Batteries Characteristics: energy and power

Batteries with high specific energy can store large amounts of electricity for their weight. A gasoline tank has a specific energy of about 12 kWh/kg. This is roughly 100 times as great as the best batteries. Electric vehicles require batteries with high specific energy, and range is a function of energy capacity. Most batteries have a minimum charge threshold that should generally be maintained. This is often about 20% of full capacity. While the batteries can tolerate occasional discharges below this point, repeated deep discharges will damage the battery.

The computer controls in a hybrid can automatically preserve this margin.

Batteries with a high specific power can discharge their electricity quickly in powerful bursts. The power of a gasoline vehicle is determined by the engine, not the fuel tank; a typical gasoline engine has a specific power of about 150-400 W/kg,41 which is generally at the lower range of what batteries can achieve. Electric vehicles, by comparison, have their maximum power output determined by both the batteries and the motor. When the weight of the electric motor is taken into account, electric vehicles and gasoline vehicles are roughly comparable. Acceleration is largely a function of specific power, so specific power is important in hybrid vehicles.

Current hybrid-electric vehicles like the Toyota Prius are not designed for extended operation in pure electric mode, so they have typically optimize their batteries to provide high specific power. Because plug-in hybrids use the battery both for supplemental power for acceleration and for extended electric-only operation, they require high specific energy and high specific power. There is a tradeoff-for a given battery technology, higher specific power tends to increase cost per kWh of storage capacity. There are many battery technologies under investigation for use in electric vehicles. The most widely used or most promising are discussed below also, there is some barriers you should think about.

LEAD ACID

Lead acid batteries are currently used in most automobiles for starting, lighting, and ignition applications. Some electric vehicles have been made with lead-acid batteries, including General Motors' EV I and the many electric vehicles in use at the beginning of the 20th century. Lead-12 acid batteries are inexpensive, but have poor specific energy, specific power, and lifetime. Lead-acid batteries are not seen as a promising technology for EV, HEV, or PHEV vehicles.

NICKEL METAL HYDRIDE

Nickel metal hydride (NiMH) batteries are used in existing hybrid vehicles. These batteries offer a higher power density and longer deep-cycling lifetimes compared to lead-acid batteries. Toyota Prius uses NiMH batteries, covered under an 8-yr/100,000-mile warranty. NiMH batteries do suffer from self-discharge over time.42 A 2001 EPRI study projected NiMH batteries as the most plausible battery technology for PHEV vehicles, although life cycle requirements remained a concern. EPRI estimated that the lowest reasonable price for NiMH batteries in 2010 would be $250/kWh, leading to a system incremental cost of $5782 for a PHEV60 (of which $4844 is battery cost).43 Recent information from Cobasys (formerly Texas Ovonics) indicates that the 12V NiMH batteries alone can have a specific energy of 43 Wh/kg and a specific power of 1100 W/kg at 50% depth of discharge (DOD). When assembled into hybrid electric systems of higher voltage, specific energy is 27-33 Wh/kg and specific power is 667-824 W/kg (depending on size).

A report prepared for the California Energy Commision concluded that, with recent advances, the lifetime of NiMH batteries in pure EVs was over six years. 44 The report also noted that specific energy for EVs can be up to 65 Wh/kg, and that a 30-kWh pack was expected to cost $9,000 at production volume of hundreds of thousands per year. A 2001 presentation by Dr. John Heywood of MIT notes that NiMH batteries configured for use in EVs can have specific energy of 70 Wh/kg and specific power of 150 W/kg, whereas those configured for use in HEVs can have specific energy of 40 Wh/kg and specific power of 400 W/kg.45 The Cobasys design features a substantial improvement in specific power over that cited by Heywood, but only a marginal improvement in specific energy.

LITHIUM-ION

Lithium-ion batteries demonstrate considerable potential for specific energy and specific power. Although currently expensive to manufacture, and posing some safety concerns, they are a primary focus of inquiry. Sandia National Laboratory noted in 2000, "The lithium-ion battery has four times the energy density of lead-acid batteries and two to three times the energy density of nickel-cadmium and nickel-metal hydride batteries"46 Argonne National Laboratory noted in 2001 that the best laboratory designs for lithium-ion batteries provide "specific power up to 1 kW/kg and a cycle life of more than 100,000 shadow-discharge (10%) cycles. The battery operates at ambient temperatures, although at 50C (122F) the calendar life of the cell is shortened significantly."47 Lithium-ion battery modules require protective control circuitry to prevent dangerous overcharging conditions. Deep-discharge cycle lifetime is uncertain. Manufacturers claim they will be able to produce at comparable cost to NiMH. More recently, lithium-ion batteries have been observed displacing NiMH as the battery of choice for cell phones.48 The following information is provided by GlobTek, a supplier of lithium-ion batteries: 13

Lithium ion batteries are increasingly used in consumer electronics, but are not yet widely used in vehicles. An electric vehicle developer recently constructed a vehicle with a battery pack consisting of 6,800 conventional consumer-electronic lithium-ion batteries (battery designation 18650). The "tzero" vehicle has a range of 250 miles at 75-80 mph and can accelerate from 0-60 in 3.6 seconds. Because the 18650 battery is mass-produced in huge volumes, AC Propulsion found assembling a huge number to be a less expensive option that purchasing a lithium-ion battery pack designed for EV operation.49 The battery weigh 43 g each for a total of 292 kg, but the designers estimate that a conventional EV would only use about half as many cells (AC Propulsion was interested in demonstrating extremely high performance).

Lithium ion polymer batteries attempt to improve on conventional lithium ion batteries. The primary advantage is geometry, as these can be shaped in a variety of forms. They are more expensive than conventional lithium-ion batteries and have lower energy density and cycle life. They are also somewhat safer (less risk of overcharge) and lighter. Sony and Sanyo are among the principal developers of lithium-ion polymer batteries. Recent batteries have achieved a specific energy of 95 Wh/kg and specific power of up to 2000 W/kg at full charge (down to 1300 W/kg at 80% depth of discharge).

SODIUM NICKEL CHLORIDE

Sodium nickel chloride batteries include the "Zebra" batteries made by MES-DEA in Switzerland. They are currently used in the bus and commercial vehicle market.51 There is also no self-discharge and no overcharge gassing reaction (as can happen with lithium batteries). Peak power is retained down to 80% depth of discharge, and the battery has a 90% useful depth of discharge.52 Energy density is similar to that of lithium-ion batteries, at about 94 Wh/kg according to the company's documentation.53 Cost is currently at $500/kWh but with a projected $220/kWh for volume production.

One drawback to sodium nickel chloride batteries is the high-temperature operation. Batteries operate within an insulated box (300 C). However, an NREL review found that the batteries were generally unlikely to present significant public safety hazards.55 Specific power is also low, at 169 W/kg. For use in hybrids, the company seeks to increase specific power to at least 350 W/kg.56

These batteries are currently used in transit bus applications, at the 600V size. For such use, the manufacturer provides a one-year warranty. The manufacturer estimates a 75% chance of the battery lasting at least two years and 50% chance of lasting at least three years in transit bus application.57 Many other battery chemistries are being explored, and it is possible if not likely that future battery technologies will outperform the best current technologies.