BATTERY

Batteries are electrochemical devices which are widely used to supply energy for electrical and electronic products. Chemical energy stored in a battery is converted into electric current when the battery is discharged. This electric current is produced directly by chemical reactions within the battery. Many combinations of chemicals and metals have been tried (as storage systems), with varying degrees of success. Each type of battery has its advantages and disadvantages with regard to its physical and electrical characteristics. Energy density, expressed in watt-hours per pound or watt-hours per cubic inch., are important measures of merit for a battery system. Some battery types are not rechargeable but are used once and thrown away. Other types are reusable and rechargeable, such as the lead acid type.

The battery is the source of energy for the electric truck. A basic battery cell consists of two different metals placed in an acid solution. By chemical action, electrons are transferred from one metal plate to the other. A difference in potential energy (or voltage) is created between the two plates because one now has an excess of electrons, while the other has a shortage. When a circuit consisting of motors, contactors, or other electrical devices is connected between the two plates, electrons will flow in a direction to achieve a balance.

The rechargeable lead acid battery has been found most suitable for lift truck operations. In these batteries, one plate is lead and the other is lead-dioxide. The solution, or electrolyte, separating the two plates is diluted sulfuric acid. Lead acid batteries are used because they provide the optimum balance of: (1) energy output per pound: (2) cost; (3) capability of numerous recharging cycles; and (4) ability to withstand abuse. The voltage of a lead acid cell IS about 2.1 volts.

GEL BATTERIES Lead acid batteries are “flooded” with electrolyte that bubbles as electrons flow during the charge and discharge process. The gaseous portions of these bubbles are released into the atmosphere through vent caps. The venting lowers the electrolyte, necessitating routine inspection of the cells and periodical addition of water to the cells. A few battery manufacturers offer batteries in which the electrolyte is not a pool but is immobilized as a gel or in a porous media. They allege that the immobilization of the electrolyte increases power, eliminates gassing and allows the cells to be sealed. Sealing the cells eliminating both the need to add water and the possibility of acid spills.

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LEAD ACID�BATTERY

DESCRIPTION

When the battery’s terminals are connected to an external load, a stream of electrons flows from the positive lead peroxide plate through the sulfuric acid electrolyte to the negative lead plate. In this process the lead peroxide gives up oxygen, which combines with hydrogen in the electrolyte to form water. Sulfate in the electrolyte combines with both positive and negative plates to form lead sulfate. The reaotion continues until both positive and negative plates achieve a relatively similar lead sulfate composition. Because of the loss of sulfates and the additional water, the electrolyte solution is more dilute (watery). The battery is then said to be fully discharged, and its voltage is too low for practical use.

A battery is made up of a number of cells connected in series, each cell having a nominal, or approximate, voltage of 2 volts. Thus a 36-volt battery has 18 cells; a 48-volt battery has 24 cells.

As stated before, the basic battery is composed of two plates separated by an electrolyte of diluted sulfuric acid. In actual practice, however, a cell is made up of a number of plates in parallel, as illustrated in Figure There is always one more negative plate than positive plate so that both sides of each positive plate will be used in producing energy.

Each plate is constructed of a lead-antimony grid to which a paste made of the active material is added. The active material is that which produces battery action; lead paste for the negative plate and lead-dioxide paste for the positive plate.

Figure 1 : Connection of Plates in a Lead Acid Cell

Highly porous insulation material is placed between the plates to provide physical separation without interfering with the chemical action. All positive plates are electrically connected to each other as are the negative plates. The whole assembly is placed in a high-impact case made of insulating material.

During the life of a battery, some of the active material of the plates goes into the solution and settles at the bottom of the cell case. To prevent this

CONSTRUCTION

OPERATION

ICE BATTERY

ELECTRIC LIFT TRUCK

BATTERY

RATING

sediment from short-circuiting the plates, a support is placed between the plates and the bottom of the case.

Two terminals are brought out of the top of each cell to which external connections are made. Figure 2 shows how a number of cells could be connected to make a 36-volt battery.

Figure 2: Cell Connection for 36- Volt, 18 Cell Battery

A filler cap is located in the top of each cell through which the sulfuric acid is initially introduced and through which water is added prior to charging. A typical battery construction is shown in Figure 3.

The internal combustion powered trucks use an automotive type battery for starting. These trucks also have a charging system similar to an automobile. This battery is a 12-volt, 6 cell lead acid type.

The electric lift truck battery is the source of all of the truck’s power. It provides the energy needed to perform lifting, steering, and travel functions. Typically, the batteries for Electric Rider trucks are 24, 36, or 48 volts. The batteries used in Narrow Aisle and Motorized Hand Trucks are usually 12, 24, or 36 volts.

When a battery is delivering current to a circuit, it is being discharged. When a certain state of discharge is reached, any further discharge will only cause damage to the battery. The amount of electricity that can be supplied in one discharge cycle is expressed in ampere-hours. The ampere-hour rating is governed by the size and number of plates. A 23-plate battery will have 11 positive plates. If each positive plate is rated at 75 ampere-hours, the battery will have a rating of 75 x 11 = 825 ampere-hours. Battery plates in motive

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power application have ratings from 55 to 160 ampere-hours per positive plate.

The voltage of a battery is nominally 2 volts multiplied by the number of cells in series. In actual usage, the cell voltage varies from 2.1 volts for a fully-charged cell to 1.75 for a discharged cell. Thus, a 36-volt battery will measure from 37.8 to 31.5 volts over its useful range.

Motive power batteries are typically rated in amp-hours, usually for a 6-hour discharge period. This means a 600 amp-hour rated battery, if fully charged to begin with, could deliver 100 amperes for 6 hours if it is allowed to fully discharge.

Figure 3: Typical Battery Construction

Motive power cells are rated at For a given discharge rate, at operating temperatures higher than F, the capacity is greater, and at lower temperatures, less.

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F,

LEAD BUSHING

CELL COVER

POSITIVE PLATE

SEPARATOR

VENT AND FILLER CAP

NEGATIVE PLA TE

TERMINAL POST

CONTAINER

Figure 4: Motive Power Cell

40 0

2.' O

2.00

1.90

1

1.70

Typical Voltage Profile When Discharged at 6-Hour Rate at 77° F

1 2 4 5 6

Time, Hours

Under normal conditions, motive power cells are operated at ambient temperatures between 50° and 90°. Figure 5 shows the percentage variation of capacity at the 6-hour discharge rate versus temperature between 50° and 110° F. The graph shows that at the 6-hour rate, the capacity becomes zero when the electrolyte temperature reaches -38°F. At discharge rates lower than the 6-hour rate, the capacity would be greater throughout the entire temperature range.

Figure 5. Motive Power Battery

Capacity as a Percentage of 6-Hour-Rate Capacity vs. Temperature

10 70 90 110

Temperature,

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Volta

ge, V

olts

3 0

Perc

ent o

f 6-H

our-R

ate

Cap

acity

-50 -30 -10 30 50

Motive power batteries are specifically designed for, and are generally used at, the 6- or 8-hour discharge rate. They can also be discharged at other rates, with the capacity varying with the discharge rate. Figure 6 shows the relationship between discharge rate and capacity.

Because the capacity and operating voltage are affected by the discharge rate, so are available energy and energy densities. As the discharge rate increases, the energy content is reduced. The relationship between energy and discharge rate is illustrated in Figure 7.

Figure 6. Motive Power Battery Capacity as a Percentage of 6-Hour-Rate

Capacity vs. Time

120

60

40

20 7 8 9 10

Time, Hours

Figure 7. Motive Power Battery

Energy Ouput as a Percentage of 6-Hour-Rate Capacity vs. Time

00

80

60

40

8 9 10

Time, Hours

Perc

ent o

f 6-H

our-R

ate

Cap

acity

80

0 1 2 3 4 5 6

0 1 2 3 4 5 6 7

Perc

ent o

f 6-H

our-R

ate

Cap

acity

A lead acid battery is capable of being charged and used again. To charge a battery, it is necessary to supply electric current to it from a power source having a voltage somewhat higher than the battery voltage so that the direction of current flow is opposite to that during discharge. Because of losses, it is usually necessary in charging to supply 20% more ampere-hours than were removed during discharge.

Charging of a battery creates heat within the cells. It is important to limit this heat to a safe level to prevent internal damage to the battery. This is accomplished by limiting the charging current. The recommended starting rate is .225 amperes for each ampere-hour of capacity.

The recommended finishing rate is .05 amps per ampere-hour. A 600 ampere- hour battery should have an initial charging current of .225 x 600 or 135 amps and a final charging current of .05 x 600 or 30 amps.

During battery charging, chemical action is reversed; the sulfuric acid content of the electrolyte increases the specific gravity. Cell voltage increases also. When the charge is approximately 80% complete, a sharp rise can be observed in the cell voltage. This occurs at about 2.35 charging volts per cell. At this time “gassing” occurs; oxygen and hydrogen are released. Because of the explosive nature of these gases, no open flames should be allowed in a

charging area. When gassing starts, the charge rate should be reduced.

The energy of a battery or, the work that it can perform, is measured in kilowatt-hours. The kilowatt-hour rating of a battery is the product of the voltage multiplied by the ampere-hours and divided by 1000. A 36-volt, 600 ampere-hour battery can deliver 36 x 600/1000 = 21.6 kilowatt-hours (abbreviated KWH). One kilowatt-hour is equivalent to 2,654,000 ft./lbs. of work or 1.34 horsepower hours.

If a battery is discharged beyond a certain limit, damage to the plates will result and battery life will be severely shortened. Optimum battery life is achieved if the battery is never discharged below 80% of capacity. It is important, therefore, to be able to measure the state of charge of a cell.

As a cell is discharged, its voltage decreases. As illustrated in Figure 6, a fully- charged cell measures about 2.1 volts; a discharged cell measures about 1.75 volts. Measuring the battery voltage, then, provides a means of determining state of charge. Although not as accurate as other means, it is a convenient measurement. It is used in most discharge indicators found on fork lift trucks. In these devices, when the battery voltage reaches a certain level, a light is energized to signal the operator. Additionally, some trucks have a “lift interrupt” circuit which prevents hoist motor operation when the battery is 80% discharged.

A more accurate measure of state of charge is specific gravity, often referred to as s.g. As a cell is discharged, the acid content of the electrolyte is decreased and the water content increased. Specific gravity is a measure of the ratio of the weight of a given amount of electrolyte to the weight of the same amount of water. At full charge, the specific gravity is usually 1.280 (sometimes referred to as s.g. of twelve-eighty). A discharged cell would have a s.g. of 1.120. Specific gravity is measured with a device called a hydrometer (Figure 8). Electrolyte is drawn up into the glass tube which contains a float. The float has a graduated scale on its stem. For high specific gravity liquids the float will ride

BATTERY CHARGING

STATE OF CHARGEDISCHARGE

high in the solution. The specific gravity can be read from the scale at the surface of the solution.

Specific gravity changes with temperature and a correction must be made if the temperature is different from that for which the hydrometer was calibrated, usually 80°F. For each 10°F above this temperature, 0.004 must be added to the hydrometer reading. For each 10°F below this, 0.004 must be subtracted.

Figure 8: Hydrometer

Hydrometer

- SPECIFIC GRAVITY

- LIQUID LEVEL

SCALE

FLOAT

Self-discharge of a wet, fully-charged, fresh battery is about 7 to 10 percent per month at room temperature. For dry-charged batteries, the self-discharge rate is between 2 and 4 percent per month. Because self-discharge is accelerated at higher temperatures, wet batteries should be stored at room temperature or below whenever possible. Dry-charged batteries should also be stored in cool areas and at low humidity to minimize self-discharge.

A fully charged battery left on open-circuit long enough to have a specific gravity drop of 0.025 points should be charged at the finishing rate until the specific gravity remains constant for 4 hours. As explained in the Battery Charging section, the finishing rate is about .05 amps per ampere hour. A low charging rate is necessary because of the formation of a hard, gritty sulfate on the negative plate, a condition which closes the pores of the plate. Charging becomes very difficult when a battery is stored in a discharged state for a long time. To avoid such problems, batteries should be charged as soon as possible after a discharge, and not be left in a discharged condition for more than 24 hours, or below 32° F.

SELF-DISCHARGE

BATTERY CHARGERS

There are two basic types of chargers: constant potential and constant current. In the constant potential charger, a fixed voltage is applied to the battery. The charging current is determined by the difference between battery voltage and charger voltage and the resistance of the circuit between them. As the battery voltage increases during charging, the charging current decreases. As it nears the end of charge, the current reduces to the finishing rate. Often this is done when the gassing voltage is sensed, the current is then changed abruptly to the finishing rate.

The constant current charger provides a fixed charging current, starting at about the recommended starting rate and cutting back to the finishing rate when gassing voltage is sensed. Both types of chargers are equipped with timers to prevent excess charging. Usual charging time is 8 hours. An occasional 12 hour charge at a low rate is recommended to equalize the charge between cells.

The following is a list of items for proper battery care:

1. Keep batteries clean and free of acid spills.

2. Do not over-discharge. Excessive discharge shortens the cycle life.

3. Do not overcharge. Overcharging produces corrosion of positive grids and excessive gassing, which loosens the active material of the plates.

4. Charge batteries in a well-ventilated area to remove the explosive gases and acid fumes.

5. Maintain electrolytes at the proper level. If low, add water. Before charging, make sure tops of plates are covered; after charge, fill to recommended level. Do not add acid.

6. Keep batteries from freezing.

7. Keep batteries in a charged state.

8. Do not charge a battery from a charger with an ampere-hour rating higher than that of the battery. This will give too high a charging current and excessive heating. Charging from a lower ampere-hour charger will cause no harm, but may require longer than 8 hours to fully charge.

9. When moving batteries, be careful not to short-circuit battery terminals.

The battery discharge indicator is basically a fuel gauge for the electric truck. It indicates the state of charge and warns the operator when the battery is in need of recharging. Some BDIs have the option of being wired to “lock out” the lifting functions of the truck. This feature is know as “lift interrupt.” When lift interrupt is engaged, the operator must take the truck to a charging station.

CONSTANTPOTENTIAL

CONSTANTCURRENT

BATTERY CARE

BDI OPTION

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The benefits of the BDI option are as follows:

Protection of equipment, cargo, and operator from damage associated with discharged or weak batteries.

2. Extension of battery life by warning the operator to recharge the batteries when they are 80% discharged.

3. Sustained productivity by warning the operator at 75% discharge level-- before lock out occurs.

4. Continuous usage sustained by checking the no load voltage of the battery. The BDI system checks to see if a newly installed battery is, in fact, properly charged and, if so, indicates a full battery,

As stated, battery capacity is usually measured in ampere-hours. By multiplying the rate of flow in amperes by the time in hours, one can determine the battery's capacity; the quantity of electricity the battery is capable of delivering before it needs recharging. Industrial truck battery capacities are figured at a 6 or 8 hour discharge rate. For example, if a battery is rated at 640 AH (ampere-hours), it will deliver 80 amperes of work power for 8 hours.

To accurately size a battery, one must analyze the job to be accomplished and then, after all calculations are finished, add 25% extra kilowatt-hours to protect the battery from over-discharging. Truck batteries should never be discharged past 80% of their rated capacity.

to the flow of water from one tank to another.

HIGH PRESSURE GIVES VELOCITY (DISTANCE)HIGH VOLTAGE GIVES SPEED

To select the proper battery, determine the following:

1. Weight of truck, with battery. 2. Average weight of load handled. 3. Average length of one-way trip in feet, including ramps. 4. Elevation in feet of any ramps negotiated 5. Number of trips per hour. 6. Average lifting height. 7. Number of lifts per hour. 8. Attachment type, if any.

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1.

Dischage of a storage battery is similar

GALLONSNEEDED equalsAMP HRS.NEEDED

BATTERYSELECTION

SAMPLE BATTERY CALCULATION

TRAVEL POWER

COMPUTATION

Assume a Yale 4000 Ib. capacity electric truck will make 20 trips per hour over a one-way average distance of 200 ft.; during the trip, a ramp with a 5 ft. rise must be negotiated with the average load weight. The average lift is 12 feet, and a sideshifter is utilized. The truck with its standard battery, Triplex mast and sideshifter weighs 10,000 pounds, and the average load is 3,000 pounds.

NOTE: The number of trips per hour is determined by the speed of the truck. Higher voltages on a given truck will produce higher speeds and more trips per hour. Thus, a 48 volt truck will make 25% more trips per hour in a given work shift than a 36 volt truck.

Refer to Figure 9. Locate 10,000 pounds on the left side of the chart and follow horizontally to the diagonal 200 foot “distance” line. Read straight down to the watt hour line which should indicate about 38 watt hours. Do the same for the return trip with the average load handled increasing the total truck and load weight to 13,000 pounds. Watt hours reading is 47. Total round trip travel power is 85 watt hours.

Figure 9. Travel Power

LIFT, TILT AND ATTACHMENT

POWER COMPUTATION

According to the example, an average of 3,000 pounds is lifted an average of 12 feet with a sideshifter. Follow along the 3,000 pound line from the “Pay Load” side to its intersection with the 12 foot diagonal lift line. To allow for the sideshift function add 2 more feet for a total of 14 feet. Read down directly. It should read about 43 watt-hours.

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RAMP POWER

Figure 10. Lift, Tilt, and Attachment Power

The example includes a ramp with a 5-foot rise to be climbed with the (In figure. 11) ramp power computation average load. In figure 11, we find 13,000 pounds and follow over to the 5-foot “elevation of ramp” diagonal, then down to the watt hour line. The chart reads about 42 watt-hours.

Figure 11. Ramp Power

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From the three charts, it is known the trips will consume 85 (travel), plus 43 (lift, tilt and attachment) plus 42 (ramp) for a total of 170 watt hours. Since the assumption is 20 trips per hour, an eight hour shift will result in 160 trips. Thus, in this eight hour shift, the truck will use 27,200 watt hours (160 x 170). Dividing by 1000 to convert KWH (1000 watt hours equal 1 KWH) equals 27.2 KWH. This represents the minimum battery that can be utilized to do the work the example requires. KWH should be likened to the work capacity or fuel capacity. When it is depleted, a refill (or in the case of a battery, a recharge) is required. But don’t stop here. Refer to the over-discharge factor to complete the example.

For attachments, an allowance must be added to the average lift that is involved and calculated as part of the lift and tilt power. The following additions reflect 100% utilization of the attachment, or, for instance, a clamping and unclamping operation during each cycle. If this is not the case, and the attachment is used only on alternate cycles, or half the time, then the additions should be adjusted proportionately (in this case by 50%).

The table below specifies attachments and the allowance necessary.

Attachment Allowance

Sideshifter 2 feet Clamp feet Sideshift Clamp 7 feet Rotator 10 feet

12 feet Rotating Clamp 15 feet

Items like power steering and dual hydraulic pump motors are already factored in. If other than standard low consumption electric compound rubber or poly tires are used, up to 20% must be added to the travel power figure obtained from the power chart.

After all these calculations are complete, 25% extra KWH should be added. This is to protect the battery from over-discharging as batteries should not be discharged past 80% of their rated capacity, or 1.130 specific gravity. At this point, the discharge indicator activates as well as the lift interrupt system, if employed. When the 25% factor is added to the above example, the KWH requirement becomes 34.0 (1.25 x 27.2).

To convert the required KWH to ampere-hours, first multiply the KWH by 1000 to get watt-hours. Second, divide the watt-hours by the battery voltage.

For example:

34 KWH x 1000 = 34.000 watt-hours

34,000 watt - hours 36 volts

= 944 ampere - hours

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BATTERYREQUIREMENTS

EQUIPMENT FACTORCOMPUTATION

OVERDISCHARGEFACTOR

Push/Pull

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