lead acid battery operation

14

Review of Literature

Operation and Maintenance of the 125 VDC Station Battery and Charger System at BCFTPP-I

A. HISTORY OF BATTERIES

Although Alessandro Volta in Italy is usually credited with being the inventor of

the modern battery (Silver-Zinc), ancient cells have been discovered in Sumerian ruins,

origin around 250 BC. The first evidence of batteries comes from archaeological digs in

Baghdad, Iraq. This first "battery" was dated to around 250 B.C. and was used in simple

operations to electroplate objects with a thin layer of metal, much like the process used

now to plate inexpensive gold and silver jewelry, one of the first uses for batteries.

Batteries were rediscovered much later by a man named Alessandro Volta after which the

unit of electrical potential was named, the volt. The jar was found in Khujut Rabu just

outside Baghdad and is composed of a clay jar with a stopper made of asphalt. Sticking

through the asphalt is an iron rod surrounded by a copper cylinder. When filled with

vinegar - or any other electrolytic solution- the jar produces about 1.1 volts.

Figure 5. Alessandro Volta

In 1859, Gaston Planté invented the lead acid batteries and first demonstrated it to

the French Academy of Sciences in 1860. The Lead Acid is the oldest rechargeable

battery.

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Figure 6. Earliest form of battery

B. BATTERY CONSTRUCTION

The lead-acid battery is one of the most common batteries in use today. Figure 7

shows the makeup of a lead-acid battery. The container houses the separate cells. Most

containers are hard rubber, plastic, or some other material that is resistant to the

electrolyte and mechanical shock and will withstand extreme temperatures. The container

(battery case) is vented through vent plugs to allow the gases that form within the cells to

escape. The plates in the battery are the cathodes and anodes. In figure 8, the negative

plate group is the cathode of the individual cells and the positive plate group is the anode.

As shown in the figure, the plates are interlaced with a terminal attached to each plate

group. The terminals of the individual cells are connected together by link connectors as

shown in figure 7. The cells are connected in series in the battery and the positive

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terminal of one end cell becomes the positive terminal of the battery. The negative

terminal of the opposite end cell becomes the negative terminal of the battery.

Figure 7. Lead-acid battery construction

Figure 8. Lead-acid battery plate arrangement

The terminals of a lead-acid battery are usually identified from one another by

their size and markings. The positive terminal marked (+) is sometimes colored red and is

physically larger than the negative terminal, marked (-).

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C. COMPOSITION AND OPERATION

A battery provides electrical power by converting its stored chemical energy into

electrical energy. This energy conversion is achieved by a chemical reaction in the

battery that releases electrons. The process is reversible in a stationary battery. If a load is

placed across the battery terminals, the chemical reaction produces electrical power. If

electrical energy is directed into the battery (charging the battery), the chemical reaction

reverses and restores the battery to a fully charged condition.

The lead acid battery, in the charged state, is made up of electrodes of lead metal

(Pb) and lead dioxide (PbO2) in an electrolyte of 35% sulphuric acid and 65% water

solution. The electrolyte causes a chemical reaction that produces electrons.

C.1 ELECTROLYTE

The electrolyte in a lead-acid battery is a mixture of sulfuric acid and water.

Sulfuric acid, H2SO4, is a very active compound of hydrogen, sulfur, and oxygen. When

added to water, the sulfuric acid does not stay intact as individual H2SO4, molecules.

Instead, the sulfuric acid molecules split into two ions, hydrogen and sulfate. Each

hydrogen ion carries one positive electrical charge and each sulfate ion carries two

negative electrical charges.

Sulfuric acid is highly reactive and ionizes almost completely in water. The ions

are in constant motion, attracted or repelled by one another. This constant random motion

tends to cause the ions to diffuse throughout the electrolyte. This diffusion process is not

immediate and can take a relatively long time to reach equilibrium throughout the

electrolyte.

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Specific gravity is a measure of the density of a liquid. Pure water has a specific

gravity of 1.0. The specific gravity of other liquids is usually expressed in relation to that

of water. The lead-acid cell electrolyte specific gravity typically varies from 1.210 to

1.300, depending on the particular cell design.

The specific gravity range of 1.210 to 1.240 is usually adequate for vented cells.

The capacity of a vented cell is often limited by the quantity of active material in the

positive or negative plates. A higher specific gravity electrolyte would generally be used

only if the quantity of sulfuric acid in the electrolyte is potentially limiting.

C.2 ELECTROCHEMICAL PROCESS

The generation of electrical current from a cell originates from a difference in

electrochemical potential between two compounds inside the cell that are not in direct

contact, but are electrically connected by a conducting medium. The two compounds are

installed in the cell as positive and negative plates, and the conducting medium between

the two plates is referred to as the electrolyte.

As the plate materials chemically react with the electrolyte, a potential difference

is created between the plates and the electrolyte. The positive plates have a positive

potential in relation to the electrolyte; the negative plates have a negative potential in

relation to the electrolyte. The electrochemical process between the plates and electrolyte

creates a voltage between the positive and negative plates of the cell. This voltage

between the plates constitutes an electromotive force that causes electrons to flow from

the negative plates to the positive plates if the plates are connected together by an

external conductor (a load). The flow of electrons disrupts the electrochemical

equilibrium between the plates and the electrolyte, which initiates further chemical

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reaction as the cell attempts to maintain electrochemical equilibrium. The battery’s

chemical reaction continues to generate electrical current until the materials involved in

the reaction are depleted or the external connection (the load) is removed.

In a lead-acid battery, the positive plate material is lead dioxide (PbO2) and the

negative plate material is lead (Pb). The plate material is often referred to as the active

material. The electrolyte is a sulfuric acid solution (H2SO4). The chemical reaction in a

lead-acid cell can be described in terms of the reaction occurring at each plate:

The above half-cell reactions offer greater insight into the actual processes

occurring at each plate and within the electrolyte. From left to right, these equations

represent the discharge process, and from right to left, the charging process. The sum of

the plus and minus charges on the left side of each equation equals the total charge on the

right side.

C.2.1 NEGATIVE PLATE REACTION

The negative plate reaction during discharge is depicted in Figure 9. When lead

from the negative plate comes into contact with the electrolyte, the chemical interaction

between the two compounds casts lead ions into solution; the ions carry a charge of plus

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2 (Pb+2

). Each positively charged lead ion entering solution leaves behind two negative

charges (electrons). Hence, the chemical reaction gives the negative plate an excess of

electrons and a net negative charge relative to the electrolyte.

Once in solution, the lead ions combine with sulfate ions (SO4-2

), which have

charges of equal magnitude but opposite sign, to form lead sulfate, which has a neutral

charge. The sulfate ions are created when the dilute sulfuric acid disassociates into

positively charged hydrogen ions (H+) and negatively charged sulfate ions (SO4

-2). The

lead sulfate is highly insoluble in the electrolyte and is immediately deposited as a solid

substance on the negative plate. This reaction occurs within molecular distances from the

plate.

Figure 9. Negative plate reaction

If an external conduction path exists, the excess electrons at the negative plate

migrate toward the positive plate, creating a flow of electrical current. During recharge,

electrons are forced toward the negative plate and the reaction is reversed.

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C.2.2 POSITIVE PLATE REACTION

The positive plate reaction during discharge is depicted in Figure 10. As shown,

the positive plate reaction is more complicated than the negative plate reaction. When

lead dioxide from the positive plate comes into contact with the electrolyte, the lead

dioxide combines with water to form lead ions with a charge of plus 4 (Pb+4

) and

hydroxyl ions with a charge of minus 1 (OH-1

). The chemical reaction causes the positive

plate to acquire an overall positive charge in relation to the electrolyte.

Figure 10. Positive plate reaction

When current is allowed to flow through an external circuit, the lead ions (Pb+4

)

combine with electrons that are migrating to the positive plate via the external circuit.

After combining with the electrons, the lead ions now carry a charge of plus 2 (Pb+2

) and

combine with sulfate ions to form lead sulfate as described above for the negative plate

reaction. The hydroxyl ions combine with the positively charged hydrogen ions from the

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sulfuric acid to form water. The net effect, as shown by the positive plate half-cell

reaction, is that lead dioxide combines with the sulfuric acid to form lead sulfate and

water.

The overall chemical reaction within a lead-acid battery is best described in one

equation by combining the two half-cell reactions. The single equation, referred to as the

full-cell reaction, is:

Positive Plate Electrolyte Negative Plate Positive Plate Electrolyte Negative Plate

𝑷𝒃𝑶𝟐 + 𝟐𝑯𝟐𝑺𝑶𝟒 + 𝑷𝒃 𝑷𝒃𝑺𝑶𝟒 + 𝟐𝑯𝟐𝑶 + 𝑷𝒃𝑺𝑶𝟒

CHARGED CONDITION DISCHARGED CONDITION

D. CHARGING SYSTEM

One of the most common charge methods, the IU characteristic charges the

battery at a constant current until a set cell voltage is reached. The charger then switches

to a constant voltage charge, where the charge current decreases until the trickle charge

level is reached. This type of charger is often used in standby applications and can be

kept connected to the battery for an indefinite period of time.

Figure 11 shows the charge stages of lead acid batteries following the IU

characteristic charging. Stage 1 takes about 5 hours and the battery is charged to 70%.

During the topping charge in Stage 2 that follows, the charge current is gradually reduced

as the cell is being saturated. The topping charge takes another 5 hours and is essential

for the well being of the battery. If omitted, the battery would eventually lose the ability

to accept a full charge. Full charge is attained after the voltage has reached the threshold

and the current has dropped to 3% of the rated current or has leveled off. The final Stage

3 is the float charge, which compensates for the self-discharge.

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Figure 11. Charge stages of a lead-acid battery. The battery charges at a constant current to a set voltage threshold (Stage 1). As the battery

saturates, the current drops (Stage 2). The float charge compensates for

the self-discharge (Stage 3).

Correct settings of the voltage limits are critical and range from 2.30V to 2.45V.

Setting the voltage limit is a compromise. On one end, the battery wants to be fully

charged to get maximum capacity and avoid sulfation on the negative plate. A continually

oversaturated condition at the other end, however, would cause grid corrosion on the

positive plate. It also promotes gassing, which results in venting and loss of electrolyte.

E. THE IDEAL LIFE CURVE OF A LEAD-ACID BATTERY

If properly designed, built, and maintained, a battery can provide several years of

reliable service. The ideal profile of capacity during a lead-acid battery's operational life

is shown in Figure 12. A new battery might not initially provide 100% capacity. The

capacity typically improves over the first few years of service, reaches a peak, and

declines until the battery reaches its end of life. A reduction to 80% of the rated capacity

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is usually defined as the end of life for a lead-acid battery. Below 80%, the rate of battery

deterioration accelerates, and it is more prone to sudden failure resulting from a

mechanical shock (such as a seismic event) or a high discharge rate. Note that even under

ideal conditions, a battery is expected to eventually wear out.

Figure 12. The ideal life curve of a lead-acid battery

F. PREDICTIVE MAINTENANCE

A reliable battery system is tantamount to a reliable power plant. The lifespan of a

battery will vary considerably with how it is used, charged and how it is maintained.

Predictive maintenance (PdM) techniques help determine the condition of in-

service equipment in order to predict when maintenance should be performed. This

approach offers cost savings over routine or time-based preventive maintenance, because

tasks are performed only when warranted.

PdM, or condition-based maintenance, attempts to evaluate the condition of

equipment by performing periodic or continuous (online) equipment condition

monitoring. The ultimate goal of PdM is to perform maintenance at a scheduled point in

http://en.wikipedia.org/wiki/Preventive_maintenance

http://en.wikipedia.org/wiki/Condition_monitoring

http://en.wikipedia.org/wiki/Condition_monitoring

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time when the maintenance activity is most cost-effective and before the equipment loses

optimum performance. This is in contrast to time- and/or operation count-based

maintenance, where a piece of equipment gets maintained whether it needs it or not.

Time-based maintenance is labor intensive, ineffective in identifying problems that

develop between scheduled inspections, and is not cost-effective.

The "predictive" component of predictive maintenance stems from the goal of

predicting the future trend of the equipment's condition. This approach uses principles of

statistical process control to determine at what point in the future maintenance activities

will be appropriate. Most PdM inspections are performed while equipment is in service,

thereby minimizing disruption of normal system operations. Adoption of PdM can result

in substantial cost savings and higher system reliability.

Since the battery system is the primarily used to ensure that the normal system

operation continues without interruption in case of power failure, a good predictive

maintenance program is a must.

G. BATTERY PARAMETERS

G.1 BATTERY VOLTAGES

All Lead-Acid batteries supply about 2.15 volts per cell when fully charged.

Batteries that are stored for long periods will eventually lose all their charge. This

"leakage" or self discharge varies considerably with battery type, age, & temperature. It

can range from about 1% to 15% per month. In systems that are continually connected to

some type charging source, whether it is solar, wind, or an AC powered charger this is

seldom a problem. However, one of the biggest killers of batteries is sitting stored in a

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partly discharged state for a few months. A "float" charge should be maintained on the

batteries even if they are not used.

The purpose of the float voltage check is to verify that the battery is maintained

within the manufacturer’s recommended limits for optimum performance; either too high

or too low, a voltage will have a detrimental effect on battery performance.

G.2 SPECIFIC GRAVITY

The electrolyte participates in the battery chemical reaction to produce current.

When a cell discharges, the sulfuric acid combines with lead dioxide from the positive

plates and lead from the negative plates to form lead sulfate in the plates and water in the

electrolyte. During discharge, the electrolyte sulfuric acid concentration and specific

gravity decrease. Conversely, during recharge, the sulfuric acid concentration and

specific gravity increase. Based on the interaction between cell plates and electrolyte, a

low specific gravity measurement typically indicates a cell is not fully charged, which

may require corrective action (e.g., recharge) to restore specific gravity to the expected

range.

If the specific gravity of a cell’s electrolyte is low, the cell is not fully charged.

The interpretation of a low specific gravity is that the cell might not have the capacity

needed to perform its design basis discharge because it is not fully charged. In this case,

corrective action is needed to restore the cell to a fully charged condition. But, a perfectly

normal specific gravity does not alone mean that a cell has its full capacity; it only means

that it is fully charged. This is an important distinction; fully charged does not mean full-

rated capacity.

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Specific gravity is measured with a hydrometer. Simply draw the battery acid into

the hydrometer so that the float is not touching the sides, top or bottom of the barrel.

Take the reading with your eyes level with the surface of the drawn up liquid, and then

subtract 0.004 for each 5°C above or below 25°C. A table of the specific gravity

measurements to be expected at 75°F is shown on Table 1.

Table 1. Equivalent State of Charge at 75°F

State of Charge Specific Gravity

100% charged 1.265

75% charged 1.239

50% charged 1.200

25% charged 1.170

Fully discharged 1.110

G.3 TEMPERATURE

A lead-acid battery is an electrochemical device whose characteristics vary with

temperature; heat accelerates chemical activity and cold slows it down. A change in the

electrolyte temperature from the reference temperature has two significant effects on

lead-acid battery performance:

• Battery capacity decreases as the temperature drops below 77°F (25°C).

• Battery life decreases as the temperature rises above 77°F (25°C).

At temperatures lower than 77°F (25°C), the battery cannot provide its rated

capacity. In general, a lower temperature increases the viscosity of the electrolyte, and

thus restricts its ability to circulate into the plates. Also, the efficiency of the chemical

reaction decreases as temperature decreases. Table 2 provides typical capacity factors for

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performance at temperatures other than the reference temperature of 77°F (25°C). For

example, at 45°F (7.2°C) a battery can provide only 80% of its rated capacity. The data

contained in Table 2 is based on a vented lead-acid battery with a specific gravity of

1.215.

Table 2. Effect of Temperature on Capacity (Adapted from IEEE 485)

As the temperature of the electrolyte increases, the internal resistance decreases

and the electrochemical reaction rates increase requiring the charging current to increase

in order to maintain a constant cell voltage. Therefore, cells in a battery at a higher

temperature than the other cells will require higher current. The voltage of the warmer

cells will be lower than the average. Also, the increased temperature causes faster

positive grid corrosion as well as other failure modes.

G.4 LEVEL OF ELECTROLYTE

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Keeping the battery fully charged during normal float operation means that there

is always a slight amount of charging current flow in excess of that required to offset the

self-discharge of the cells. The excess charging current through a fully charged cell

causes electrolysis of water into hydrogen and oxygen gas. These gases are often referred

to as charge gases. This dissociation of water into gas results in a gradual and predictable

decline in the electrolyte level. Evaporation also contributes to a loss of electrolyte water.

If the plates are exposed because of a low electrolyte level, the exposed portion can

experience accelerated sulfation, which results in loss of battery capacity. Permanent

damage to the plates can also occur if a low level is allowed to persist. Too high

electrolyte level could result in an overflow of electrolyte during an equalizing charge

when the cell is gassing vigorously. Overflowing electrolyte can result in shock and short

circuit hazards, and a loss of electrolyte acid.

Since only the water in the electrolyte is lost to the atmosphere by the electrolysis

process, the sulfuric acid concentration and the electrolyte specific gravity increase as the

level drops.

G.5 BATTERY STORAGE CAPACITY

The Amp-hour (Ah) Capacity of a battery tries to quantify the amount of usable

energy it can store at a nominal voltage. All things equal, the greater the physical volume

of a battery, the larger its total storage capacity. Storage capacity is additive when

batteries are wired in parallel but not if they are wired in series.

G.6 C-RATE

The charge and discharge current of a battery is measured in C-rate. Most portable

batteries, with the exception of the lead acid, are rated at 1C. A discharge of 1C draws a

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current equal to the rated capacity. For example, a battery rated at 1000mAh provides

1000mA for one hour if discharged at 1C rate. The same battery discharged at 0.5C

provides 500mA for two hours. At 2C, the same battery delivers 2000mA for 30 minutes.

1C is often referred to as a one-hour discharge; a 0.5C would be a two-hour, and a 0.1C a

10 hour discharge.

The capacity of a battery is commonly measured with a battery analyzer. If the

analyzer’s capacity readout is displayed in percentage of the nominal rating, 100 percent

is shown if 1000mA can be drawn for one hour from a battery that is rated at 1000mAh.

If the battery only lasts for 30 minutes before cut-off, 50 percent is indicated. A new

battery sometimes provides more than 100 percent capacity. In such a case, the battery is

conservatively rated and can endure a longer discharge time than specified by the

manufacturer.

G.7 DEPTH OF DISCHARGE (DOD)

The Depth of Discharge (DOD) is a measure of how deeply a battery is

discharged. When a battery is 100% full, then the DOD is 0%. Conversely, when a

battery is 100% empty, the DOD is 100%. The deeper batteries are discharged on

average, the shorter their so-called cycle life.

H. TYPES OF BATTERIES

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H.1 FLOODED-TYPE

This is the traditional engine start and traction style battery. The liquid electrolyte

is free to move in the cell compartment. The user has access to the individual cells and

can add distilled water as the battery dries out.

H.2 SEALED-TYPE

This term can refer to a number of different constructions, including only a slight

modification to the flooded style. In that case, even though the user does not have access

to the cell compartments, the internal structure is still basically the same as a flooded

battery. The only difference is that the manufacturer has ensured that a sufficient amount

of acid is the battery to sustain the chemical reaction under normal use throughout the

battery warranty period.

H.3 VRLA

This stands for Valve Regulated Lead Acid battery. This is also a sealed battery.

The valve regulating mechanism allows for a safe escape of hydrogen and oxygen gasses

during charging.

H.4 AGM

As stated earlier, the Absorbed Glass Matte construction allows the electrolyte to

be suspended in close proximity with the plate’s active material. In theory, this enhances

both the discharge and recharge efficiency. This particular style has recently become very

popular in much engine start and power sports applications.

H.5 MAINTENANCE FREE

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This term is very generic and refers to basically all of the battery types except

flooded batteries that have accessible individual cells so that the end user can add water.

Since any sealed construction prevents the user from adding water to the individual cells,

then by default it becomes maintenance free.

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lead acid battery operation