design for prius c plug-in conversion
Post on 25-Feb-2016
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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
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