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.
