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Ni-MH Rechargeable Batteries
able of Contents
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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
4.1 Basic Cell Construction
4.2 Cylindrical Cell Construction
4.3 Prismatic Cell Construction
5 Performance Characteristics
5.1 General Characteristics 5.5 Constant Power Discharge Characteristics
5.2 Discharge Characteristics: Effect of Discharge Rate 5.6 Polarity R
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Ni-MH Rechargeable Batteries Introduction 1 1 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, portabl
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Ni-MH Rechargeable Batteries Composition and Chemistry 3 3 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. 3.1 Active Components: Positive and Negative Electrodes Nickel oxyhydroxide (NiOOH) is the active mate- and AB alloys, of which TiMn or ZrMn are examples. 2 2 2 rial in the positive electrode of the nickel-metal hydride DURACELL nicke
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Ni-MH Rechargeable Batteries Composition and Chemistry (cont.) The sealed nickel-metal hydride cell uses the FIGURE 3.3.1 “oxygen-recombination” mechanism to prevent a build- up of pressure that may result from the generation of Positive Electrode oxygen towards the end of charge and overcharge. This mechanism requires the use of a negative electrode NiOOH/Ni(OH) 2 (the metal hydride/metal electrode) which has a higher effective capacity than the positive (nickel oxyhydrox- Useful Capacity ide/
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Ni-MH Rechargeable Batteries Battery Construction 4 4 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
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Ni-MH Rechargeable Batteries Battery Construction (cont.) 4.3 Prismatic Cell Construction FIGURE 4.3.1 The basic differences between the prismatic cell and the cylindrical cell are the construction of (+) Positive Terminal the electrodes and the shape of the can. Prismatic Safety Vent cells are designed to meet the needs of compact Heat Shrink Tube Metal Lid Cosmetic Disk equipment where space for the battery is limited. The rectangular shape of the prismatic cell permits more efficient batt
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Ni-MH Rechargeable Batteries Performance Characteristics 5 5 FIGURE 5.2.1 5.1 General Characteristics 8.5
Temperature: 45°C (113°F) The discharge characteristics of the nickel-metal 8.0
hydride cell are very similar to those of the nickel- cadmium cell. The charged open circuit voltage of both 7.5
C/5 (0.48A) systems ranges from 1.25 to 1.35 volts per cell. On C (2.4A) discharge, the nominal voltage is 1.2 volts per cell and 7.0
the typical end voltage is 1.0 volt per cell. 6.5
Fi
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Ni-MH Rechargeable Batteries Performance Characteristics (cont.) Typically, when the current is higher and the FIGURE 5.3.1 temperature is lower, the operating voltage will be 2.5
lower. This is due to the higher “IR” drop that °C (70°F) 21 occurs with increasing current and the cell’s increas- ing resistance at the lower temperatures. However, 45°C (113°F) 2.0
at moderate discharge rates (» C/5), the effect of low temperature on the capacity of the nickel-metal 0°C (32°F) hydride battery
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Ni-MH Rechargeable Batteries Performance Characteristics (cont.) Figure 5.4.1 compares the gravimetric and FIGURE 5.4.1 volumetric energy density of nickel-metal hydride 200 and nickel-cadmium cells. As indicated, nickel-metal hydride cells deliver more energy per weight or Wh/L 150 volume than nickel-cadmium cells. Wh/L 100 5.5 Constant Power Discharge Characteristics The output energy characteristic of nickel-metal Wh/kg 50 hydride batteries under the constant power mode at Wh/kg different
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Ni-MH Rechargeable Batteries Performance Characteristics (cont.) 5.7 Internal Impedance FIGURE 5.7.1
180 DURACELL nickel-metal hydride batteries have low internal impedance because they are manufactured using cells designed with thin plate electrodes which offer large 175 surface areas and good conductivity. Figure 5.7.1 shows the change in internal impedance with depth of discharge. DR30 As demonstrated, the impedance remains relatively constant 170 during most of the discharge. Towards th
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Ni-MH Rechargeable Batteries Performance Characteristics (cont.) 5.9 Voltage Depression (“Memory Effect”) FIGURE 5.9.1 Although many years of premium performance 1.35
can be enjoyed from a nickel-metal hydride battery that 1.25
is properly handled, the capacity delivered in each charge/discharge cycle will eventually begin to decrease. 1.15
This inevitable decrease in capacity can be accelerated by Cycle #2
1.05
overcharging, storage or
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Ni-MH Rechargeable Batteries Charging Sealed Nickel-Metal Hydride Batteries 6 6 6.1 General Principles FIGURE 6.1.1 2.0 Recharging is the process of replacing energy Ni-Cd that has been discharged from the battery. The subse- 1.8 quent performance of the battery, as well as its overall 1.6 life, is dependent on effective charging. The main crite- Ni-MH 1.4 ria for effective charging are: 1.2 Choosing the appropriate rate • 1.0 0 20 40 60 80 100 120
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Ni-MH Rechargeable Batteries Charging Sealed Nickel-Metal Hydride Batteries (cont.) Duracell recommends the charge termination method FIGURE 6.1.3 described in Section 6.3.1. 10.0
The voltage of the nickel-metal hydride battery during charge depends on a number of conditions, 9.5
including charge current and temperature. Figures 0°C (32° F) 6.1.3 and 6.1.4 show the voltage profile of the nickel- 9.0
metal hydride battery at different ambient temperatures 21°C (70° F) and charge rates, res
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Ni-MH Rechargeable Batteries Charging Sealed Nickel-Metal Hydride Batteries (cont.) The following summary explains some of the FIGURE 6.2.1 recommended methods for charge control. The charac- teristics of each of these methods are illustrated in -ΔV Figure 6.2.1. In many cases, several methods are employed, particularly for high rate charging. Voltage (V) TCO 6.2.1 Timed Charge Under the timed charge control method, the Temperature (T) charge is terminated after the bat
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Ni-MH Rechargeable Batteries Charging Sealed Nickel-Metal Hydride Batteries (cont.) 6.2.4 Temperature Cutoff (cont.) Usually this method is used in conjunction with activate. A charge rate of 1C and a temperature cutoff other charge control techniques primarily to terminate at 60 C (140 F) is recommended. A top-up charge is the charge in the event that the battery reaches exces- not recommended if this termination method is used. sive temperatures before the other charge controls 6.2.5 Delta
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Ni-MH Rechargeable Batteries Charging Sealed Nickel-Metal Hydride Batteries (cont.) 6.3.1 Duracell’s Recommendation: Three-Step Charge Procedure For fast charging and optimum performance, 1) Charge at the 1C rate, terminated by using Duracell recommends a three-step procedure that pro- dT/dt = 1 C(1.8 F) /minute. vides a means of rapidly charging a nickel-metal hydride 2) Apply a C/10 top-up charge, terminated by battery to full charge without excessive overcharging or a timer after 1/2 hour
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Ni-MH Rechargeable Batteries Charging Sealed Nickel-Metal Hydride Batteries (cont.) 6.3.5 Trickle Charge A number of applications require the use of recommended. The preferred temperature range for batteries which are maintained in a fully-charged state. trickle charging is between 10 C to 35 C (50 F to This is accomplished by trickle charging at a rate that 95 F). Trickle charge may be used following any of the will replace the loss in capacity due to self-discharge. previously discussed
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Ni-MH Rechargeable Batteries Cycle and Battery Life 7 7 7.1 Cycle Life FIGURE 7.1.1 Temperature (°F) The cycle life of nickel-metal hydride batteries 32 50 68 86 104 122 depends on the many conditions to which the battery has been exposed, as is true for all types of recharge- 100
able batteries. These include such variables as: 90
80
Temperature during charge and discharge • 70
Charge and discharge current •
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Ni-MH Rechargeable Batteries Cycle and Battery Life (cont.) Charge rate and amount of charge input during Cycle life is also affected by the depth of dis- overcharging are also important factors affecting cycle charge. Depending upon the charge termination method, life. If the battery is charged at a rate that exceeds the up to 500 cycles can be obtained with the battery being oxygen recombination rate, oxygen that is generated fully discharged on each cycle (100 percent depth of dis- during ov
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Ni-MH Rechargeable Batteries Safety Considerations 8 8 Duracell’s nickel-metal hydride batteries are designed to ensure maximum safety. Each cell includes a resealable pressure relief mechanism (safety vent) to prevent excessive build-up of pressure in the cell in the event it is overcharged excessively, exposed to extreme high temperatures, or otherwise abused. Duracell’s nickel-metal hydride batteries contain protective devices, as discussed in Section 6.4, to prevent excessive heating durin
