■FIGURE 5. The display unit
in the Prius dashboard.
Charge %, Charge Current Limit,
Discharge Current Limit, Max Battery
Temperature, and Min Battery
Temperature. In this way, the whole
system could be built up and tested
away from the car.
The second stage of the program
used only one of the items (SOC%)
and gave out one of two outputs —
high or low — depending on the
value of SOC. In order to maintain
the existing battery SOC at around
70%, a simple pair of decision
instructions in Flowcode are put on
the low output if SOC% > 70 (and
disconnected the Li-Ion cells from the
charge circuit) and put on the high
output if SOC%< 65 (which switched
the Li-Ion cells into the circuit and
charged the NiMh Prius battery). In
each case, the opposite output
would be turned off.
One additional output was used
to drive a relay which, in turn,
energized the four main contactors.
This output would come on five
seconds after the system powered
up and would go off in response to
the additional battery pack becoming
discharged.
There was no need for a display
on the final controller and this now
lives in an enclosure in the boot
(or trunk for us Yanks -Ed) next to
the extra batteries and power
contactors.
The additional battery pack is
a set of 56 Thunder Sky Li-Ion cells.
These cells are about two years
old and vary in capacity; the worst
being about 50 Ah at 20 degrees
C when discharged at 25 amps. The
worst cell defines the pack capacity,
so with the current limit set to 25
amps the car will run for two to
three hours in assist mode until the
battery pack switches off. The car
then runs in normal hybrid mode
as before.
One drawback of the system is
these batteries are still very expensive
and physically quite large. Another
is that the batteries take up some of
the boot space as you can see in
Figure 6.
Figure 8 shows the Flowcode
program. Flowcode allows users to
develop a program by dragging
standard flow chart icons onto the
workspace and then clicking on them
to enter the properties. Users can
simulate this on screen, and when
the program is debugged can
compile the flow chart to machine
code which can be downloaded to
a PIC microcontroller. Flowcode
includes built-in routines and dialogs
for CANbus communication which
allows you to easily transmit and
receive packets of CAN data.
Conclusion
In summer, the car will return
■FIGURE 6. Photos showing before and after pictures ofthe battery system.
■ FIGURE 7. The inverter in the boot.
August 2008 59
