Battery Types
Chemical batteries are individual cells filled with a
conducting medium-electrolyte that, when connected together, form a battery.
Multiple batteries connected together form a battery bank. At present, there
are two main types of batteries:
Primary batteries (non-rechargeable)
Secondary batteries (rechargeable).
Secondary batteries are further divided into two categories
based on the operating temperature of the electrolyte. Ambient operating
temperature batteries have either aqueous (flooded) or nonaqueous electrolytes.
High operating temperature batteries (molten electrodes)
have either solid or molten electrolytes. Batteries in EVs are the
secondary-rechargeable-type and are in either of the two sub-categories. A
battery for an EV must meet certain performance goals.
These goals include quick discharge and recharge capability,
long cycle life (the number of discharges before becoming unserviceable), low
cost, recyclability, high specific energy (amount of usable energy, measured in
watt-hours per pound [lb] or kilogram [kg]), high energy density (amount of
energy stored per unit volume), specific power (determines the potential for
acceleration), and the ability to work in extreme heat or cold.
No battery currently available meets all these criteria.
Lead–Acid Batteries
Lead–acid starting batteries (shallow-cycle lead–acid
secondary batteries) are the most common battery used in vehicles today. This
battery is an ambient temperature, aqueous electrolyte battery.
A cousin to this battery is the deep-cycle lead–acid
battery, now widely used in golf carts and forklifts. The first electric cars
built also used this technology. Although the lead–acid battery is relatively
inexpensive, it is very heavy, with a limited usable energy by weight (specific
energy).
The battery’s low specific energy and poor energy density
make for a very large and heavy battery pack, which cannot power a vehicle as
far as an equivalent gas-powered vehicle.
Lead–acid batteries should not be discharged by more than
80% of their rated capacity or depth of discharge (DOD). Exceeding the 80% DOD
shortens the life of the battery. Lead–acid batteries are inexpensive, readily
available, and are highly recyclable, using the elaborate recycling system
already in place.
Nickel Iron and Nickel Cadmium Batteries
Nickel iron (Edison cells) and nickel cadmium (nicad) pocket
and sintered plate batteries have been in use for many years. Both of these
batteries have a specific energy of around 25 Wh/lb (55 Wh/kg), which is higher
than advanced lead–acid batteries.
These batteries also have a long cycle life. Both of these
batteries are recyclable. Nickel iron batteries are non-toxic, while nicads are
toxic. They can also be discharged to 100% DOD without damage.
The biggest drawback to these batteries is their cost.
Depending on the size of battery bank in the vehicle, it may cost between
$20,000 and $60,000 for the batteries. The batteries should last at least
100,000 mi (160,900 km) in normal service.
Nickel Metal Hydride Batteries
Nickel metal hydride batteries are offered as the best of
the next generation of batteries. They have a high specific energy: around 40.8
Wh/lb (90 Wh/kg).
According to a U.S. DOE report, the batteries are benign to
the environment and are recyclable. They also are reported to have a very long
cycle life.
Nickel metal hydride batteries have a high self-discharge
rate: they lose their charge when stored for long periods of time. They are
already commercially available as “AA” and “C” cell batteries, for small
consumer appliances and toys.
This battery is a high-temperature battery, with the
electrolyte operating at temperatures of 572°F
(300°C). The sodium component of this battery explodes on
contact with water, which raises certain
safety concerns.
Lithium Iron and Lithium Polymer Batteries
The USABC considers lithium iron batteries to be the
long-term battery solution for EVs. The batteries have a very high specific
energy: 68 Wh/lb (150 Wh/kg).
They have a molten-salt electrolyte and share many features
of a sealed bipolar battery. Lithium iron batteries are also reported to have a
very long cycle life.
These are widely used in laptop computers. These batteries
will allow a vehicle to travel distances
and accelerate at a rate comparable to conventional
gasoline-powered vehicles.
Lithium polymer batteries eliminate liquid electrolytes.
They are thin and flexible, and can be molded into a variety of shapes and
sizes.
Zinc and Aluminum Air Batteries
Zinc air batteries are currently being tested in postal
trucks in Germany. These batteries use either aluminum or zinc as a sacrificial
anode.
As the battery produces electricity, the anode dissolves
into the electrolyte. When the anode is completely dissolved, a new anode is
placed in the vehicle.
The aluminum or zinc and the electrolyte are removed and
sent to a recycling facility. These batteries have a specific energy of over 97
Wh/lb (200 Wh/kg).
The German postal vans currently carry 80 kWh of energy in
their battery, giving them about the same range as 13 gallons (49.2 liters) of
gasoline. In their tests, the vans have achieved a range of 615 mi (990 km) at
25 miles per hour (40 km/h).
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