Batteries: Charge of the Electrolyte Brigade
When you?ve got a battery you?ve got the power ? until it goes flat of course. Chris Long takes a look at the developments in battery technology
Next time you use your laptop, a torch, a mobile phone or any finely designed sex aid, think frog legs. Not the food, not the things you kiss to make a beautiful prince, just frog legs. Without them it is likely we wouldn?t have any of the above items since frog legs were an important part of the development of the battery ? and without the battery we wouldn?t have a lot of things we take for granted today. Of course this also means that without frog legs we wouldn?t face losing our data at 45,000 feet over the Atlantic as our laptop keels over as the battery goes flat.
It started in the 1760s with some frog legs hanging on a rail at a Bologna university ? alas the identity of the frog they were previously attached to has been lost to oblivion. At the university, Luigi Galvani a professor of anatomy, discovered he could make them twitch by touching them with different metals. He also noticed that a greater reaction was obtained when two dissimilar metals were used. He attributed the effect to ?animal electricity?.
This work inspired his chum Alessandro Volta to investigate further. Born of noble blood in Como, Italy, Volta was a physicist and pioneer in the study of electricity ? although it is sometimes forgotten how much else he was involved in. He became a professor of physics at the Royal School in Como in 1774. Shortly after that, in 1775, he invented the electrophorus, a device to generate static electricity; in 1778 he discovered methane gas, and around the same time was appointed professor of physics at Italy?s finest university in Pavia.
Inspired by the work of Galvani, Volta investigated reactions between dissimilar metals, and developed the Voltaic Pile, the first electric battery, in 1800. His name is given to the unit of electric potential, the Volt, which, by the way, is a unit of electric potential and electromotive force equal to 1.00034 times the International System unit (this information is always useful). Galvani was honoured by having the galvanometer named after him and in 1801 Volta was made a count by Napoleon in honour of his work in the field of electricity.
The overwhelming irony of this is that despite the blinding development of the computer and its associated technologies, the battery hasn?t really developed all that much since Volta found out about those twitching legs 200 years ago.
The smallest ?unit? of a battery is a cell. This is the device where the chemical reaction that produces the electricity happens. It is worth noting that the term battery comes from talking about ?a battery? (that is, a number) of cells, so, strictly speaking, any battery should consist of two or more cells connected together, although the term is also used for single cells.
A cell consists of a positive and negative electrode, a liquid or paste electrolyte that the electrodes sit in, and a separator which electrically isolates the positive and negative electrodes. In some designs, the physical distance between the electrodes provides enough electrical isolation and a separator isn?t needed.
Effectively, the electrolyte creates a chemical reaction involving the two electrodes (if you want to impress your friends the electrolyte is called an ionic conductor). One of the electrodes is positive and reacts with the electrolyte by producing electrons, while the other, negative, electrode accepts electrons (thus producing a current flow). Therefore when the electrodes are connected to a device, called a load, an electrical current flows and your device bursts into life. But you don?t get something for nothing ? as the process continues it depletes the materials in the battery and the reactions slow down until it is no longer capable of supplying electrons. At this point the battery is discharged.
There are two types of battery: rechargeable and non- rechargeable. Non-rechargeable batteries are known as primary cells (or voltaic cells or dry cells), where the chemicals that are used can?t be reconstituted into their original form once the energy has been converted (when the battery is discharged).
Rechargeable cells (or secondary cells, storage cells or accumulators) are batteries where the chemicals can be reconstituted by passing an electric current through them in the opposite direction.
The most common primary cell is the Leclanche cell, invented by the French chemist Georges Leclanche in the 1860s. The electrolyte consists of a mixture of ammonium chloride and zinc chloride. The negative electrode is made of zinc, as is the outside shell of the cell, and the positive electrode is a carbon rod surrounded by a mixture of carbon and manganese dioxide.
The original version was a wet cell with the electrodes immersed in a pool of electrolyte. It became popular because it was rugged, easy to manufacture, and had a good shelf-life. Soon after, the original design was improved by turning the electrolyte into a paste. As a
result, the cell could be produced as a sealed unit with no free liquid electrolyte. The carbon-zinc ?dry? cell is still the mainstay of the primary battery market and it says a lot about its technology that the Leclanche cell in use today is very similar to the original invention.
The rechargeable cell was invented in 1859 by a French physicist called Gaston Plante. His cell used two thin lead plates, separated by rubber sheets, rolled up and immersed in dilute sulphuric acid. The initial capacity was extremely limited, but around 1881 other people working in the area developed more efficient batteries by using a paste of lead oxides for the positive plate active materials.
Another widely used rechargeable cell is the alkaline cell, or nickel iron battery, developed by the American inventor Thomas Edison in the 1900s. The principle of operation is the same as in the lead acid cell except that the negative electrode consists of iron, the positive electrode is nickel oxide, and the electrolyte is a solution of potassium hydroxide.
Despite all this invention, batteries have not changed much from 100 years ago, especially when compared with the change in the computer industry. All the same they are having this change forced upon them.
Nowadays we want Pentium processors, lots of memory, TFT screens, modems and hard disks in multiples of gigabytes. As a result, the power consumption of a typical new portable computer has risen over the past year or so from about 16W to about 24W. This is somewhat forcing the notebook manufacturers to force the battery manufacturers to force the development of more powerful and lighter batteries.
Currently it is a two-horse race with the third horse ? the original leader ? looking a bit lame. The original leader, similar to the old Edison nickel iron battery, is the nickel cadmium battery (NiCad), in which the iron electrode in Edison?s battery is replaced by cadmium. Recently nickel metal hydride (NiMH) took over, not least because NiMH batteries have about 20 per cent more volumetric energy (power to weight) than NiCad but also create fewer disposal problems (cadmium is very toxic). Typically, NiCad batteries provide an energy density of about 50W hours per kilogram (Wh/kg) and 150W hours per litre (Wh/l). NiMH batteries provide about 75Wh/kg and almost 200Wh/l.
And now we have Lithium ion (Li-ion) which delivers more than 100Wh/kg and about 300Wh/l translating into 50 per cent more volumetric energy than NiMH cells and 80 per cent more energy per unit of weight ? seriously good news for long distance travellers. The cells also discharge themselves very little and do not suffer from the so-called ?memory effect? that reduces the life of NiCad and NiMH batteries.
But because of space and weight limitations imposed by computer manufacturers, the capacity of a typical lithium battery pack runs only in the 40Wh range. The answer is easy of course: just make the battery bigger ? and heavier. Perhaps one that will drive a full spec notebook for seven hours? Alas the battery will be about as big and heavy as the notebook ? not a good idea.
So we return to the original conundrum: how to produce lighter, more powerful batteries.
One such potential new technology is zinc air, something that has been hanging around in the background for a while, and, at first glance at least, is a seductive offer. zinc air batteries use normal oxygen to fuel a chemical reaction that generates its charge. Whereas most notebook computers run for only three to five hours on a fully charged NiCad or Li-Ion battery, a zinc air battery promises life for around 10 to 12 hours ? that?s if the developers are to be believed of course.
There is, not surprisingly, a downside: for a start they are large ? almost as big as the computers they power, but manufacturers promise reductions in size and weight. But the really big downside is that you can only recharge the batteries about 60 times before they lose their efficiency. Compared with about 400 charges for NiCad batteries and about 750 charges for Li-ion batteries, zinc air doesn?t look that good ? although time will tell.
Whatever the cell technology, there is a still a need for safety during battery charging ? so why not build that safety into the battery itself and call it a smart battery?
With Li-ion batteries, in particular, it is important to build in safety and protection systems. Lithium protection circuits need to come into play when there are extremely high surge currents, DC currents (like a shorted battery), or when the charger attempts to overcharge the battery. Any of these conditions will cause the smart battery to protect itself rather than rely on external limiting by the battery charger.
The first level of protection takes the form of the battery asking the computer to correct the problem. If this fails to sort things out the smart battery can take a series of independent actions to protect from danger. First would be to throw an electronic switch in the battery, switching itself off (the electronic switch is easily reset once the problem is resolved). Next would be for the battery to open a mechanical switch, such as a temperature thermostat. Once the battery has cooled down the thermostat closes, allowing resumption of normal operation. The final level of protection is a fuse that blows at a specified temperature or current level. This is irreversible and the battery cannot return to normal operation after.
There are currently two standards for implementing the smart battery to host communications. They are the SMBus, an Intel-Duracell initiative and the older single-wire bus. Both SMBus and single-wire have emerged as strong standards for implementing smart battery systems. Single-wire was adopted early by Sony and as such has a large installed base. SMBus is based on the Philips I2C standard and has been adopted for use by the smart battery system (SBS) data specification. This is both endorsed and guided by a number of battery makers such as Energizer, Duracell, and Toshiba, and semiconductor manufacturers such as Intel, National Semiconductor, Benchmarq, Linear Technology, Maxim and Mitsubishi. The fight isn?t over but SMBus has the upper hand.
When we talk about batteries the conversation mostly revolves around notebooks and their problems. But there is another ? arguably more critical ? area that batteries support: uninterruptible power supplies (UPS). Standby power is an area of technology that has existed for decades. Until the 1950s, the typical standby power source that provided emergency power to critical locations was the ?battery room?. This was a room full of large storage batteries, usually lead acid cells, each of which might be more than 6ft tall. The technology was known and they were reliable units that performed well and in some areas still do. But because of their size and construction, they were not suitable for jobs where people were working nearby ? large quantities of sulphuric acid not being part of a friendly environment.
The need for smaller UPS units has developed because of, and along with, the miniaturisation of computers. A key development in lead acid technology was the introduction of valve regulated lead acid (VRLA) batteries in the early 1970s. VRLA batteries are smaller and more powerful than the earlier, flooded lead acid batteries, and they are mostly maintenance-free. They have no free electrolyte, which means they can be safely used in and around offices.
The rapid development of the small, standalone personal computer of the 1980s was also the start of the growth of the UPS. The result has been the classic high-speed development seen throughout the industry in response to users demanding shorter, higher power discharges. One such development is the technology called thin metal film (TMF).
The TMF cell is considered to be ideal for UPS applications because it is rugged and is tolerant of high charge currents and voltages. Its design allows it to recover to a full state of charge following discharge in around five to 10 minutes although typically, it would be recharged in 30 to 45 minutes, depending on the current available. Confidence in the TMF technology is such that manufacturers are talking about retrofitting it to current legacy systems.
All this goes to show that, despite the seemingly slow development of the battery, there is life in it yet. We mustn?t forget that this is an area of technology pushed along by the computer industry. This is a place that doesn?t respect the phrase ?this is the end of the line for this technology?. Thus it will come as no surprise that, while we look forward to new technologies and new ways of doing things, that an old technology should tap us on the shoulder and ask us if it can rejoin the game.
Well, it?s happened. Sanyo has just announced that it has developed a new NiMH battery that offers more energy than an equivalent Li-ion battery of equivalent size.
Currently it is slated for use in cellular phones and mobile communications, but it is only time before we see it in the laptop. Basically, for the same weight, NiMH batteries are going from under 200Wh/l to 240Wh/l.
So, no change there then, the instant you think it is plain sailing from here we end up with the unexpected happening and all bets are off. Remember this the next time you buy batteries for your notebook - or have frog legs for dinner.