Moltech: Breaking Rules In 1997, the performance boundaries of NiMH were broken when Moltech
Power Systems pioneered the use of Nickel-Metal Hydride in high
drain devices, dispelling the belief that NiMH was only suited to
low current applications. Today, Moltech's high drain products power
professional cordless powertools all over the world with discharge
rates as high as those delivered by the best NiCd solutions. Moltech's
NiMH technology received further endorsement when it was used to power
an electric car which broke the landspeed record for an electric
vehicle in 1998 at an earth shattering 409kph (254mph).
When Moltech identified the market need for a NiMH product that
could operate under the extreme conditions of continuous charge
at high temperature seen in the sub-50Wh backup market, their high
drain NiMH provided the ideal first building block. It was recognised
that existing NiMH platforms which had been developed for occasional
use at elevated temperatures such as those used in notebook computers
could not provide the performance required for prolonged use in
this market, and new platforms would need to be developed.
If you can't stand the heat… Continuous overcharge at elevated temperatures causes two main problems
within a NiMH cell which can lead to premature failure - electrode
and separator degradation through oxidisation. During the charging
phase, oxygen generated at the positive electrode is recombined
at the negative electrode - maintaining an internal equilibrium
(see figure 1), however, this oxygen cycle degrades the electrode
materials - a process which is accelerated by higher temperatures.
Figure 1 - Relative Electrode Balances
for NiMH cell during discharge/charge/overcharge
The separator which maintains electrical isolation between electrodes
also oxidises in the caustic electrolyte - contributing to cell
degradation. The decomposition of the nylon separator initially
results in increased concentrations of ammonium ion, accelerating
self discharge through a nitrate shuttle reaction. After further
use, the integrity of the separator is compromised causing the plates
to short circuit, rendering the cell useless.
One simple fix for the issue of separator degradation is to substitute
the more common nylon separator material for a more resilient polypropylene
one, and this serves well for cells used in applications where transient
increases in cell temperature are seen (notebook computer batteries
for example where the battery is close to the processor) - however,
this alone cannot solve the problems generated during continuous
overcharge. - Quite simply, cells which are designed to optimise
'out of the box' capacity cannot claim to also champion the causes
of continuous charge and elevated temperature, even those designed
for transient high temperature such as cells used in notebooks.
They are therefore not a 'drop in' replacement for high temperature
NiCd cells.
The Moltech solution has been to develop a high temperature NiMH
cell, the active components of which have all been designed for
the specific purpose of providing reliable backup power under the
harsh conditions of high temperature and continuous charge. The
main breakthrough lies in Moltech's enhanced positive electrode
and proprietary electrolyte formulation. These platforms allow efficient
charging of the positive electrode's Ni(OH)2 active material at
high temperatures, while minimising oxygen evolution and reducing
the effects of electrode degradation through oxidisation. Figure
2 illustrates the improvements in charge efficiency that these component
changes provide. "Standard" and "High Temperature"
Moltech NiMH cells were charged at 55°C (a common ambient for
many emergency lighting applications) at their C/10 rate for 24
and subsequently 48 hours. The standard product fails to accept
the charge input applied due to it's inefficiency at higher temperatures,
whereas the high temperature product reaches a capacity in excess
of 100% after 24Hrs.
Figure 2 - NiMH Charge Acceptance
@55°C
Getting The Balance Right… As with all sealed NiMH designs, the Moltech cell is designed to
be positive limited - i.e. the positive electrode has a lower capacity
than the negative, so that the positive electrode is fully charged
before the negative - allowing adequate capacity in the negative
electrode for oxygen recombination under overcharge conditions.
In a cell operating under continuous charge at elevated temperatures,
careful selection and matching of electrode capacities is required
to ensure cell capacities are met; while ensuring adequate reserve
capacity in the negative electrode to provide a prolonged service
life. Again, Moltech's research in this field has lead to the development
of a product which can accept continuous charge, and provide backup
power after 4 years at elevated temperature.
The future is NiMH… Moltech's high temperature Nickel-Metal hydride platform
is initially being released in the popular Cs (HRT23/43) cell size
(22mm diameter, x 42mm height); known as the EMT-2000Cs, it provides
25% more energy than high temperature NiCd cells of the same size.
EMT-2000Cs has already passed the requirements of IEC61436 (Sealed
NiMH Rechargeable Single Cells), a standard which will include a
provision for high temperature NiMH when it is rereleased in the
near future.
As designers and marketers alike look forward to using the latest
technological advances in their field to develop new back-up products
which will outperform their competitors, Moltech Power Systems are
providing leading edge innovative battery solutions which are designed
to meet the most stringent UPS requirements. The EMT-2000Cs will
at last allow the rules written by NiCd and SLA to be broken for
good.
About The Author… Neil Oliver, Battery Projects Manager has worked for Ever Ready,
Energizer & Moltech Power Systems for over twelve years holding
various Design, Application Engineering and Technical Marketing
positions. He can be contacted at neil.oliver@moltechpower.co.uk
