This article is copyright David Gibson, 2010. It is an edited version of an article that first appeared in CREG journal 70 in March 2008, see http://bcra.org.uk/creg/jnl/index.html?j=70
A new type of nickel metal hydride rechargeable battery with a very low self-discharge and a new type of primary battery using a lithium iron disulphide technology are both now available at high-street retail outlets. Between them, these AA-zize cells present the best performance of all small batteries and are a strong candidate for the preferred battery to be designed into new caving electronic equipment.
Keywords: battery technology, caving equipment, AA cells, nickel metal hydride, hybrid, hybrio, low self-discharge, lithium iron disulphide, Energiser Ultimate, environment, rare-earth metal.
I suppose the title of this article should really be "Batteries with which to start the 21st century", since it is unlikely that the technologies I am about to describe will last more than a few years. Practical low-cost fuel cells are almost ready to launch into the consumer market-place and low-cost printable solar panels http://google.com/search?q=printable+solar are not far behind. In fact, such is the huge consumer-driven demand for portable power that this article might be out-of-date very soon.
Use AA cells for preference
I will begin, not by discussing the new 'low self discharge' (l.s.d.) NiMH or the new lithium iron disulphide (Li/FeS2) technologies, but by presenting an argument for using AA-size cells in the equipment you design. Battery technology is moving forward all the time, and the newest, most efficient processes tend to be first used in consumer products, for which AAs are predominant. This means that for energy density (Wh/cm^3 or Wh/kg) AA cells tend to lead the other sizes of cells. Coupled with this is the fact that 'fast' battery chargers are now flooding the consumer market and, for under £25, you can now buy a 'universal' charger complete with four 2.7 Ah AA NiMH cells. The charger will run off 100 V mains as well as the UK's 230 V or a 12 V car battery. Wherever in the world you are caving, you can now easily and cheaply charge your AA cells. If you lose the charger, you can buy alkaline manganese AA cells (e.g. Duracell) from even the most remote shack or - importantly, if you are stuck in the middle of nowhere, or down a cave with a failed light - you can use your friends' batteries.
These are substantial arguments for using AA cell for caving lamps instead of any more exotic technology. Such was the thesis of a talk I gave at BCRA's 2007 Cave Technology Symposium when I suggested that, for anyone designing new equipment - not just caving lamps but any portable equipment - there is now only one sensible choice of battery to use. Whether your figure of merit is Wh/cm^3, Wh/kg, Wh/Euro or a more complicated overall 'star rating', the standard NiMH AA cell out-performs many if not most of the more specialist cells.
It is interesting to see how technology has progressed. At the time when the FX2 lamp was launched in the mid 1980s a nickel-cadmium AA cell was rated at 0.5 Ah; the latest NiMH AA cells are rated at 2.9 Ah. For the larger cells, 4.0 Ah NiCd D-cells and 7 Ah F-cells now have NiMH equivalents of 9 Ah and 13 Ah respectively. The salient point, however, is not the large increase in capacity, but that you will find it extremely difficult to buy a 9 Ah D-cell in a retail outlet. Most consumer NiMH D-cells are merely re-packaged AA cells. This again demonstrates the market domination of AA cells. Of course, there are times when an AA battery will not do. For extremely high-current discharge - the obvious caving application being a portable drill - you will either need a larger NiMH cell (the Uniross Industrial range can be discharged at 3C, i.e. 39 A for the 13 Ah F-cell) or a lead-acid battery. But for most applications - certainly cap-lamps and radios of all types - the AA cell really is the only sensible choice for new equipment.
If the above arguments do not convince you, there are two battery technologies now available that will surely tip the balance. Of course, these technologies can be applied to any size of battery but, because they are consumer-driven, they have appeared in AA cells and there will be little demand to migrate the technology to other sizes. The two new technologies that I am about to describe provide the ultimate in performance - at least for the next year or two until something better comes along.
Low Self-Discharge NiMH
Improvements to the NiMH process have resulted in a version that has a significantly lower self-discharge rate that the standard variant. The various marketing departments refer to this cell as a 'hybrid' -meaning that it combines the advantage of a primary alkaline cell (low self-discharge) with that of a secondary NiMH cell (which is essentially that it is rechargeable). A conventional NiMH cell has rather a high self-discharge rate, typically 10% a month and often higher than this. In other words, if you leave a set in your digital camera for a few months, they will be almost flat when you come to use the device. However, the new NiMH cells have a self-discharge as low as 15%/year which means they are poised to all but replace primary cells completely in many applications. The salient point is that, because of the low self-discharge, they can be sold pre-charged and ready to use. This is, of course, impossible with a conventional NiMH cell, with a shelf-life of only a few months. There are three manufacturers of these new low self-discharge (l.s.d.) NiMH cells - Sanyo, Panasonic and Yuasa-Delta. However, the cells are marketed with over a dozen different brand names. For more info on NiMH cells in general see http://wikipedia.org/wiki/Nickel_metal_hydride_battery but remember to be cautious of what you read in the Wikipedia. These cells can be found in specialist electronics shops but they have not yet reached the supermarket. In the UK, Maplin sell their own branded product called 'hybrid' and the Uniross 'hybrio' cell is available from several sellers at Amazon. Try http://maplin.co.uk/Search.aspx?criteri ... %20battery and http://amazon.co.uk/s/ref=nb_sb_noss?ur ... rds=hybrio
Lithium / Iron Disulphide
There is a confusing range of primary lithium-based technologies (see http://wikipedia.org/wiki/Lithium_battery), for which the features are generally a very low self-discharge and an elevated terminal voltage of 3 to 4 V. Lithium cells are often specified for only a very low current drain, although they can still produce a dangerously high current when short-circuited. Typically the low-current rating limits them to applications such as clocks or computer memory backup where, for example, a 30 uA drain would allow a 3 Ah cell to last for over ten years.
Some lithium technologies, however, can supply a current measured in amps rather than milliamps. The Energiser Ultimate Lithium battery (now found in many camera shops and supermarkets) is a high-current primary (i.e. non-rechargeable) AA cell with a 3 Ah capacity and a 1.5 V terminal voltage. It utilises a lithium iron disulphide (Li/FeS2) chemistry. This battery should not be confused with others in the Energiser Ultimate range, which use a different chemistry. Being a lithium cell, it has a very low self-discharge (5% in 15 years) but it allows a continuous drain of 2 A. This feature allows it to be used in a number of applications for which we would otherwise struggle to find a suitable battery.
One example - in a caving context - could be that of a cap-lamp in an emergency underground first-aid dump. Such dumps (containing emergency food, clothing and first aid) are located in remote parts of large cave systems or used on expeditions. Clearly a cap-lamp - or a through-rock radio - that still functions after many years in storage is an advantage. A professional application I currently have for this battery is a remote data-logger, for which the specification demands a 10-year unattended lifetime, but with the high pulsed power need to drive a radio transmitter. It requires a large pack of batteries but Li/FeS2 is the only technology capable of meeting the specification at reasonable cost.
The future?
Iron disulphide is a very cheap material and so Li/FeS2 cells are likely to gradually replace other high-current alkaline primary cells for many consumer applications. However, they are not rechargeable so, in an environmentally-conscious society, perhaps they may be eclipsed by l.s.d. NiMH cells? Unfortunately, the NiMH cell also suffers from an environmental problem. Although the toxic cadmium cathode has been dispensed with, its replacement - an inter-metallic hydride comprising a mixture of rare-earth metals - depends on a continuing supply of these increasingly rare materials. This is the case throughout the electronics industry (Cohen, 2007) and could lead to a number of products rising steeply in price - but that's another story. Fuel cells and printable solar panels will be with us before too long. For now, though, l.s.d. NiMH cells and Li/FS2 cells represent awesome technical advances. My assertion is that for the many applications where lithium-ion technology is not required, electronic designers of portable equipment should think hard before choosing a power source other than the AA batteries I have described in this article.
References
- Cohen, David (2007), Earth's natural wealth: an audit, New Scientist 2605, pp 34-41, 26 May 2007. Available online at http://newscientist.com
- http://google.com/search?q=printable+solar
- http://wikipedia.org/wiki/Lithium_battery
- http://wikipedia.org/wiki/Nickel_metal_hydride_battery
- http://maplin.co.uk/Search.aspx?criteri ... %20battery
- http://amazon.co.uk/s/ref=nb_sb_noss?ur ... rds=hybrio
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