Chapter 22 Hybrid Drive Systems 423
Hybrid Battery Voltage
Hybrid batteries operate off of high voltage because
higher voltages improve motor-generator effi ciency. Many
hybrids use a high horsepower or high torque electric motor
to propel the vehicle.
Hybrid battery pack voltage, depending on the make
and model of the vehicle and number of cells used, can vary
from about 150 volts up to approximately 300 volts DC.
Since an HV battery pack has several cell modules wired in
series, you can use Ohm’s law to calculate HV battery volt-
age output. For example, if one battery pack module supplies
7.2 volts DC and it contains 38 modules, you would simply
multiply the voltage by the number of modules. This battery
pack would therefore supply 274 volts DC. A 28-module
battery pack would produce 201 volts DC. Another manu-
facturer HV battery pack might have 30 modules of 9.6 volts
each wired in series for a voltage output of 288 volts DC.
Hybrid Battery Rating
Hybrid batteries are often rated by their energy to
weight ratio. It is a very important measurement because a
hybrid battery pack can be heavy and affect fuel economy.
A hybrid battery must be lightweight but provide high-
energy output for extended periods of time.
A hybrid battery pack is often rated for watt-hours per
kilogram (Wh/kg). The higher the number in the Wh/kg rat-
ing, the more power the battery has per pound. Conventional
12-volt lead acid batteries have a Wh/kg rating of about
30 Wh/kg. Hybrid NiMH batteries have a Wh/kg rating of
70 Wh/kg and have a much longer service life than other
battery types. Lithium-ion batteries have double the Wh/kg
rating of NiMH at 140 Wh/kg and are found in hybrids and
in all electric vehicles.
Hybrid Battery Pack Operation
The HV battery pack operates the power control mod-
ule. It sends high-voltage DC into the power control module
for inversion into AC or conversion to a higher or lower
voltage. This high-power electrical energy is then fed to the
motor-generator for propelling the vehicle or starting the
gas engine when the HV battery pack becomes discharged.
High-voltage batteries are needed in hybrid vehicles because
higher voltage improves motor-generator effi ciency.
The hybrid battery normally supplies the DC voltage
to an inverter circuit that converts the electrical energy into
600 to 650 volt 3-phase AC for the electric drive motor.
Refer to Figure 22-6. AC current is much more effi cient
than DC for high-power demands found in a hybrid drive
system. The inverter module also changes AC output from
the large generator back into DC to recharge the HV bat-
tery pack. Many inverter modules step-up battery voltage
to almost 600 volts AC for increasing drive motor power on
rapid acceleration.
HV Battery Pack Ventilation
Battery pack ventilation is used to cool the battery pack
and to vent any electrolyte gases away from the vehicle.
Routing cool, fresh air over the battery pack can also keep the
battery temperature from increasing too much under high-
load conditions. When a hybrid initially accelerates under
all-electric power, the battery pack and large power cable
can get hot. Forced air ventilation helps remove some of this
heat to avoid overheating damage to the battery cells.
Forced battery pack ventilation is often provided by an
electric blower motor and fan assembly that directs outside
air through the battery pack enclosure. The 12-volt blower
motor is similar to that used in a passenger compartment
Figure 22-6. Converter inside the power control module can also reduce high voltage to 12 volts DC for charging the auxiliary
battery. (Toyota)
DC/DC Converter
Input
Filter
DC 288 V
AMD
GND
Auxiliary
Battery (12 V)
NODD VLO
Motor-Generator ECU Hybrid Drive ECU
Converter Control Circuit
High Voltage
Low Voltage Fuse
HV Relay
Step-Down
Transformer