422 Auto Electricity and Electronics
energy is changed into electrical energy and forced into
the HV battery pack. The battery pack stores this energy
and sends it to the motor-generator during acceleration
to move the vehicle. Braking may feel different since the
motor-generator drag slows the vehicle without applying
hydraulic friction brakes.
Note: In some hybrid systems, the internal
combustion engine often aids the
regenerative braking by opening the engine
valves so the engine freewheels and engine
compression does not absorb as much power during
deceleration and braking.
Engine Starting Mode
In the engine starting mode, the motor-generator
spins the engine crankshaft to start the internal combustion
engine. The engine then runs to propel the vehicle at cruis-
ing speeds or to drive the motor-generator when the battery
pack has become partially discharged.
The hybrid ECU energizes transmission solenoids
that apply and release clutches in the transmission so the
engine crankshaft and motor-generator are locked and turn
together. The power control module then sends just enough
current to the motor-generator to spin the engine crankshaft
at about 300 rpm.
The hybrid ECU also signals the engine ECU to provide
“spark and fuel” to the engine. The engine ECU electrically
energizes the engine’s fuel pumps, electronic fuel injectors,
and ignition coil while the motor-generator spins the crank-
shaft until the engine “fi res” and runs on its own power.
Battery Pack Charging Mode
After driving a few miles on all-electric mode, the HV
battery pack becomes partially discharged. The hybrid ECU
detects the lower voltage (about 70% discharge) and starts
the internal combustion engine to propel the vehicle and
recharge the HV battery pack. Refer to Figure 22-4D.
The engine crankshaft spins the motor-generator so
it can send high current fl ow to the power control mod-
ule and battery pack. As soon as the battery pack is fully
charged, the hybrid ECU and power control module shut
the motor-generator off so it does not place any unwanted
drag or friction on the drive train.
Hybrid Battery Pack
A hybrid battery pack is several battery modules wired
in series to produce a high-voltage power source. The
hybrid battery pack normally consists of high-effi ciency
NiMH (sealed nickel metal hydride) or Li-Ion (lithium ion)
battery cells stacked in a sealed enclosure. A Li-Ion battery
pack contains the same type of sealed cells found in laptop
computers and cell phones, but much larger. Both types of
battery packs are designed to handle very high-current fl ow
rates during charging and discharging.
The hybrid battery pack is normally mounted in the
rear of the chassis, often behind or under the rear seat or in
the trunk area. See Figure 22-5.
A battery pack housing encloses and protects the series
of battery modules from damage. It also protects people
from electric shock. Some hybrids have the power control
module and battery pack housed together; others do not.
Machine screws hold the cover(s) on the battery pack.
Warning: Never remove the cover from a
hybrid battery pack. The manufacturer
warranty may become void, and battery
packs are not normally serviceable in the fi eld. If you
touch a battery pack cable with your bare hands, you
can easily be electrocuted and killed!
Battery pack cables allow all of the series cell modules
to be fastened to the power control module. Positive and
negative cables are bolted to the power control module to
assure a good electrical connection.
Battery pack temperature sensors are used to protect
against battery thermal damage. When being recharged
by regenerative braking and when powering the motor-
generator(s), a tremendous amount of current fl ows in and
out of the battery pack. If a battery temperature sensor detects
cell overheating, it signals the hybrid ECU to disconnect the
power control module and prevent further overheating.
If a condition exists (extended battery service life,
impact damage from a collision, hit from lightning, short in
cell), the ECU instantly shuts the electric drive system down
and triggers a malfunction indicator light in the dash.
Figure 22-5. High-voltage battery pack is enclosed in a metal
housing. Note the large orange power cables coming out of the
battery pack. They go to the power control module. (Toyota)