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Design for Prius C Plug-In Conversion

Design for Prius C Plug-In Conversion. Objective. Add an additional battery and charger to compliment the Prius C’s existing hybrid drive system to improve overall efficiency. Outline. Will roughly follow what is known as the “contactor method” already proven in full-sized Prius conversions

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Design for Prius C Plug-In Conversion

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

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