Synopsis…
With an increased awareness of the environmental issues raised by rechargeable batteries containing Cadmium, Lead and Mercury, organisations involved in the design, manufacture and marketing of small (<50Wh) back-up or UPS systems need a competitive, more environmentally benign alternative which can provide improved energy densities at the extremes of temperature seen in these devices. The answer partially lies in existing technology - but as Neil Oliver, Battery Projects Manager for Moltech Power Systems explains, there are a number of hurdles to understand and overcome before such a product can be released which fully meets the challenges posed by this extremely challenging market. - Something Moltech Power Systems have succeeds in doing with the introduction of their EMT series.

Imagine running a marathon through the Sahara desert - doesn't sound like fun does it? - Now imagine yourself as a battery, subjected to constant charging next to a hot ballast in a hospital emergency lighting system - or crammed inside a microwave base station bolted to the side of an embassy in Saudi Arabia - your stroll through North Africa doesn't sound so bad now does it?

It is these harsh environments that backup batteries are subjected to, day in, day out, all over the world. Whilst many of today's battery technologies are happy partying at the "sexy" end of the market - in mobile phones, notebook computers and MP3 players. Backup batteries are the true workhorses, providing reliable UPS power to a myriad of devices in the event of mains power failure. However, things are changing, and with recent developments in battery technology, backup battery systems are set to get smaller and lighter. Moltech Power Systems, a battery company used to developing innovative technologies is leading the way in this technically challenging market with it's enhanced Nickel-Metal Hydride technology.

Small Is Beautiful For Back-Up Too…
Most backup applications are not portable. Emergency lighting units are screwed to ceilings and walls, microwave transceivers sit on masts and on the side of buildings - however, the need for device manufacturers to make these units smaller and less obtrusive has never been more of a driver than it is now.

The majority of the devices in this low voltage backup market (using batteries of <50Wh) currently rely on Sealed Lead Acid or Nickel-Cadmium batteries for their power source in times of primary power failure. although reliable, they have not received the development resources enjoyed by their more sexy cousins in the 'mobile' market - for this reason, the technology has basically stayed the same for decades with uninspiring roadmaps for the future. As the devices they provide backup power to become smaller, (driven by ever shrinking electronic systems), the bulk and weight of these old battery technologies has become a limiting factor, preventing the backup market from reaching its full market potential.

Environmental Considerations
although no battery technology can ever really be described as "environmentally friendly", the more recently developed technologies are certainly more benign. Attempts to introduce recycling schemes for small battery types have met with limited success - with only a very small percentage ever collected (automotive batteries being a notable exception).

The draft Council Directive 91/157/EEC (aka: European Battery Directive) mandates an EU wide ban on the marketing of batteries containing all but the smallest trace elements of Cadmium, Lead and Mercury by the 1st January 2008 (see table 1). Member States are free to introduce legislation to remove these batteries from the market prior to this date if they wish.



Various interested parties are currently lobbying the European Parliament to incorporate within the new battery directive a list of applications exempted from the NiCd ban due to the current lack of a viable alternative. However, the battery industry is not insular, and it is the market which will dictate that such alternatives are developed, made available - and that they are done so at a competitive (if not equal) price level.

Faced with this uncertain future, power system designers can no longer ignore the environmental impact that these battery components have on their products. These issues should be addressed carefully during the planning stages of any new product as failure to do so could lead to costly re-iterations of design further into the project.

With all corporations aware of their of environmental responsibilities and the increasing move towards 'zero environmental impact' policies, many are now excluding certain battery types (such as NiCd) from their sourcing lists. This has an obvious impact on the specifying and purchasing behaviour of individuals within such an organisation, however more importantly it effects the way that organisations address their markets. As many customers move towards these policies, suppliers which fail to respond may find themselves unable to react fast enough and will fail to deliver to customer requirements in the future.

Which Technology…
With the desire to move towards slimmer backup devices and the requirement for more environmentally preferred battery technologies, the simple answer would seem to be to use one of the advanced battery technologies readily available in the so called "mobile professional" market. In practice, the usage patterns in these two markets could not be more different and "application/technology mismatches" must be understood and eliminated before any new technology can be endorsed - if it is not, then the possibility of publicised backup failure could prevent the use of any new battery technology in this crucial market for years to come.

The main two candidate technologies available today are Lithium Ion (Li-Ion), and Nickel-Metal Hydride (NiMH).

The former 3.6V Li-Ion system is the favoured technology in almost all portable electronic devices available today, offering energy densities in the 210Wh/l 90Wh/Kg region. However the use of Li-Ion in the backup market has a number of drawbacks. The first is it’s cost; although much reduced in recent years, the materials used and the complex manufacturing techniques employed in Li-Ion manufacture put it at a premium cost over other technologies. A second cost is related to the technology's sensitivity to over-charge and over-discharge; In the charge phase, the current and voltage profiles resemble that of sealed lead acid cells, and a relatively simple constant potential, (typically 4.2V) current limited (typically C/2) charger can be employed. However, the dangers of gassing from oxygen evolution and the plating-out of metallic lithium in an over-voltage situation require that active protection circuitry (figure 1) is used to monitor individual cell voltages and isolate the charge path if the cell voltage is too high (typically >4.25-4.35V).

Figure 1 - Typical Li-Ion Protection Circuit



In discharge, the same protection circuitry prevents an under-voltage situation occurring - typically isolating the discharge path when one or more cell voltages reach 2.2-2.5V. These issues, combined with unreliable performance under continuous charge conditions and only a moderate rate ability (typically <2C) mean that existing Li-Ion technology is unlikely to seriously impact the backup market over the next few years.

The 1.2V Nickel-Metal Hydride system evolved from NiCd technology in the late 1980's and soon became the predominant power source for handheld mobile devices because of it's improved energy density, low cost and more environmentally benign technology (cadmium in the negative electrode is replaced with a hydrogen absorbing alloy). Fuelled by the growth in portable electronic devices and the need for increased runtimes, manufacturers pushed the technology forward - optimising the cell capacity. However, this optimisation for mAh's was achieved through a 'blend' of innovation and compromise - which overall resulted in higher capacities being obtained, while other features such as cycle life, long term storage performance, rate dependency and temperature performance, in the main, remained underdeveloped.