employ nickel metal hydride (NiMH).
This chemistry is lighter and environmentally friendlier than lead-based
chemistries. The battery consists of
cylindrical cells that are connected in
series to attain several hundred
volts. The cell strings are suspended to
allow air-cooling. Figure 1 shows a
demonstration pack of an early Toyota
hybrid car battery.
One of the critical battery requirements for hybrid applications is
longevity. Rechargeable batteries for
consumer products typically last for
two to three years. This short service
life is no major drawback with cell
phones, laptops, and digital cameras
because the devices become obsolete
quickly. At $2,000 to $3,000 per
battery pack, the replacement cost of
an HEV battery would constitute a
Most batteries for HEVs are
guaranteed for eight years. To meet
this long service life, the cells are
optimized for longevity and not size
and weight, as is the case with
portable applications. Since the battery runs on wheels, the increased
weight and size are not too critical.
The NiMH pack of an HEV can be
charged and discharged 1,000 times if
only taken to 80% depth-of-discharge.
In a hybrid vehicle, a full discharge
seldom occurs except if the owner
lives on a mountain and requires all
available battery power to commute
home. Such a routine would add
stress to the battery and the life would
be shortened. In most other applications, the hybrid car uses only 10% of
the rated battery capacity. This allows
thousands of charge/discharge cycles.
Batteries in satellites use a similar
system in which the battery discharges
less than 10% during a satellite night.
NASA achieves this by over-sizing
One of the limitations of NiMH
is moderate energy conversion
efficiency. This translates to the battery
becoming hot during charge and
discharge. The charge efficiency is best
at 50%-70% state-of-charge. Above
70%, the battery cannot absorb the
charge well and much of the charging
energy is lost in heat. Operating a
battery with a partial charge requires
a larger mass that lowers the energy-to-weight ratio and efficiency.
The Japanese car manufacturers
have tried several battery chemistries,
including going back to lead acid.
Today, the focus is on lithium-ion.
Cobalt-based lithium-ion is one of the
first chemistries in the lithium family
and offers a very high energy density.
Unfortunately, this battery system
cannot deliver high currents and is
restricted to portable applications.
HEV manufacturers are experimenting with manganese (spinel) and
phosphate versions. These lithium-ion
systems offer an extremely low
internal resistance, deliver high load
currents, and accept rapid charge.
Unlike the cobalt version, the
resistance stays low throughout the
life of the battery.
To verify the characteristic
of manganese-based lithium-ion, a
research lab applied 30,000 discharge/
charge cycles over a period of seven
years. Although the capacity dropped
from 100% to 20%, the cell retained
its low internal resistance.
The drawback of manganese and
phosphate is lower energy density,
but these systems provide 20% more
capacity per weight than NiMH and
three times more than lead acid.
Figure 2 illustrates the energy densities
of the lead, nickel, and lithium-ion
systems. It should be noted that
lithium-ion systems have
the potential of higher
energy densities but at
the cost of lower safety
and reduced cycle life.
The lithium-ion sys-
FIGURE 2. Energy
densities of common
the highest energy
density. Manganese and
phosphate systems are
thermally more stable
and deliver higher load
currents than cobalt.
tems are promising candidates for
both the HEV and plug-in HEV but
require more research. Here are
some of the roadblocks that need to
• Durability. The buyer requests a
warranty of 10 years and more.
Currently, the battery manufacturer
for hybrid electric vehicles can only
give eight years on NiMH. The
longevity of lithium-ion has not yet
been proven and honoring eight years
will be a challenge.
• Cost. If the $2,000 to $3,000
replacement cost of a nickel-metal-hydride pack is prohibitive, lithium-ion
will be higher. These systems are more
expensive to produce than most other
chemistries but have the potential for
price reductions through improved
manufacturing methods. NiMH
has reached the low cost plateau and
cannot be reduced further because of
high nickel prices.
• Safety. Manganese and phosphate-based lithium-ion batteries are
inherently safer than cobalt. Cobalt
gets thermally unstable at a moderate
temperature of 150°C (300°F).
Manganese and phosphate cells can
reach 250°C (480°F) before becoming
unsafe. In spite of the increased
thermal stability, the battery requires
expensive protection circuits to
supervise the cell voltages and
limit the current in fail conditions.
The safety circuit will also need to
August 2007 51