Nickel-Cadmium Battery

In consumer electronic equipment, the most popular rechargeable/storage batteries have been nickel-cadmium cells, often called NiCads, but they are now being replaced by Nickel-Metal Hydride batteries in part because cadmium is a toxic heavy metal. Ni-Cad batteries use cathodes made from nickel and anodes from cadmium, as the name implies. Their most endearing characteristic is the capability to withstand a huge number of full charge/discharge cycles, in the range of 500 to 1000, without deteriorating past the point of usefulness. 

NiCads are also relatively lightweight, have a good energy storage density (although about half that of alkaline cells), and tolerate trickle charging (when properly designed). On the downside, cadmium is toxic thus the warning labels that implore you to be cautious with them and properly dispose of them. The output voltage of most chemical cells declines as the cell discharges because the reactions within the cell increase its internal resistance. NiCads have a very low internal resistance--meaning they can create high currents--which changes little as the cell discharges. Consequently, the NiCad cell produces a nearly constant voltage until it becomes almost completely discharged, at which point its output voltage falls precipitously. This constant voltage is an advantage to the circuit designer because fewer allowances need to be made for voltage variations. However, the constant voltage also makes determining the state of a NiCad's charge nearly impossible. As a result, most battery-powered computers deduce the battery power they have remaining from the time they have been operating and known battery capacity rather than by actually checking the battery state.

 NiCads are known for another drawback: memory. When some NiCads are partly discharged, then later recharged, they may lose capacity. Chemically, recharging NiCads before they are fully discharged often results in the formation of cadmium crystals on the anodes of the cell. The crystals act like a chemical memory system, marking a second discharge state for the cell. When the cell gets discharged to this secondary discharge state, its output abruptly falls despite further capacity being available within the cell. In subsequent cycles, the cell remembers this second discharge level, which further aggravate the situation by reinforcing the memory of the second discharge state. The full capacity of the cell can only be recovered by nudging the cell past this second discharge state. This will erase the memory and restores full cell capacity. As a practical matter, the cure for the memory problem is deep discharge--discharging the battery to its minimum working level and then charging the battery again. Deep discharge does not mean totally discharging the battery, however. Draining nearly any storage battery absolutely dry will damage it and shorten its life. If you discharge a NiCad battery so that it produces less than about one volt (its nominal output is 1.2 volts), it may suffer such damage. Notebook computers are designed to switch off before their batteries are drained too far, and deep discharge utilities do not push any farther so you need not worry in using them. But don't try to deeply discharge your system's batteries by shorting them out--you risk damaging the battery and even starting a fire. 

According to battery makers, newer NiCads and nickel-metal hydride cells are free from memory effects, although this has not been proven in practice. Some lithium battery makers claim that the memory effect results from the use of nickel rather than cadmium (a view not supported by the chemistry) and some users also report contrary experiences with both nickel-based battery types. In any case, to get the longest life from NiCads the best strategy is to operate them between extremes--operate the battery through its complete cycle. Charge the battery fully; run it until it is normally discharged; then fully charge it again. Preventing Electrolysis As with lead-acid batteries, nickel-cadmium cells are also prone to electrolysis breaking down water in the electrolyte into potentially explosive hydrogen and oxygen. Battery makers take great steps to reduce this effect. Commercially available NiCads are sealed to prevent leakage. They are also designed so that they produce oxygen before hydrogen, which reacts internally to shutdown the electrolysis reaction. To prevent sealed cells from exploding should gas somehow build up inside them, their designs usually include resealable vents. You risk the chance of explosion if you encase a NiCad cell in such a way it cannot vent. The vents are tiny and usually go unnoticed. They operate automatically. The warning against blocking the vents applies mostly to equipment makers. Standard battery holders won't block the vents, but encapsulating the battery epoxy to make a solid power module certainly will. 


Nickel-Metal Hydride Battery

 Chemically, one of the best cathode materials for battery cells would be hydrogen. But hydrogen is problematic as a material for batteries. At normal temperatures and pressures, hydrogen is a lighter-than-air gas, as hard to hold to as grabbing your breath in your hands.

In the late 1960's, however, scientists discovered that some metal alloys had the ability to store atomic hydrogen 1000 times their own volume. These metallic alloys are termed hydrides and typically are based on compounds such as LiNi5 or ZrNi2. In properly designed systems, hydrides can provide a storage sink of hydrogen that can reversibly react in battery cell chemistry. The most common cells that use hydride cathodes carry over the nickel anodes from NiCad cell designs. These cells typically have an electrolyte of a diluted solution of potassium hydroxide, which is alkaline in nature. Substituting hydrides for cadmium in battery cells has several advantages. The most obvious is that such cells eliminate one major toxic material, cadmium. No cadmium also means that the cells should be free from the memory effect that plagues NiCad cells. In addition, hydrogen is so much better as a cathode material that cells based on nickel and metal hydrides have a storage density about fifty percent higher than nickel-cadmium cells. In practical terms, that means cells of the same size and about the same weight can power a notebook computer for about fifty percent longer. 

Cells based on nickel and metal hydrides--often abbreviated as Ni-MH cells--are not perfect. Their chief drawback is that most such cells have a substantially higher self-discharge rate than do NiCad cells. Some NiMH cells lose as much as five percent of their capacity per day, although this figure is coming down with more refined cell designs. As with NiCads, NiMH cells have a nominal output voltage of 1.2 volts that remains relatively flat throughout the discharge cycle, falling precipitously only at the end of the useful charge of the cell. (Fully charged, a NiMH cell produces about 1.4 volts, but this quickly falls to 1.2 volts where it remains throughout the majority of the discharge cycle.) In many ways NiMH cells are interchangeable with NiCads. They have a similar ability to supply high currents, although not quite as much as NiCads. NiMH cells also endure many charge/discharge cycles, typically up to 500 full cycles, but they are not a match for NiCads. Although the discharge characteristics of NiMH and NiCads are similar, the two cell types react differently during charging. Specifically, NiCads are essentially endothermic while being charged and NiMH cells are exothermic--they produce heat. As the NiMH cell approaches full charge, its temperature can rise dramatically. Consequently, chargers are best designed for one or the other type of cell. NiMH cells work best in chargers designed for them. NiMH cells do, however, readily accept trickle charging. 


Lithium Ion Battery

Lithium is the most chemically reactive metal and provides the basis for today's most compact energy storage for notebook computer power systems. Nearly all high-density storage systems use lithium because it has an inherent chemical advantage. Lithium has a specific capacity to store 3860 Ampere-hours per kilogram of mass compared to 820 Ah/kg for zinc and 260 Ah/kg for lead. Lithium is also very reactive. Depending on the anode, cells with lithium cathodes can produce anywhere from 1.5 volts to 3.6 volts per cell, higher voltage than any other chemistry. The problem with lithium is that it is too reactive. It reacts violently with water and can ignite into flame. 

Batteries based on lithium metal were developed and manufactured in the 1970's and in the 1980's some companies introduced commercial rechargeable cells based on metallic lithium. Such batteries quickly earned a reputation for doubtful safety. To prevent problems caused by reactive metallic lithium, battery makers refined their designs to keep the lithium in its ionic state. In this way they were able to reap the electrochemical benefits of lithium-based cells without the safety issues associated with the pure metal. In lithium-ion cells, the lithium ions are absorbed into the active material of the electrodes rather than being plated out as metal. The typical lithium-ion cells use carbon for its anode and lithium cobalt dioxide as the cathode. The electrolyte is usually based on a lithium salt in solution. Lithium batteries offer higher storage densities than nickel-metal hydride cells, which equates to using them in notebook computers system for about fifty percent longer without a recharge. Lithium-ion cells also lack the memory effect that plagued early nickel-cad cells. On the other hand, current lithium cells have a higher internal resistance than nickel-cadmium cells and consequently cannot deliver high currents. A NiCad could melt a screwdriver, but a lithium cell cannot--that's why lithium cells don't wear the same warnings as NiCads. The available power is sufficient for a properly designed notebook computer that minimizes surge requirements (meaning that certain devices such as disk drives may require a fair surge of power during certain phases of operation, notably spinup). Moreover, the life of lithium cells is more limited than that of nickel-based designs, although lithium-ion cells withstand hundreds of charge/recharge cycles. Because lithium-ion cells use a liquid electrolyte (although one that may be constrained in a fabric separator), cells designs are limited to the familiar cylindrical battery form. Although such designs are no more handicapped than they are with other battery chemistries, the lithium-ion chemistry lends itself to other, space-saving designs based on polymerized electrolytes. Lithium Polymer, Lithium Iron Disulfide and Zinc Air Lithium Polymer Today's brightest new battery technology is a refinement of familiar lithium chemistry called the lithium solid polymer cell. In fact, most battery makers and computer makers are switching to the lithium solid polymer cell design. 

 Where conventional lithium ion cells require liquid electrolytes, solid polymer cells integrate the electrolyte into a polymer plastic separator between the anode and cathode of the cell. As an electrolyte, lithium polymer cells use a polymer composite such as polyacrylonitrile containing a lithium salt. Because there's no liquid, the solid polymer cell does not require the chunky cylindrical cases of conventional batteries. Instead the solid polymer cells can be formed into flat sheets or prismatic (rectangular) packages better able to fit the nooks and crannies of notebook computers. Although the energy density of solid polymer cells is similar to ordinary lithium ion cells, PC manufacturers can shape them to better fit the space available in a PC, squeezing more capacity into each machine. For example, simply by filling the empty space that would appear in the corners around a cylindrical cell, a solid polymer battery can fit in about 22% more chemistry and energy capacity. In addition, solid polymer batteries are environmentally friendly, lighter because they have no metal shell and safer because they contain no flammable solvent. 


Lithium-Iron Disulfide Battery

Unlike other lithium cells that have chemistries tuned to obtaining the greatest capacity in a given package, lithium-iron disulfide cells are a compromise. 

To match to existing equipment and circuits, their chemistry has been tailored to the standard nominal 1.5-volt output (whereas other lithium technologies produce double that). These cells are consequently sometimes termed voltage-compatible lithium batteries. Unlike other lithium technologies, lithium-iron disulfide cells are not rechargeable. Internally, the lithium-iron disulfide cell is a sandwich of a lithium anode, a separator, and iron disulfide cathode with an aluminum cathode collector. The cells are sealed but vented. 

Compared to the alkaline cells with which they are meant to compete, lithium-iron disulfide cells are lighter (weighing about 66% of same-size alkaline cells), higher in capacity, and longer in life. Even after ten years of shelf storage, lithium-iron disulfide cells still retain most of their capacity. Lithium iron-disulfide cells operate best under heavier loads. In high current applications, they can supply power for about 260% the time of a same-size alkaline cell. This advantage is less at lower loads, however, and at very light loads may disappear entirely. For example, under a 20 mA load one manufacturer rates its AA-size lithium-disulfide cells to provide power for about 122 hours while its alkaline cells will last for 135 hours. With a one ampere load, however, the lithium disulfide cells last for 2.1 hours versus only 0.8 for alkaline. You can use lithium iron-disulfide cells wherever you might use zinc-carbon batteries, although they are cost-effective only under high-current loads--flashlights, motor-driven devices, and powerful electronics. They are not a wise choice for clocks and portable radios. 


Zinc-Air Battery

Of the current battery technologies, the one offering the densest storage is zinc-air. One reason is that one of the components of its chemical reaction is external to the battery. Zinc-air batteries use atmospheric oxygen as their cathode reactant, hence the "air" in the name. Small holes in the battery casing allow air in to react with a powered zinc anode through a highly conductive potassium hydroxide electrolyte. Originally created for use in primary batteries, zinc-air batteries were characterized by long stable storage life, at least when kept sealed from the air and thus inactive. A sealed zinc-air cells loses only about 2% of its capacity after a year of storage. Once air infiltrates the cell, zinc-air primary cells last only for months, whether under discharge or not. Some battery makers have adapted zinc-air technology for secondary storage. Zinc-air cells work best when frequently or continuously used in low-drain situations. 

The chief drawback of zinc-air batteries is, however, a high internal resistance which means zinc air batteries must be huge to satisfy high current needs--for notebook PCs that means an auxiliary battery pack about the size of the PC itself. Zinc-air secondary cells are have been only crudely adapted to portable PC applications. One of the first products was the PowerSlice XL, developed jointly by Hewlett-Packard Co. and AER Energy Resources Inc. demonstrated the shortcomings of zinc-air technology for high-current computer use. The product developed for the HP OmniBook 600 notebook PC weighed 7.3-pounds--more than the computer itself--but could power the OmniBook only for about 12 hours. Energizer has also adapted zinc-air technology to small button cells aimed at hearing aids.