The energy produced by the piezo elements is stored in batteries. The battery used here is a 3.6V/300mAh nickel cadmium battery (usually used in cordless phones).
1.2 LAYOUT OF BATTERY CHARGING CIRCUIT
The dc power from the vibrations is stored in Nickel Cadmium batteries. The circuit used to charge the batteries is very simple, and it consists of a full wave bridge rectifier, capacitor and the battery intended to be charged, as shown in Figure 4.1. The voltage produced by the PZT is first full wave rectified, then accumulated in a large capacitor, typically greater than 1000μF, then the battery intended to be charged is placed in parallel with the capacitor. The simplicity of this circuit allows it to be constructed very compactly and without additional components that would result in additional power dissipation.
1.3 NICKEL CADMIUM BATTERY
Swedish Waldmar Jungner invented the nickel-cadmium battery in 1899. At that time, the materials were expensive compared to other battery types available and its use was limited to special applications. In 1932, the active materials were deposited inside a porous nickel-plated electrode and in 1947 research began on a sealed nickel-cadmium battery.Nickel Cadmium batteries were the first rechargable batteries to be widely commercially available. Nickel Cadmium batteries work by oxidizing nickelic hydroxide into nickelous hydroxide, which produces two free electronics for every transaction. The Nickel Cadmium (Ni-Cd) battery is a type of battery commonly used in portable computers, camcorders, portable drills, and other small battery-powered devices, having an effective and even power discharge. Although Nickel-Metal Hydride and Lithium Ion batteries are becoming more popular in mobile computers, Ni-Cd batteries are still the most widely sold. The Table 4.1 gives the material of the electrodes and the electrolyte used in a nickel cadmium battery.
Nickel-cadmium prefers fast charge to slow charge and pulse charge to DC charge. It is a strong and silent worker; hard labor poses little problem. In fact, nickel-cadmium is the only battery type that performs well under rigorous working conditions. All other chemistries prefer a shallow discharge and moderate load currents. Nickel-cadmium does not like to be pampered by sitting in chargers for days and being used only occasionally for brief periods.
A periodic full discharge is so important that, if omitted, large crystals will form on the cell plates (also referred to as memory) and the nickel-cadmium will gradually lose its performance. The Electrodes and Electrolytes used in Nickel Cadmium batteries is shown below.
Among rechargeable batteries, nickel-cadmium remains a popular choice for two-way radios, emergency medical equipment and power tools.
There is shift towards batteries with higher energy densities and less toxic metals but alternative chemistries cannot always match the superior durability and low cost of NiCd.
1.3.1 ADVANTAGES OF NICKEL CADMIUM BATTERY
Ø Fast and simple charge, even after prolonged storage.
Ø High number of charge/discharge cycles – if properly maintained, nickel-cadmium provides over 1000 charge/discharge cycles.
Ø Good load performance – nickel-cadmium allows recharging at low temperatures.
Ø Long shelf life – five-year storage is possible. Some priming prior to use will be required.
Ø Simple storage and transportation – most airfreight companies accept nickel-cadmium without special conditions.
Ø Good low temperature performance.
Ø Forgiving if abused – nickel-cadmium is one of the most rugged rechargeable batteries.
Ø Economically priced – nickel-cadmium is lowest in terms of cost per cycle.
Ø Available in a wide range of sizes and performance options – most nickel-cadmium cells are cylindrical.
1.3.2 APPLICATIONS OF NICKEL CADMIUM BATTERY
Ø Motorised equipment
Ø Power tools
Ø Two way radios
Ø Electric razors
Ø Commercial and industrial portable products
Ø Medical instrumentation
Ø Emergency lighting