintenance free and avoid any hassles of checking specific gravity,

adding water or acid, etc.

These batteries have a relatively lesser life of approx. 3-5 years. The life expectancy typically

depends on the number of charge/discharge cycle experienced by the batteries and the ambient

temperature in which the batteries are used. These batteries are primarily the most popular for

commercial applications due to Install and forget approach.

The Performance and service life of these batteries can be maximized by observing the

following guidelines:

1. Permissible operating temperature range of SMF batteries is 15 deg C to 50 deg C, but using

within an operating range 5 C to 35 C will extend service life. Below15 deg C, the battery

changes its chemical composition and cannot hold a charge. You will enjoy longer service life, if

batteries are operated in ambient temperature range of 20 deg C to 25 deg C (68F to 77F). At

lower temperature they have longer life and lower capacity while at higher temperature they

have higher capacity and lower life.

2. A good rule of thumb when determining battery service life in relation to temperature is that

for every 8.3 deg C (15F) average annual temperature above 25 deg C (77F), the life of the

battery is reduced by 50%. Therefore warranty of the battery should be ideally reduced to 50%

for every 8.3 deg C (15F) increase in operating temperature above 25 deg C (77F).

3. SMF batteries are designed to have a float voltage of 2.3 V/cell. This means that a 12 V

battery (with 6 internal cells) has a float voltage of 13.8 Volts. Most of the battery manufacturers

recommend float voltage of 2.252.3 volts per cell. When there are more cells (generally >120)

in series, to compensate for higher temperatures, float voltage should be decreased by approx.

3 mV per cell per deg C above 25 deg C. It should be increased by the same amount when
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operated at a temperature less than 25 deg C to avoid undercharge. The Cutoff voltage is 1.67

V/cell for high rate of discharge (

4. It is recommended that SMF batteries should not be left in totally discharged state more than

72 hrs. The batteries may get partially or fully damaged due to SULPHATION if charging does

not start within 72 Hrs from totally discharged state. Sulphation is the formation of lead sulphate

on negative plates which acts as an insulator and has a detrimental effect on charge

acceptance.

5. In normal float / equalize use (2.25 to 2.35V/cell), gas generated inside battery is recombined

into negative plates, and return to water content of the electrolyte. Thus electrical capacity is not

lost because of this recombination. There is no need to add external water, but due the

corrosion of the electrodes battery will eventually lose capacity.

6. At ambient temperature of 3040 deg C, the shelf life of batteries is 5-6 months only. A

freshening charge must be given to the batteries every 6 months, if needed to be stored for

longer periods. Batteries should be kept in dry, cool place. At ambient temperature of 20 deg C

(68F), the self-discharge rate is 3-4% (approx.) of rated capacity per month. The self-discharge

rate varies with ambient temperature.

7. SMF batteries are equipped with a safe, low pressure venting system, which operates at 7 psi

to 10 psi (can vary slightly from manufacturer to manufacturer), automatically releasing excess

gas in the event that gas pressure rises to a level above the normal rate ensuring no excessive

buildup of gas in the batteries. Resealing is automatic once the pressure is returned to normal.

8. Cyclic life of the battery depends on ambient operating temperature, the discharge rate, the

depth of discharge, and the manner in which the battery is recharged. The most important factor

is the depth of discharge. At a given discharge rate and time, the shallower the depth of

discharge, the longer is the cyclic life.

9. Failure mode at the end of life includes:

Capacity decrease

Internal short circuit

Damage to container/lid

Terminal corrosion

Reduced open circuit voltage.

10. The IEEE defines B (Bend of useful life) for a UPS battery as being the point when it can

no longer supply 80 percent of its rated capacity in ampere-hours (AH). The relationship

between AH capacity and runtime time is not linear, a 20% reduction in capacity results in a

much greater reduction in runtime. When battery reaches 80% of its ratedcapacity,the aging

process accelerates and the battery should be replaced. Some UPS/ Battery manufacturers
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define B (Bend of useful life) for a UPS battery when battery capacity reaches 50-60% of its

rated capacity.

11. Mixed use of batteries with different capacities, different makes should be avoided as it will

cause accelerated aging of the whole string.

12. If two or more battery groups are to be used, connected in parallel, they must be connected

to the UPS through lengths of wires, cables or busbars that have the same loop line resistance

as each other. This makes sure that each parallel bank of batteries presents the same

impedance to the UPS as any other of the parallel banks thereby ensuring correct equalization

of the source to allow for maximum energy transfer to the UPS load.

13. The normal life SMF battery will support approx. 200 charge/discharge cycles at 25 deg C

(77F) and 100% depth of discharge.

14. The term B (Bend of useful life) for a UPS battery refers to the fact that these batteries do

not require fluid. But preventive maintenance like checking for cracks and deformation of the

container & lid, electrolyte leakage/spills tightening of the connection etc, particularly for higherAH capacity batteries should be done to prevent any damage.

Nickel Cadmium Batteries (Ni-Cd)

Ni-cd batteries do emit hydrogen and oxygen gas, products of electrolysis, but there are no

corrosive gases as lead acid batteries, so these can be installed near electronic equipment.

Water consumption is relatively low and so therefore maintenance is low. Normal service life is

20-25 years. These are most expensive of the various types of batteries previously discussed.

Initial cost may be approximately three times that of lead acid battery depending upon their AH

capacity.

These batteries do not experience the severe shortening of life when operated at elevated

temperatures and perform better at low temperatures than do the lead acid batteries. Nominal

cell voltage is 1.2 V/cell. The battery chargers and inverters have to be designed to operate with

low end cell cutoff voltages and higher recharging voltages needed for such batteries.

These batteries occasionally demand boost charging and typically find their applications

wherein UPSs support critical equipment in hazardous environment such as chemical, fertilizer,

cement industry.

Merits/demerits

As spelled earlier, all the above discussed types of batteries have their own merits and

demerits. Let us now look at them individually.

A) Vented / flooded Lead acid batteries


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Merits

Most economical among three types of batteries.

Life is higher than SMF batteries.

Robust- not much sensitive to temperature.

Demerits

Needs periodic maintenance- twice a month.

Emits corrosive fumes.

Needs special battery room with acid proof tilling.

Cannot be transported in charged condition, initial charging takes 55 to 90 hours.

Needs specially trained persons for handling due highly hazardous sulphuric acid.

B) Sealed maintenance free batteries / Valve Regulated Lead Acid

Merits

No maintenance as far as water filling, specific gravity check etc is concerned.

Can be shipped in charged conditions so ready to use.

User friendly.

Demerits

Leaving batteries in discharged state for longer life will reduce life significantly or can damage

them permanently.

Very sensitive to temperature

Service life lowest among the three types

Costlier than flooded / vented lead acid battery

C) Nickel Cadmium (Ni-cd) batteries

Merits

Moderate maintenance

Higher service life

less sensitive to temperature

Fumes not corrosive so can be installed near electronic equipment


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Demerits

Most expensive among three types

Cannot be transported in charged conditions.

Compatibility with respect to charger and inverter needed to be considered.

Summary of Techno-Commercial Analysis between Ni-Cd & Lead acid Batteries

No. Description Ni-Cd Lead acid Remarks

1 Reliability Criteria Worst case failure is

cell short circuit.

Result is reduced

performance. The

battery will continue to

support the system.

Worst case failure

is cell open circuit.

Result is complete

loss of battery,

known as sudden

death, resulting in

an unpredicted

system failure.

This point makes

NiCd to be superior in

terms of purpose

2 Requirement of

Air-Conditioning

& additional cost

of Electrical

Energy

Nil Yes Additional Cost due to

Air conditioning

equipment and

running cost-For

VRLA

3 Battery Life as

claimed by UK

manufacturers

(Industrial

Batteries)

20 Years 5-7 Years Replacing cost of

VRLA after every 5-7

years will involve

manufacturing cost

escalation during that

year. Eg.at 6th year,

12th year and 18th

year.

4 Battery Cost Approximately 3 times

than VRLA

- -

5 Relative Costs

Initial & Life

Cycle

Ni-Cd cells have a

higher initial cost than

lead acid but superior

lifetime and

characteristics, giving

Lead Acid has a

low initial cost but a

restricted lifetime.

In many

applications they

-


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a lower life cycle cost

in many applications.

can have a poor life

cycle cost.

6 Physical Size Ni-Cd cells are

generally larger than

VRLA cells.

In terms of Ah,

Lead acid (VRLA)

is the most

compact battery.

7 Water Topping Occasionally Required Not Required

Summary of Comparison between tubular flooded (ordinary Lead-acid) battery with SMF Lead

Acid Battery

No. Tubular Lead Acid battery VRLA Battery

1 Tubular positive Plates:

The positive active material is held in a

polyester tube. This does not allow the

materials to shed during charge-discharge

cycling resulting in long cyclic life-

minimum 1200 cycles at 80 % depth of

discharge.

Flat pasted positive plates:

The positive active material is a paste form

pasted over a lead alloy grid.? This results in

a shorter cyclic life compared to tubular

construction.- minimum 500 cycles at 80%

depth of discharge

2 Electrolyte:? Stationary batteries of UPS

and Power plant back up works on low

specific gravity (1.200) electrolyte andlarger in volume. This results in less

corrosion of grids and longer life. The

larger volume keeps the battery

comparatively cooler which also adds to

life.

Electrolyte: VRLA batteries work on high

specific gravity electrolyte (1.280 to 1.300)

and less volume. The cells get heated upduring charging and high rate discharges

which affects the service life.

3 Separators: Micro porous poly ethylene

separators are used. This permits the ion

movement and positive to negative plate

separation.

Separators: Adsorptive glass mat separators

are used in these batteries with a closer

spacing between the positive and negative

plates. Chances of cell shorting are

therefore more.

4 Charging compatibility: Tubular stationary

lead acid batteries can be charged with

constant current and constant voltage

mode. Flooded electrolyte batteries can

withstand more abuses during charging.

Water lost in electrolyte during such

Charging compatibility:? VRLA batteries

require constant voltage charging with a

specified limit only specifically to avoid

overcharging.? If by any chance, the

charging conditions are altered, the battery

will get heated up which will deteriorate the


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abusive overcharging could be easily

made up manually by periodic top-up.

Lagging cells could be brought to normal

life by an extended equalizing charging

without significantly affecting the other

cells.

battery life. Though there is no need to top-

up due to the recombination principle, it is

not always 100% efficient, so some water

loss during use is not avoidable. This results

in a further increase in electrolyte specific

gravity and life reduction. Equalizing(Extended) charging in VRLA batteries

results in some cells getting heated up and

life reduction.

5 Containers: We use transparent SAN

containers. The electrolyte level is easily

seen for topping up needs and ease of

maintenance. Low maintenance.

Not applicable: Cannot monitor Electrolyte

6 Need for temperature control: Due to the

large electrolyte volume the temperature ofthe cells generally do not rise abnormally

during charging. No need for Air

conditioning.

Need for temperature control:? The close

packing arrangement and compactness ofthe stack in the cell assembly rises the

temperature sharply during charging. Due to

this, VRLA battery manufacturers invariably

recommend the need for air conditioned

environment for the battery compartments.

This adds to the cost of maintenance.

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