1. Design for Prius C Plug-In Conversion

2. Objective
Add an additional battery and charger to compliment the Prius C’s existing hybrid drive system to improve overall efficiency

3. Outline
Will roughly follow what is known as the “contactor method” already proven in full-sized Prius conversions
LiFePO4 pack as extra battery with higher voltage than hybrid battery to avoid using a DC-DC converter
Connected to Hybrid Drive system in parallel with original hybrid battery
Battery connection controlled by an electrically controlled contactor
Contactor controlled by an Arduino microcontroller
Arduino monitors state of charge, current, voltage and cell under/over voltage and sets state of the contactor

4. Build Steps
Get Arduino to read the CANbus, specifically the State of Charge (SOC) of the Prius’ battery
Build the hybrid battery pack, including Battery Monitoring System
Build the interface board between Arduino and the battery as well as instrumentation and control to include
LiFePO4 battery current
Relay for charge control
Main contactor control
Display system for hybrid pack information
Display hybrid pack SOC
Warning for system faults
Incorporate LiFePO4 pack battery charger
Develop Arduino code for system control

5. Arduino and CANBus
CANBus Shield gives Arduino the ability to read and log CANBus data
Reading of CANBus is necessary to find the Hybrid Battery’s SOC to know when to open and close the contactor between it and the LiFePO4 battery to prevent over/under charging

6. Arduino and CANBus
I have already developed and Arduino sketch (program) to read and log CANBus data

7. Arduino and CANBus
No publicly available data identifies PID codes for Prius C’s unique attributes
Reverse engineering was necessary to find the Hybrid Battery’s SOC on the CANBus

8. Battery
LiFePO4 chemistry chosen due to proven use in full-EV conversions
Long cycle life
Flat discharge curve
High power/weight
Hybrid battery is 144V nominal
LiFePO4 nominal voltage will be 154V to allow for low-rate charge of Hybrid battery when connected in parallel
48 Cell, 20AH GBS Batteries, 3KWH pack

9. Battery Management System
To provide LiFePO4 cell under/overvoltage (UCV/OCV) protection and alarm as well as inter-cell balancing, a Battery Management System(BMS) is necessary
Ready made systems for full-EV conversions are expensive (~$1000 for my application) and redundant to capabilities inherent to Arduino

10. Battery Management System
Maxim MAX11068 IC chosen for my application
Provides UCV/OCV alarms
Total pack voltage
Inter-cell balancing
Pack temperature
Two wire interface (I2C) to Arduino to provide alerts
MAX11068 Evaluation Kit (~$250) will be used to reduce time and cost in producing PCB

11. Interface Board
A small PCB will be necessary to support several interface features
Provide 12V Battery power to Arduino and interface systems
Transistor interface to activate contactor
Allegro MicroSystems ACS758 IC chosen to measure bidirectional current for LiFePO4 pack
Relay for controlling LiFePO4 battery charger
Relay for sensing if AC is still plugged in

12. Display System
A small LCD will be mounted in view of the driver to provide information about the system
LiFePO4 pack voltage, current, SOC
State of contactor
Warning for system faults

13. Battery Charger Elcon PFC 1500 chosen
Mounted onboard to allow for charging away from home
Will recharge a fully discharged pack within three hours via 120VAC

14. Safety Features Numerous software and hardware features
Software trip of contactor
OCV/UCV
Abnormally high charge/discharge current
Over-temperature
Hardware
Fuses for main cabling
Barrel switch near driver to allow for manual disconnection of LiFePO4 pack
Inertial switch to trip contactor in event of a crash

15. Pseudo-Code Initialization
Determine SOC of Hybrid battery
Verify system health
If Hybrid SOC <80% and LiFePO4 pack healthy (>20% SOC, no OCV/UCV or over-temp)
Shut main contactor
If Hybrid SOC >90% or any fault detected
Open main contactor
SOC for shutting and opening contactor will be modified after initial testing to optimize use of stored energy in LiFePO4 pack

16. Performance Estimates
Stock Prius C advertises ½ mile on EV only mode
0.9KWH NIMH pack, max DOD 45%
With addition of 3KWH LiFePO4 pack, max 80% DOD, up to 6 additional miles in EV only mode
In blended mode, full sized Prius conversions have resulted in >80MPG during normal commuting
Due to smaller vehicle size and larger proportional pack size, expect as good or better than 80MPG

17. Follow my progress at: http://www.100mpgpriusc.com
Conclusion
Follow my progress at: