Duracell Ni-MH User Manual

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Ni-MH Rechargeable Batteries

able of Contents

1 Introduction

2 General Characteristics

3 Composition and Chemistry

3.1 Active Components: Positive and Negative Electrodes

3.2 Electrolyte

3.3 Cell Reactions

4 Battery Construction

.1 Basic Cell Construction

4.2 Cylindrical Cell Construction

4.3 Prismatic Cell Construction

5.1 General Characteristics

5.2 Discharge Characteristics: Effect of Discharge Rate

and Temperature

5.3 Capacity: Effect of Discharge Rate

and Temperature

5.4 Energy Density

5.5 Constant Power Discharge Characteristics

5.6 Polarity Reversal During Overdischarge

5.7 Internal Impedance

5.8 Self-Discharge and Charge Retention

5.9 Voltage Depression (“Memory Effect”)

6.1 General Principles

6.2 Techniques for Charge Control

6.2.1 Timed Charge

6.2.2 Voltage Drop (-∆V)

6.2.3 Voltage Plateau (zero ∆V)

6.2.4 Temperature Cutoff

6.2.5 Delta Temperature Cutoff (∆TCO)

6.2.6 Rate of Temperature Increase (dT/dt)

6.3 Charging Methods

6.3.1 Duracell’s Recommendation:

Three-Step Charge Procedure

6.3.2 Low-Rate Charge

6.3.3 Quick Charge

6.3.4 Fast Charge

6.3.5 Trickle Charge

6.4 Thermal Devices

5 Performance Characteristics

6 Charging Sealed Nickel-Metal Hydride Batteries

7 Cycle and Battery Life

7.1 Cycle Life

7.2 Battery Life

8 Safety Considerations

9 Proper Use and Handling

9.1

Care and Handling

9.2 Transportation

9.3 Waste Management: Recycling and Disposal

Ni-MH Rechargeable Batteries

Introduction

Rapid advancements in electronic technology have expanded the number of battery-powered portable

devices in recent years, stimulating consumer demand for higher-energy rechargeable batteries capable of

delivering longer service between recharges or battery replacement.

The trend towards smaller, lighter more portable battery-powered devices is expected to continue well

into the future, with the so-called “3C” applications — cellular phones, portable computers and consumer

electronics — expanding rapidly beyond the traditional business user and into the consumer marketplace.

As with other battery-powered consumer devices, battery performance and convenience will influence

the rate of consumer acceptance for 3C devices. Yet conventional rechargeable batteries often fail to meet the

needs of consumers, as well as equipment designers, in terms of their size and weight, operating time, ease-

of-use, availability and environmental acceptability. New battery systems are needed to meet their growing list

of demands.

The sealed nickel-metal hydride (Ni-MH) battery is one rechargeable battery system that is responding

to these demands by offering significant improvements over conventional rechargeable batteries in terms of

performance and environmental friendliness. First introduced to the commercial market in 1988, nickel-metal

hydride battery technology is at a very early stage of maturity and manufacturers such as Duracell have

identified many opportunities to improve battery performance. These improvements will make DURACELL

nickel-metal hydride batteries an attractive power source for 3C devices for many years to come.

General Characteristics

Many of the operating characteristics of the sealed nickel-metal hydride rechargeable battery are simi-

lar to those of the sealed nickel-cadmium rechargeable battery. The nickel-metal hydride battery, however, has

the advantage of higher energy density (or capacity) which translates into longer service life. In addition, the

nickel-metal hydride battery is environmentally friendlier than nickel-cadmium and other battery systems

because it contains no added cadmium, mercury or lead.

Features of the sealed nickel-metal hydride battery include:

•

Higher capacity — Up to 40 percent longer service

life than ordinary nickel-cadmium batteries of

equivalent size.

•

High rate discharge — Efficient discharge at rates

as high as 2C.

•

Fast charge — Can be charged in approximately

one hour.

•

Safe — Designed to safely withstand abusive

conditions in consumer devices.

•

Long cycle life — Up to 500 charge/discharge cycles.

•

Performs at extreme temperatures — Capable of

operation on discharge from -20°C to 50°C (-4°Fto

122°F) and charge from 0°C to 45°C (32°F to

113°F).

•

Environmentally friendlier than nickel-cadmium

batteries — Zero percent cadmium.

•

Similar operating voltage to nickel-cadmium

batteries — Allows user to upgrade easily to longer

lasting nickel-metal hydride batteries.

Ni-MH Rechargeable Batteries

Composition and Chemistry

A rechargeable battery is based on the principle that the charge/discharge process is reversible, that is, the

energy delivered by the battery during discharge can be replaced or restored by recharging.

Nickel oxyhydroxide (NiOOH) is the active mate-

rial in the positive electrode of the nickel-metal hydride

battery in the charged state, the same as in the nickel-

cadmium battery.

The negative active material, in the charged state,

is hydrogen in the form of a metal hydride. This metal

alloy is capable of undergoing a reversible hydrogen

absorbing/desorbing reaction as the battery is charged

and discharged, respectively.

The unique attribute of the hydrogen storage

alloy is its ability to store hundreds of times its own

volume of hydrogen gas at a pressure less than atmos-

pheric pressure. Many different intermetallic com-

pounds have been evaluated as electrode materials for

nickel-metal hydride batteries. Typically, these fall into

two classes: AB

alloys, of which LaNi

is an example,

and AB

alloys, of which TiMn

or ZrMn

are examples.

DURACELL nickel-metal hydride battery technol-

ogy is based on the use of AB

instead of AB

alloys.

alloys offer better corrosion resistance characteris-

tics, resulting in longer cycle life and better recharge-

ability following storage. The composition of the metal

alloy is formulated for optimal stability over a large

number of charge/discharge cycles. Other important

properties of the alloy include:

•

Large hydrogen storage capability for high energy

density and battery capacity.

•

Favorable kinetic properties for high rate capability

during charge and discharge.

•

Low hydrogen pressure alloy and high purity mate-

rials to minimize self-discharge.

An aqueous solution of potassium hydroxide is

the major component of the electrolyte of a nickel-

metal hydride battery. A minimum amount of elec-

trolyte is used in this sealed cell design, with most of

this liquid being absorbed by the separator and the

electrodes. This “starved electrolyte” design facilitates

the diffusion of oxygen to the negative electrode at the

end-of-charge for the “oxygen recombination” reaction.

During discharge, the nickel oxyhydroxide is

reduced to nickel hydroxide

NiOOH + H

O + e

——> Ni(OH)

+ OH

and the metal hydride (MH) is oxidized to the metal

alloy (M).

MH + OH

——> M + H

O + e

The overall reaction on discharge is:

MH + NiOOH ——> M + Ni(OH)

The process is reversed during charge.

3.3 Cell Reactions

3.2 Electrolyte

3.1 Active Components: Positive and Negative Electrodes

The sealed nickel-metal hydride cell uses the

“oxygen-recombination” mechanism to prevent a build-

up of pressure that may result from the generation of

oxygen towards the end of charge and overcharge.

This mechanism requires the use of a negative electrode

(the metal hydride/metal electrode) which has a higher

effective capacity than the positive (nickel oxyhydrox-

ide/nickel hydroxide electrode) electrode. A schematic

drawing of the electrodes is shown in Figure 3.3.1.

During charge, the positive electrode reaches

full charge before the negative electrode which causes

the evolution of oxygen to begin:

2OH

- _____

> H

O +

+ 2e

The oxygen gas diffuses through the separator

to the negative electrode, a process which is facilitated

by the “starved-electrolyte” design and the selection of

an appropriate separator system.

At the negative electrode, the oxygen reacts

with the metal hydride and oxidizes or discharges the

metal hydride to produce water:

2MH +

_____

> 2M + H

Thus, the negative electrode does not become fully

charged and pressure does not build up.

The charge current, however, must be con-

trolled at the end of charge and during overcharge to

limit the generation of oxygen to below the rate of

recombination. Thus, charge control is required to pre-

vent the build-up of gases and pressure. Duracell rec-

ommends that continuous overcharge not exceed C/300

for optimal performance.

As shown in Figure 3.3.1, the nickel-metal

hydride cell is designed with a discharge and charge

reserve in the negative electrode. The discharge

reserve minimizes gassing and degradation of the cell in

the event of overdischarge. The charge reserve ensures

that the cell maintains low internal pressure on over-

charge.

The negative electrode has excess capacity

compared to the positive electrode and is used to

handle both overcharge and overdischarge. Thus,

the useful capacity of the battery is determined by

the positive electrode.

Ni-MH Rechargeable Batteries

Charge 

Reserve

Schematic representation of the electrodes, divided

into useful capacity, charge reserve and discharge

reserve.

NiOOH/Ni(OH)

Positive Electrode

FIGURE 3.3.1

Negative Electrode

Useful Capacity

Discharge 

Reserve

MH/Alloy

Composition and Chemistry (cont.)

Ni-MH Rechargeable Batteries

Battery Construction

DURACELL standard-sized nickel-metal hydride batteries are constructed with cylindrical and prismatic nickel-

metal hydride cells. DURACELL nickel-metal hydride batteries are a sealed construction designed for optimal perfor-

mance and maximum safety. The batteries are manufactured to strict quality control standards to ensure reliability

and consumer satisfaction and offer such features as:

•

High energy density — Minimizes battery volume

and weight

•

Wide voltage range — Meets operating voltage

requirements of 3C devices

•

Thin profiles — Innovative wall-less design

•

Advanced interconnect — Self securing, voltage-

keyed interconnect provides a highly reliable battery-

to-device contact

•

Durability — Manufactured with LEXAN

and

LUSTRAN

polycarbonate high impact and flame

retardant polymers

•

UL listing — Independent approval of battery use

in devices

4.1 Basic Cell Construction

4.2 Cylindrical Cell Construction

The electrodes in both cylindrical and prismatic

cell configurations are designed with highly porous

structures which have large surface areas to provide low

internal resistance which results in superior high rate

performance. The positive electrode in the cylindrical

nickel-metal hydride cell is a highly porous nickel-felt

substrate into whichthe nickel compounds are pasted.

Similarly, the negative electrode is a perforated nickel-

plated steel foil onto which the plastic-bonded, active

hydrogen storage alloy is coated.

The assembly of a cylindrical cell is shown in

Figure 4.2.1. The electrodes are separated by the sepa-

rator which is a synthetic, non-woven material that

serves as an insulator between the two electrodes and as

a medium for absorbing the electrolyte. The electrodes

are spirally-wound and inserted into a cylindrical nickel-

plated steel can. The electrolyte is added and contained

within the pores of the electrodes and separator.

The positive electrode is connected to the metal

lid with a tab. The cell is then sealed by crimping the

top assembly to the can. The top assembly incorporates

a resealable safety vent, a metal lid and a plastic gasket.

A heat-shrink tube is placed over the metal can. The

bottom of the metal can serves as the negative terminal

and the metal lid as the positive terminal. The insulator

and gasket insulate the terminals from each other. The

vent provides additional safety by releasing any excess

pressure that may build up if the battery is subjected to

abusive conditions.

FIGURE 4.2.1

LEXAN

is a registered trademark of the General Electric Company.

LUSTRAN

is a registered trademark of the Monsanto Company.

Metal Can

Separator

Negative Electrode

Insulator

Gasket

Metal Lid

Cosmetic Disk

Heat Shrink Tube

Positive Electrode

Positive Tab

(-) Negative Terminal

(+) Positive Terminal

Safety Vent

Ni-MH Rechargeable Batteries

The basic differences between the prismatic

ell and the cylindrical cell are the

construction of

the electrodes and the shape of the

can. Prismatic

cells are designed to meet the needs

of compact

equipment where space for the battery is

limited.

The rectangular shape of the prismatic cell

permits

more efficient battery assembly by eliminating

the

voids that occur in a battery constructed with

cylindrical cells. Thus, the volumetric energy density

of a battery can be increased by constructing it with

prismatic instead of cylindrical cells.

Figure 4.3.1 shows the structure of the pris-

matic nickel-metal hydride cell. The electrodes are

manufactured in a manner similar to those of the

cylindrical cell, except that the finished electrodes are

flat and rectangular in shape. The positive and

negative electrodes are interspaced by separator

sheets. The assembly is then placed in a nickel-plated

steel can and the electrolyte is added. The positive

electrodes are connected to the metal lid with a tab.

The cell is then sealed by crimping the top assembly to

the can. The top assembly incorporates a resealable

safety vent, a metal lid and a plastic gasket that is

similar to the one used in the cylindrical cell. A heat-

shrink tube is placed over the metal can. The bottom

of the metal can serves as the negative terminal and

the top metal lid as the positive terminal. The insula-

tor and gasket insulate the terminals from each

other. The vent provides additional safety by re-

leasing any excess pressure that may build up if the

battery is subjected to abusive conditions.

4.3 Prismatic Cell Construction

FIGURE 4.3.1

Battery Construction (cont.)

Metal Lid

Gasket

Insulator

Positive Tab

Cosmetic

Disk

Safety Vent

Positive Electrode

Negative Electrode

(-) Negative Terminal

Separator

(+) Positive Terminal

Heat Shrink Tube

Ni-MH Rechargeable Batteries

5.1 General Characteristics

The discharge characteristics of the nickel-metal

hydride cell are very similar to those of the nickel-

cadmium cell. The charged open circuit voltage of both

systems ranges from 1.25 to 1.35 volts per cell. On

discharge, the nominal voltage is 1.2 volts per cell and

the typical end voltage is 1.0 volt per cell.

Figure 5.1.1 illustrates the discharge character-

istics of nickel-metal hydride and nickel-cadmium

rechargeable cells of the same size. As shown, the volt-

age profile of both types of cells is flat throughout most

of the discharge. The midpoint voltage can range from

1.25 to 1.1 volts per cell, depending on the discharge

load. Figure 5.1.1 can also be used to compare the

capacity of the two rechargeable types. Note that the

capacity of the nickel-metal hydride cell is typically up to

40 percent higher than that of a nickel-cadmium cell of

equivalent size.

5.2 Discharge Characteristics: Effect of

Discharge Rate and Temperature

Typical discharge curves for DURACELL nickel-

metal hydride batteries under constant current loads at

various temperatures are shown in Figures 5.2.1 to

5.2.3. Discharge voltage is dependent on discharge

current and discharge temperature.

FIGURE 5.1.1

1.5

1.4

1.3

1.2

1.1

1.0

Voltage (V)

Ampere-Hour Capacity (%)

Comparison of discharge voltage and capacity of

same-size Ni-MH and Ni-Cd cells. 

[Conditions: Charge: C/3 for 5 hours, Temperature: 21°C (70°F)]

0 20 40 60 80 100 120 140 160

Ni-MH

C/5

Ni-Cd

Performance Characteristics



FIGURE 5.2.1

Voltage (V)

Discharge Capacity (Ah)



FIGURE 5.2.2

Discharge Capacity (Ah)

Voltage (V)

C (2.4A)

Voltage (V)

Discharge Capacity (Ah)

8.5

8.0

7.5

7.0

6.5

6.0

5.5

0 0.5 1.0 1.5 2.0 2.5

C/5 (0.48A)

C (2.4A)

FIGURE 5.2.3

Voltage and capacity of DURACELL DR30 Ni-MH

batteries at various discharge temperatures 

and rates.

[Conditions: Charge: 1C to -∆V = 60mV @ 21°C (70°F)]

Temperature: -20°C (-4°F)

8.5

8.0

7.5

7.0

6.5

6.0

5.5



0 0.5 1.0 1.5 2.0 2.5

C/5 (0.48A)

C (2.4A)

Temperature: 45°C (113°F)

8.5

8.0

7.5

7.0

6.5

6.0

5.5

C/5 (0.48A)

C (2.4A)

0 0.5 1.0 1.5 2.0 2.5

Temperature: 21°C (70°F)

C/5 (0.48A)



Temperature: 0°C (32°F)

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