The Hybrid Car Battery: A Definitive Guide

Published November 6, 2008

What Is a Hybrid Car Battery?

A hybrid car battery is like any other battery—except that it is rechargeable and has enough juice to move a large heavy vehicle down the road for a few feet or a few miles.

How Does It Work?

Like all batteries, hybrid batteries have two electrodes (which collect or emit an electric charge) that sit in an ion-rich solution called the electrolyte. (An ion, by the way, is an atom or group of atoms with an electrical charge.)

The electrodes are typically very close, so a polymer film, called a separator, prevents them from touching, which would create a short circuit. An on-off switch in whatever device is powered by the battery—your phone or laptop—bridges the cell’s electrodes to generate power. That’s when the electrochemical reaction begins.

Keep in mind: What we commonly call “a battery” is actually a battery pack that houses many individual cells. Your mobile phone battery is just one single cell, but anything larger—even a laptop battery—uses multiple cells working together.

Ionized elements in one electrode are in a chemical state where they are easily attracted to combine with other molecules, emitting electrons (energy) in the process. Those elements are tugged through the electrolyte and the separator toward the opposing electrode. The ions of the negative electrode (anode) give up electrons; the positive ions coming toward the anode accept them. The electrons released during this process travel through the external circuit (e.g. your phone), producing a flow of charge in the opposite direction to the flow of ions. During recharge, current is forced into the cell, reversing the process.

As we take a tour of hybrid batteries, remember one thing: Total energy determines the vehicle’s electric range, whereas available power determines its acceleration.

Today's Hybrid Car Battery: Nickel Metal Hydride

Toyota Prius Hybrid Battery

The battery pack of the second generation Toyota Prius consists of 28 Panasonic prismatic nickel metal hydride modules—each containing six 1.2 volt cells—connected in series to produce a nominal voltage of 201.6 volts. The total number of cells is 168, compared with 228 cells packaged in 38 modules in the first generation Prius. The pack is positioned behind the back seat.

The weight of the complete battery pack is 53.3 kg. The discharge power capability of the Prius pack is about 20 kW at 50 percent state-of-charge. The power capability increases with higher temperatures and decreases at lower temperatures. The Prius has a computer that’s solely dedicated to keeping the Prius battery at the optimum temperature and optimum charge level. The Prius supplies conditioned air from the cabin as thermal management for cooling the batteries. The air is drawn by a 12-volt blower installed above the driver’s side rear tire well.

(Photos courtesy of the Automotive Career Development Center.)

Toyota Highlander Hybrid Battery

The nickel metal hydride battery used in Highlander Hybrid—and the Lexus RX 400h—is packaged in a newly developed metal battery casing. The 240 cells can deliver high voltage of 288 volts—but the motor-generators units can operate on variable voltage anywhere from 280 volts to 650 volts. The battery pack supplies 288 volts, but the boost converter, a part of the inverter above the transaxle, changes this to 500 volts. This battery pack provides 40 percent more power than the Prius battery, despite being 18 percent smaller.

Each of the modules has its own monitoring and cooling control system. The cooling performance reduces efficiency losses due to excessive heat, ensuring that the battery can supply required electric power to the motors at all times. The battery-monitoring unit manages discharge and recharging by the generator and motors to keep the charge level constant while the car is running. The battery pack is stowed under the rear seats.

Ford Escape Hybrid Battery

The Ford Escape Hybrid’s battery pack, made by Sanyo, consists of 250 individual nickel metal hydride cells. As with other hybrid battery packs, the cells are similar in shape to a size D flashlight battery. Each individual battery cell, contained in a stainless steel case, is 1.3 volts. The cells are welded and wrapped together in groups of five to form a module. There are 50 modules in the battery pack. The total voltage of the battery pack is 330 volts.

Honda Insight Battery

The Honda Insight’s battery pack, made up of 120 Panasonic 1.2-volt nickel metal hydride D cells is capable of 100A discharge, and 50A charge rates. The system limits the usable capacity to 4ah to extend battery life. Total battery pack output is 144 volts. The batteries are located under the cargo compartment floor, along with the Honda Integrated Motor Assist’s power control unit. Honda used technology developed for its EV Plus electric car for the original development of the Insight’s battery system.

Saturn Vue Hybrid Battery

The Saturn Vue Green Line’s 36-volt nickel metal hydride battery, designed and made in America by Cobasys, is capable of delivering and receiving more than 14.5 kW of peak power. The hybrid system is used to provide both 12-volt accessory power and power to charge the battery pack. The pack fits under the cargo area, leaving cargo room unchanged from the standard Vue—but drivers lose the benefit of a spare tire.

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Lithium Ion Battery - For Next Generation Hybrids and Electric Cars

Lithium ion (or Li-ion) batteries are important because they have a higher energy density—the amount of energy they hold by weight, or by volume—than any other type. The rule of thumb is that Li-ion cells hold roughly twice as much energy per pound as do the previous generation of advanced batteries, nickel-metal-hydride (NiMH)—which are used in all current hybrids including the Toyota Prius. NiMH, in turn, holds about twice the energy per pound of the conventional lead-acid (PbA) 12-Volt battery that powers your car’s starter motor. It’s Li-ion’s ability to carry so much energy that makes electric cars possible.

Compare the batteries from GM’s legendary EV1 to those for its upcoming Volt extended-range EV. The 1997 EV1 pack used lead-acid cells; it was almost 8 feet long and weighed 1200 pounds. But today’s Volt pack, using lithium-ion cells, stores the same amount of energy (16 kilowatt-hours) in a 5-foot-long container weighing just 400 pounds.

There’s Not One Lithium Ion Battery

Crucially, there is no one lithium-ion battery, although this mistake is often seen in the press. Several different chemical formulations for the electrodes compete; each has its pros and cons. “No chemistry will be the perfect one,” says Klaus Brandt, the chief executive of Gaia, a German cell maker. The anode (or negative electrode) is typically made of graphite, but the cathode (positive electrode) chemistry varies widely. As much as any other factor, what the cathode is made from determines the cell’s capacity. The critical feature is the rate at which the cathode can absorb and emit free lithium ions. Each of several competing cathode materials offers a different mix of cost, durability, performance, and safety. Let's take a look at the most important cathode contenders.

Cobalt Dioxide

Cobalt Dioxide is the most popular choice today for small cells (those in your mobile phone or laptop). It’s been on the market for 15 years, so it’s proven and its costs are known, though like nickel, cobalt is pricey. Cobalt is more reactive than nickel or manganese, meaning it offers high electrical potential when paired with graphite anodes, giving higher voltage. It has the highest energy density—but when fully charged, it is the most prone to oxidation (fire) caused by internal shorts. This can lead to thermal runaway, where one cell causes its neighbors to combust, igniting the whole pack almost instantly (think YouTube videos of burning laptops). Also, the internal impedance of a cobalt cell—the extent to which it “pushes back” against an alternating current—increases not just with use but with time as well. That means an unused five-year-old cobalt cell holds less energy than a brand-new one.

Cobalt dioxide cells are manufactured by dozens of Japanese, South Korean, and Chinese companies, but only Tesla Motors uses them—6,831 of them to be specific—in an electric car. Their pack uses sensors, cell isolation, and liquid cooling to ensure that any energy released if a cell shorts out can’t ignite any of its neighbors.

Nickel-cobalt-manganese (NCM)

Nickel-cobalt-manganese (NCM) is somewhat easier to make. Manganese is cheaper than cobalt, but it dissolves slightly in electrolytes—which gives it a shorter life. Substituting nickel and manganese for some of the cobalt lets manufacturers tune the cell either for higher power (voltage) or for greater energy density, though not both at the same time. NCM remains susceptible to thermal runaway, though less so than cobalt dioxide. Its long-term durability is still unclear, and nickel and manganese are both still expensive now. Manufacturers include Hitachi, Panasonic, and Sanyo.

Nickel-cobalt-aluminum (NCA)

Nickel-cobalt-aluminum (NCA) is similar to NCM, with lower-cost aluminum replacing the manganese. Companies that make NCA cells include Toyota and Johnson Controls–Saft, a joint venture between a Milwaukee automotive supplier and a French battery firm.

Manganese oxide spinel (MnO)

Manganese oxide spinel (MnO) offers higher power at a lower cost than cobalt, because its three-dimensional crystalline structure provides more surface area, permitting better ion flow between electrodes. But the drawback is a much lower energy density. GS Yuasa, LG Chem, NEC-Lamilion Energy, and Samsung offer cells with such cathodes; LG Chem is one of two companies competing to have its cells used in the Chevrolet Volt.

Iron phosphate (FePo)

Iron phosphate (FePo) might be the most promising new cathode, thanks to its stability and safety. The compound is inexpensive, and because the bonds between the iron, phosphate, and oxygen atoms are far stronger than those between cobalt and oxygen atoms, the oxygen is much harder to detach when overcharged. So if it fails, it can do so without overheating. Unfortunately, iron phosphate cells work at a lower voltage than cobalt, so more of them must be chained together to provide enough power to turn a motor. A123 Systems—which is competing for the Volt contract as well—uses nanostructures in their FePo cathodes, which it says produces better power and longer life. Other manufacturers include Gaia and Valence Technology.

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