Chapter 22 Hybrid Drive Systems 435
Figure 22-27. Hybrid dashboard display gives the driver information about fuel consumption and electrical power consumption so the
vehicle can be driven effi ciently in all-electric drive mode. (Toyota)
Discharge
Warning Light
Malfunction
Indicator
Lamp
Master Warning
Light
HV Battery
Warning Light
Ready Light
The power display can help the driver gently apply
the brakes so that the vehicle slows using the electric drive
generator. With practice, you can slow the vehicle in many
cases without the conventional friction brakes at each
wheel touching the rotors.
A hybrid driver ready light often blinks when you
press the brake pedal and turn the ignition key to the start
position with the gear selector in park. When the electric
drive is engaged, the ready light glows continuously to let
you know the vehicle is ready to be driven.
A hybrid master warning light in the combination meter
warns if there is a problem in any part of the drive train. If the
hybrid ECU detects any abnormal condition in the electric drive
circuitry or related components, it will illuminate the master
warning light to notify the driver that something is wrong.
A battery pack warning light in the combination meter
illuminates if the battery’s state of charge is below normal
limits. This could be caused by leaving the key on too
long when parked, running the air conditioning too long
on battery pack energy, a shorted battery pack, or a circuit
problem preventing the battery pack from being recharged
normally. A conventional malfunction indicator light (MIL)
turns on when any ECU detects a vehicle malfunction.
Hybrid System Voltages
Most hybrids use a dual-voltage system: a 12-volt bat-
tery and an HV battery pack. Both electrical systems are
wired separately but interface through the ECU and hybrid
power control module. The 12-volt battery is used to hold
computer memory; power the on-board computer; and
operate the headlights, radio, gauges, navigation system,
ignition system, electronic fuel injection, and other low-
voltage systems. The HV battery pack powers the electric
drive system and can also be stepped down to provide
lower voltages. The high- and low-voltage systems are iso-
lated from each other and operate independently.
A hybrid low-voltage system uses conventional 12 volts
DC for all electric circuits except the hybrid drive system.
The 12-volt battery is used to store computer memory and
provide low voltage to electrical accessories. The low-volt-
age battery provides the small amount of current needed
for the electronic fuel injectors, fuel pumps, ignition coils,
sensors, and other components during engine starting. It also
powers the dash display, stereo, and other accessories when
the engine is not running.
Some hybrids do not have a 12-volt battery but use
a step-down transformer in the power control module to
send 12 volts to the low-voltage electrical accessories.
The hybrid high-voltage system typically operates on
voltage from around 250 volts to 650 volts AC or DC. The
large conductors and electrical-electronic components in
the high-voltage system feed thousands of amps of current
through the electric drive system. This is enough energy to
power several small homes.
Note: Refer to vehicle specifi cations for
exact hybrid operating voltages. Specifi c
voltage levels will vary with each make and
model of hybrid vehicle.
Hybrid nominal operating voltage is the maximum DC
voltage available from the HV battery pack. The HV battery
pack nominal output voltages typically range from about
100 volts to more than 300 volts. Higher nominal voltages
increase the effi ciency of the electric drive system.
Hybrid maximum voltage is the three-phase AC voltage (up
to 650 volts AC) fed to the motor-generator from the power con-
trol module. For example, approximately 300 volts DC from the
battery back is converted into 650 volts AC for the drive motor.
Some hybrids have an electrically-driven air condi-
tioning compressor that operates on about 50 volts. The
hybrid power control module provides this voltage level for
operating an electric motor that drives the air conditioning