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

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

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

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

Arduino and CANBus

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

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

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

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

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

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

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

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

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

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

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

Conclusion

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