Linden D., Reddy T.B. (eds.) Handbook of batteries

Подождите немного. Документ загружается.

23.36 CHAPTER TWENTY-THREE

The cranking ability of an SLI battery is directly proportional to the geometric area of

plate surface, with the proportionality factor typically 0.155 to 0.186 cold-crank Amperes

(CCA) at

⫺17.8⬚C(0⬚F) per square centimeter of positive-plate surface. Cranking perform-

ance is generally limited by the positive plate at higher temperatures (

⬎18⬚C) and by the

negative plate at lower temperatures (

⬍5⬚C). The ratio of positive surface to negative surface

is ﬁxed by design. To maximize the cranking capacity, SLI battery designs emphasize grids

with minimum electrical resistance (using a variety of radial and expanded grid designs),

thin plates, and a higher concentration of electrolyte than motive-power or stationary batter-

ies.

Usually an ‘‘outside-negative’’ (n

⫹ 1 negative plates interspersed with 2n separators and

n positive plates) design is used. However, in order to balance the cranking rating with the

requirement or electrical load, as well as to facilitate automatic assembly, SLI batteries with

an even number of plates, or ‘‘outside-positive’’ designs, are widely produced in the United

States.

A major advance is the maintenance-free SLI battery, which has several characteristics

that distinguish it from the conventional battery. It requires no addition of water during its

life, it has signiﬁcantly improved capacity retention during storage, and it has minimal ter-

minal corrosion. The construction of a typical maintenance-free SLI battery was illustrated

in Fig. 23.8. This type of battery relies mainly on charge control to prevent electrolysis of

water and dry-out as compared to the small sealed consumer designs which rely on oxygen

recombination (see Chap. 24).

The SLI maintenance-free battery has a large acid reservoir, made possibly by the use of

smaller plates and placement of the element directly on the bottom of the container, elimi-

nating the sludge space. The positive plates are usually enveloped in a microporous separator

that prevents active material from falling to the bottom of the container and creating a short

circuit. An important feature of the maintenance-free battery is the use of nonantimonial

(such as calcium-lead) or low-antimonial lead grids. The use of these grids reduces the

overcharge current signiﬁcantly, reducing the rate at which water is lost during overcharge,

as well as improving the stand characteristics (see Sec. 23.7). The use of the expanded grid,

produced from wrought lead-calcium strip, is also shown in the ﬁgure. Most SLI batteries,

so called hybrid, are built using lead-calcium-tin grids for the negative and low-antimony

lead grids for the positive electrode.

Another reﬁnement of the SLI battery is illustrated in Fig. 23.16. In this design the plates

are approximately one-ﬁfth the width of conventional SLI battery plates and are inserted

parallel, rather than perpendicular, to the length of the battery case. This design reduces the

internal impedance of the cell and gives very high CCA ratings.

Heavy-duty SLI batteries for trucks, buses, and construction equipment are designed sim-

ilar to the passenger vehicle SLI batteries but use heavier and thicker plates with high-density

paste, premium separators often with glass mats, anchor-bonding of the element to the bottom

of the case, rubber cases, and other such features to enhance longer life. This is necessary

to provide maximum mechanical strength for the physically large (up to 530 by 285 mm)

case dimensions. Because the thick plates provide less cranking current than the thinner

plates (since fewer can be included in a cell of a given size), series or series-parallel con-

nections of batteries are used. Typically, the 12-V monoblocks are connected in series for

cranking at 24 V and in parallel for running and recharging at 12 V. A few sizes of main-

tenance-free heavy-duty batteries have also been produced.

SLI-type batteries are also used on motorcycles and boats. Batteries for recreational ma-

rine use generally have thicker plates (to give more capacity) and higher-density paste. They

have the same Battery Council International type designations as automotive batteries. See

Sec. 4.10 for a listing of BCI battery types. Marine batteries are also manufactured in four-

cell 8-V monoblocs.

Aircraft use SLI-type batteries with special spill-proof vent caps which preclude loss of

electrolyte during ﬂight.

LEAD-ACID BATTERIES 23.37

FIGURE 23.16 Cathanode lead-acid SLI battery.

23.4.3 Performance Characteristics

Discharge Performance. Discharge curves, showing the discharge proﬁle of SLI-type bat-

teries at several constant-current discharge rates, are presented in Fig. 23.17. The typical

ﬁnal or end voltages at these discharge rates are also shown. Higher service capacity is

obtained at the lower discharge rates. At the higher discharge rates, the electrolyte in the

pore structure of the plates becomes depleted and the electrolyte cannot diffuse rapidly

enough to maintain the cell voltage. Intermittent discharge, which allows time for the elec-

trolyte to recirculate, or forced circulation of the electrolyte will improve high-rate perform-

ance. In general the lead-acid cell may be discharged without harm at any rate of current it

will deliver, but the discharge should not be continued beyond the point where the cell

approaches exhaustion or where the voltage falls below a useful value.

23.38 CHAPTER TWENTY-THREE

FIGURE 23.17 Discharge curves of lead-acid SLI batteries. (a) At various hourly rates and

25⬚C. (b) At various high rates and ⫺17.8⬚C. Battery rated at 70 Ah, 20-h rate at 25⬚C.

Effect of Temperature on Performance. Figure 23.18(a) shows typical discharge curves

for a lead-acid single-cell battery at several discharge temperatures. Fig. 23.18(b) shows the

discharge characteristics of a 12 V, 60 Ah battery when discharged at 340 A at temperatures

from

⫺30 to 25⬚C. Higher discharge voltages and capacities are obtained at the higher

temperatures and lower discharge rates.

LEAD-ACID BATTERIES 23.39

FIGURE 23.18 Discharge curves of lead-acid SLI batteries at various tempera-

tures. (a)AtC / 20 rate. (b) At 340 A. 12-V battery, nominal capacity 60 Ah, 20-h

rate at 25⬚C.

The effect of discharge rate and temperature on the capacity of the lead-acid battery is

summarized in Fig. 23.19, which shows the percentage of the 20-h rate capacity delivered

under different discharge conditions. Although the battery will operate over a wide temper-

ature range, continuous operation at high temperatures may reduce life as a result of an

increase in the rate of corrosion (see Sec. 23.8.1).

The performance of the lead-acid SLI-type cell at different temperatures and loads is

given in another form in Fig. 23.20. The logarithm of the current drain is plotted against the

logarithm of the service hours, in accordance with Peukert’s relationship (Chap. 3, Sec.

3.2.6). The linear relationship is maintained over a wide range, with divergences appearing

on the high-rate and low-temperature discharges. In this ﬁgure, the data have been normalized

to unit cell weight (kilograms) and unit cell volume (liters). Figure 23.20 can be used to

approximate the performance of various size cells over the operating conditions shown or to

determine the size and weight of a battery to meet a particular service requirement.

23.40 CHAPTER TWENTY-THREE

FIGURE 23.19 Performance of lead-acid SLI batteries at various temperatures and discharge

rates to 1.75-V per cell end voltage.

FIGURE 23.20 Service life of lead-acid SLI battery to 1.75-V end voltage per cell.

LEAD-ACID BATTERIES 23.41

Internal Impedance. The high current requirement for engine cranking demands that the

batteries be designed with low resistance; for example, that conductors have large cross

sections and minimal lengths, that separators have maximum porosity and minimum backweb

thickness, and that the electrolyte be in the range of low resistance. The relationship between

plate surface area and CCA suggests the involvement of the electrochemical double layer of

the porous active materials. Low frequencies are generally necessary to evaluate the capac-

itive reactance component of battery impedance—strictly the resistance impedance compo-

nents can be evaluated by Ohm’s law by determining the voltage difference at two levels of

discharge current. The resistance of a lead-acid battery increases during a discharge almost

linearly with the decrease of the speciﬁc gravity of the electrolyte. The difference in resis-

tance between full charge and discharge is in the order of 40%. The effect of temperature

on the resistance of the battery is shown in Fig. 23.21; the battery resistance increases by

about 50% between 30 and

⫺18⬚C.

FIGURE 23.21 Comparison of lead-acid battery

designs. Effect of temperature on battery resistance.

Self-Discharge. A lead-acid battery loses capacity during open-circuit stand (self-

discharge). This loss is more severe with batteries which use antimonial lead grid alloys in

the positive plates. A comparison of the open-circuit stand loss of conventional antimonial

lead (

⬎4% Sb), low antimonial lead (⬍3% Sb), and nonantimonial lead grids is shown in

Fig. 23.6. This loss is most easily detected by a drop in the terminal voltage of the battery

and/ or the speciﬁc gravity. The sulfuric acid reacts, primarily on the surface of the negative

plate, in small local self-discharge ‘‘cells’’ where antimony and lead are in contact, becoming

a small particle of lead sulfate. In batteries using calcium-lead nonantimonial lead negative

and antimonial lead positive grids, self-discharge loss is minimized until antimony diffuses

to the negative. This is especially true for the low-antimonial alloy positives.

Life and Failure Modes. The life of SLI batteries is affected by the design, the processing,

and the operational environment of the battery. Because of the automated assembly methods

used today, SLI batteries are fairly consistent in life under ideal operating conditions, but

the wide variety of operating conditions tends to spread the failure distribution. Warranty

coverage for a failed battery is often more dependent on marketing strategy than on the

statistical expectations of the failure rate.

SLI battery design, materials, and operation have changed markedly in the past two dec-

ades; life and failure mode distribution have also changed. In Fig. 23.22a the average age

of failed batteries is plotted. Possible explanations for the shorter life in 1982 may be a

23.42 CHAPTER TWENTY-THREE

FIGURE 23.22 Failure modes of SLI batteries. (a) Average age of re-

turned batteries. (b) Failure mode of returned batteries. (From Amistadl.

)

reduction in battery size and more demanding performance requirements. These averages

include taxis, police cars, and other heavy-duty users, which account for the relatively low

age for failed batteries. Figure 23.22b shows the failure modes for these batteries, which are

described in more detail in Table 23.13. A higher incidence of short-circuited batteries used

in warmer climates suggests that grid corrosion is still a major failure mode. The ‘‘worn-

out’’ category includes low electrolyte level, and it should be noted that many maintenance-

free SLI batteries are sealed so that water, lost to evaporation and electrolysis, cannot be

replaced.

SLI batteries are not designed for deep-cycling service, and very short lives are generally

obtained with such operation. The deep-cycling capability of SLI batteries is covered in Sec.

23.5.2.

LEAD-ACID BATTERIES 23.43

TABLE 23.13 Summary of Failure Modes of Lead-Acid SLI

Batteries

1. Open circuits 4. Worn out

a. Terminal a. Worn out

b. Cell to terminal b. Undercharge

c. Cell to cell c. Low level (electrolyte)

d. Broken straps d. Terminal corrosion

e. Plates off e. Underformed

2. Short circuits 5. Serviceable

a. Plate to strap a. Serviceable

b. Plate to plate (plate fault) b. Discharged only

c. Plate to plate (separator fault) 6. Broken

d. Plate to plate (sediment /moss) a. Broken container

e. Vibration short circuit

3. Poor plates

a. Overcharge/overheat

b. Grid corrosion

c. Paste adhesion

d. Paste sulfation

e. Paste undeformed

b. Broken cover

c. Damaged terminal

d. Internal damage

e. Other

FIGURE 23.23 Correction factor for calculating cold-cranking Ampere (CCA) rating.

Standard Tests for Rating SLI Batteries. Several standard tests have been devised to eval-

uate and rate the performance of SLI batteries under conditions simulating the major re-

quirements of their applications. The cold-cranking Amperes (CCA) test evaluates the ca-

pability of the battery to deliver power to crank an engine at cold temperatures. The

cranking-test rating is the current that a fully charged battery can deliver at

⫺17.8⬚C for

30 s to a voltage of 1.2 V per cell. If the measured voltage is above or below this value at

30 s, the CCA value can be calculated by multiplying the discharge current by the correction

factor shown in Fig. 23.23. Figure 23.17b illustrates the performance of a 70-Ah cell with

a CCA rating of 550 A.

23.44 CHAPTER TWENTY-THREE

Reserve capacity is measured in a test of the battery’s ability to provide power for lights,

ignition, and the auxiliaries. The reserve capacity is deﬁned as the number of minutes a fully

charged battery can maintain a current of 25 A to 1.75 V per cell at 25

⬚C.

Other SLI tests are included in the SAE battery test standard J 537 on charge rate ac-

ceptance, overcharge life, and vibration resistance. A standard SLI life test is speciﬁed in

SAE J240A. This test consists of a shallow discharge at 25 A followed by a brief charge at

voltage and current limits for 10 min.

23.4.4 Cell and Battery Types and Sizes

SLI battery sizes have been standardized by both the automotive industry through the Society

of Automotive Engineers (SAE), Warrendale, Pa., and the battery industry through the Bat-

tery Council International (BCI), Chicago.

The BCI nomenclature follows the standards

adopted by its predecessor, the American Association of Battery Manufacturers (AABM).

The latest standards are published annually by the Battery Council International (BCI).

21,22

Internationally, standardization is handled by the International Electrotechnical Commission

(IEC). More detailed information on these standards is found in Chapter 4, Sec. 4.10 and

Table 4.10.

23.5 DEEP-CYCLE AND TRACTION BATTERIES: CONSTRUCTION

AND PERFORMANCE

23.5.1 Construction

The prime requirement for deep-cycling batteries for traction applications is maximum cycle

life, then, if possible, high energy density and low cost. In an electric forklift application,

in fact, light weight may not be advantageous because the battery’s weight usually is needed

to counterbalance the payload. The life of these batteries is improved by the use of thick

plates with high paste density, usually a high-temperature and high-humidity cure, low elec-

trolyte density formation, premium separators, and one or more layers of glass ﬁber matting

(to retain the active material in the positive plates). The major modes of failure are disin-

tegration of the PbO

positive active mass and corrosion of the positive grids. The deep-

cycling battery is usually designed to be capacity-limited when new by the amount of elec-

trolyte and not by the material in the plates. This serves to protect the plates and maximize

their life. Both negatives and positives are degraded during use, but at end of life, battery

capacity is generally limited by the positive plate. Battery failure, for cycle life rating pur-

poses, is considered to occur when the battery will no longer produce 60–80% of its initial

or rated discharge capacity.

A typical traction battery, using ﬂat-pasted plates, is shown in Fig. 23.24. Cells are always

made with an outside-negative design (e.g., n positive plates, n

⫹ 1 negative plates). Deep-

cycling traction batteries are built as an assemblage of individual cells. If the battery’s per-

formance is limited by a catastrophic failure of one (or a few) cell(s), then those cells can

be repaired or replaced in a cost-effective manner. Power requirements vary widely with the

load, distance traveled, and lifting or climbing requirements. Battery sizes are determined

by the forklift truck manufacturer and can be ‘‘calibrated’’ in the actual application by the

use of an Ampere-hour meter. A rough indication of the suitability of a traction battery for

an application is the change in the speciﬁc gravity of the electrolyte during use. A larger

battery size (or battery replacement or repair) is indicated when full operation cannot be

achieved.

LEAD-ACID BATTERIES 23.45

FIGURE 23.24 Flat-pasted-plate lead-acid traction battery. (Courtesy of C&D Technolo-

gies.)

Although the ﬂat-pasted (Faure) positive plate is typical for deep-cycling batteries in the

United States, some cycling batteries in the United States and most cycling batteries in the

rest of the world are built with tubular or gauntlet-type positives (Fig. 23.25). The tubular

construction minimizes both grid corrosion and shedding, and long life is characteristic of

these designs, but at a higher initial cost. Flat-pasted negative plates are used in conjunction

with these positives and the cells are of the outside-negative design.