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A Hybrid-Vehicle Technology Primer Bruce Bonebrake reaveals some of the reasons why a hybrid vehicle may be in your future. From "Weekend Mechanic" episode WKM-404

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	The "intelligent power unit" (IPU) consists a bank of nickel metal-hydride batteries and a computerized power control unit (PCU).

The High-Voltage Battery System

Important: For this demonstration, we removed the compartment cover in order to provide a closer look at the hybrid battery system. Under ordinary circumstances there is no reason the consumer should ever remove the unit's cover panel or need access into the battery compartment. Because of the high voltages, removing the cover can be highly dangerous. Service requiring this type of access should be performed by trained technicians at the dealership.

The battery system is located at the rear of the vehicle so, to access and inspect the system, Bruce and Randy must first remove the back seat. After removing the back seat, the shut-off switch is accessed and the high-voltage system off is turned off. (This is what emergency personnel will do in the event of an accident.)

The high-voltage battery on the Honda Civic Hybrid is 144 volts. Other hybrid high-voltage batteries range from 144 to over 350 volts.

Note: On the Civic Hybrid, the high-voltage battery system shut-off switch is accessed behind the back seat. Other hybrid makes have different shut off switch locations.

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The complete battery unit, which Honda terms the intelligent power unit, consists of the power control unit (PCU) and the battery pack.

The wiring harness comes in from the engine to the PCU, which controls the flow of electricity between the intelligent motor assist (IMA) motor and the battery pack. The PCU consists of a control module computer, a DC-to-DC converter and an AC inverter.

When the vehicle is coasting or cruising, or the brakes are applied, the system reverses it's polarity, turning the electric-assist motor into a generator. The mechanical energy is converted into electrical energy and sent to the battery pack.

An integrated fan system helps keep the battery pack and PCU cool.

The battery pack is actually a bank of nickel metal-hydride cells, much like those used in cell phones and portable computers. In the case of this Honda Civic, there are 120 of these individual 1.2 volt units. These batteries cannot be overcharged nor can they be completely drained during regular driving.

Although nickel metal-hydride batteries are expensive--around $2000--hybrid car manufacturers typically offer warranties of 8 to 10 years, 100,000 miles or the life of the vehicle.

The battery also has an integrated fan and cooling system which draws air in from the vent behind the back seat.

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