Prius PHEV Conversion



University of IDAHoPrius PHEV ConversionLead Acid Battery Add onBenjamin Barr, Ryan Litzko, Mark Eisenhuth, and Jun Li5/3/2013TABLE OF CONTENTS 1. TOC \o "1-3" \h \z \u EXECUTIVE SUMMARY3 2. BACKGROUND PAGEREF _Toc342916328 \h 4 3. SPECIFICATIONS10 4. ELECTRICAL SYSTEM DESIGN PAGEREF _Toc342916332 \h 111 System Overview11 Microcontroller12 Battery Charging Scheme Testing12 5. MECHANICAL SYSTEM DESIGN16 Battery Box Layout16 Battery Containment System17 Ventilation System17 6. BUDGET18 7. DESIGN FAILURE MODE EFFECT ANALYSIS (DFMEA)..…………………………………………………………………..19 8. FUTURE WORK20 9. APPENDICIES ..…………………………………………………………………………………………………………………………….22 APPENDIX I (Lead-Acid Specifications) APPENDIX II (NiMH Specifications) APPENDIX III (Electrical System) APPENDIX IV (Mechanical System) 1. EXECUTIVE SUMMARYThis project’s goal is to expand the range of a Toyota Prius by using a new battery developed at the University of Idaho that is composed of lead-acid. This project involves designing a prototype that contains a battery pack that can fit into the back of a Toyota Prius. The package will have a ventilation system, charging system, and a battery status indicator. In order to understand the task at hand, some background of the 2004-2009 Toyota Prius Hybrid System is required. The Prius has a gas engine, an electric motor, and a generator. The gas engine and the electric motor power the car while the generator recharges the batteries. The generator keeps the batteries charged at about 60%. The motor, generator, and engine work together during the four different driving modes of the Prius. The first mode occurs when the car is starting out (speed < 13 MPH) or when the car is in reverse. In this mode, the NiMH batteries are the sole power source. The second driving mode consists of moderate acceleration and speed above 13 MPH. In this mode, the car is only powered by the internal combustion engine and an electrical assist from the generator. In the high acceleration/speed mode, the car is powered by the internal combustion engine and an electrical assist from both the MG’s and the NiMH batteries. In Mode four when decelerating or braking, regenerative braking occurs. Regenerative braking consists of the generator converting the kinetic energy of the car into electricity in order to recharge the Prius’s NiMH batteries. Understanding each mode and the background of the Prius helps in understanding how the solution is implemented and interacts with the system.The solution starts from a wall outlet that provides power to the five lead-acid smart chargers. These chargers are denoted as “smart” because they have both overcharge protection as well as thermal protection. The chargers charge the lead-acid batteries in series for the most efficient charge. The lead-acid batteries are in parallel with the NiMH modules. When the voltage of the NiMH batteries become lower than the lead-acid batteries, current flows from the lead-acid into the NiMH, causing the NiMH batteries to charge. During different modes of operations, like for example the regenerative braking mode, the NiMH voltage can and will be higher than the lead-acid voltage at some point. This voltage difference will give a negative current flow. This negative current flow is problematic because the Prius will be essentially recharging the lead-acid batteries instead of the NiMH. For that reason, a set of diodes are included. These batteries are stored in a box that will be placed in the rear cargo part of the car. The current box is made out of plexiglass, angle iron, and steel rods. The box is fastened together by nuts and bolts. There are spacers mounted inside the box to ensure that batteries do not shift around in the box while the car is in motion and to keep the batteries desired spacing. These spacers are made out of plexiglass and are glued with epoxy to the plexiglass top and bottom of the box. Computer fans provide thermal management and venting. The overall weight of the entire system is roughly 550lb.Included in this report is a future work area as well as test data to verify that the electrical design is functional.2. BACKGROUNDIn order to understand the task at hand, some background of the 2004-2009 Toyota Prius Hybrid System is required. The main components consist of the 1NZ-FXE Internal Combustion (IC) Engine, Hybrid Transaxle, Inverter Assembly, HV Battery, and HV Electronic Control Unit (ECU). Figure 1 shows the Hybrid System Overview.Figure 1: Hybrid System Overview2.1 Prius Components2.1.1 1NZ-FXE IC EngineThe IC Engine is a 1.5-liter gas engine with electronic throttle control. Attached to the IC Engine is the Hybrid Transaxle, which consists of the Motor-Generator 1 (MG1), Motor-Generator 2 (MG2), and the Planetary Gear Set. Figure 2 shows the IC Engine attached to the Hybrid Transaxle. Figure 2: IC Engine (left), Hybrid Transaxle (right, circled in red)2.1.2 Hybrid TransaxleMG1 recharges the HV battery from the IC Engine’s unused mechanical energy as well as supplying supplemental electrical power to help drive MG2. MG2 also acts as the engine starter and the controller of the transaxle’s continuously variable transmission. MG2 acts as the vehicles primary mover at low speeds and supplements the IC Engine at higher speeds and high acceleration. MG2 also recharges the battery through the regenerative braking scheme. Planetary Gear Set acts as a power splitting device which is controlled by the ECU and splits power between the IC Engine, MG1, and MG2. The ECU acts as the “brain” of the car and controls all aspects of the car (ie: when and where to split power to). Figure 3 shows an up-close look at the Hybrid Transaxle.Figure 3: Up-close look at the Hybrid Transaxle2.1.3 Inverter AssemblyThe Inverter Assembly is an essential part of the electrical system because it controls the amount of current between MG1, MG2, and HV battery. When providing power from the HV battery to the MG’s, the assembly converts HV battery DC to AC. When either MG is generating power for recharging purposes, the assembly rectifies the HV AC power from MG1 and MG2 to an acceptable DC signal for the HV battery. Figure 4 shows the Inverter Assembly.Figure 4: Inverter Assembly2.1.4 HV BatteryThe final main component of the hybrid system is the HV battery. The battery is made up of 28 NiMH modules at 6 cells/modules, which totals to 168 cells. At nominal charge, the battery’s voltage level is 201.6 V. At max charge the battery’s voltage is around 226.4 V. The job of the HV battery is to store electrical energy generated by both the MG’s as well as supplying power to both MG’s. Figure 5 shows the HV battery. Figure 5: HV BatteryIt is also important to note that the ECU keeps the NiMH’s state of charge (SOC) at an average of 60% with a deviation of 20% as seen in figure 6.41910057785Figure 6: ECU SOC Control2.2 Hybrid ModesThe Toyota Prius has 4 hybrid modes.2.2.1 Mode 1: Starting out/ReverseThe first mode occurs when the car is starting out (speed < 13 MPH) or when the car is in reverse. In this mode, the NiMH batteries are the sole power source.Figure 7: Starting Out/Reverse2.2.2 Mode 2: Normal DrivingThe normal driving mode consists of moderate acceleration and speed above 13 MPH. In this mode, the car is only powered by the internal combustion engine and an electrical assist from the MG’s.Figure 8: Normal Driving2.2.3 Mode 3: High Acceleration/SpeedIn the high acceleration/speed mode, the car is powered by the internal combustion engine and an electrical assist from both the MG’s and the NiMH batteries.Figure 9: High Acceleration/Speed2.2.4 Mode 4: Regenerative BrakingWhen decelerating or braking, regenerative braking occurs. Regenerative braking consists of the MG’s converting the kinetic energy of the car into electricity in order to recharge the Prius’s NiMH batteries.Figure 10: Regenerative Braking3. SPECIFICATIONS3.1 Mechanical3.1.1 Battery Box The battery box must fit behind the rear seat of a Toyota Prius in a 37x34x17 inch space. It must be able to be moved and tipped over while still maintaining battery position. When lifted, the box can have no more than appropriate sagging. It must be able to be lifted by an engine hoist. 3.1.2 VentilationThe battery box must be vented to the outside without passing through air that is susceptible to sparks . The noise produced by the ventilation system must be below 80dB.3.1.3 OtherThe payload must not intrude on cars handling. 3.2 Electrical3.2.1 NiMH BatteriesThe NiMH batteries cannot charge lead acid batteries. When charging the NiMH batteries, current must not exceed 115 amps. The NiMH batteries must be protected from Surge current. 3.2.2 Lead-acid BatteriesThe lead-acid batteries must have no more than 35% energy waste. They must be protected from over charging. The lead-acid battery pack must contain at least 5 kilowatt hours of energy. The battery pack must fully charge within 8 hours.3.3 Other3.3.1 SafetyThe design must comply with NEC regulations. The LV electronics should be separated from the HV electronics. When the car is off and the lead-acid batteries are charging, the NiMH batteries cannot charge.3.3.2 BusinessThe design must be cost effective/profitable.4. ELECTRICAL SYSTEM DESIGN4.1 System OverviewIn Figure 11, the overall electrical scheme can be found. Figure 11: Electrical System OverviewStarting from the left, the 115 volt ac wall outlet provides power to the five lead-acid smart chargers. These chargers are denoted as “smart” because they have both overcharge protection as well as thermal protection. The chargers charge the lead-acid batteries in series for the most efficient charge. The 17 lead-acid batteries are in parallel with the 28 NiMH modules. When the voltage of the NiMH batteries becomes lower than the lead-acid batteries, current flows from the lead-acid into the NiMH, causing the NiMH batteries to charge. This current is monitored by the current sensor. During different modes of operations, like for example the regenerative braking mode, the NiMH voltage can and will be higher than the lead-acid voltage at some point. This voltage difference will give a negative current flow. This negative current flow is problematic because the Prius will be essentially recharging the lead-acid batteries instead of the NiMH. For that reason, a set of diodes are included.4.2 MicrocontrollerA microcontroller is added to take data from the current sensor, voltage sensors, and temperature sensors as well as turning on and off the relay and displaying the energy left in the lead-acid battery pack.Figure 12: Microcontroller Flow ChartThe microcontroller flow chart as shown in figure 12 becomes active once the car is on. Once the ignition has been turned on, the controller monitors inputs from voltage meters, temperature sensors, and a current meter. The microcontroller then evaluates the received inputs and either turns on or off the relay and adjusts the energy gauge lights according to the voltage left in the lead-acid batteries. For example, the microcontroller will turn off the relay if the temperature on the batteries is too high, if the voltages on the lead-acids are too low, or if the current between the two parallel batteries is too high.4.3 Charging Scheme TestingSince the project’s budget did not allow for the purchase of a Toyota Prius for testing, a constant resistive load bank was used to simulate the load of a Prius. Prius NiMH battery modules were also purchased so that we could test the charging scheme on a ? scale. Since the electrical design calls for 17 lead-acids, the ? ratio of NiMHs is 6.5 modules. However, this is obviously not possible. In order to solve this problem, a series of tests were ran on both 6 and 7 NiMH batteries which provided enough data to give an average of 6.5 NiMH batteries. It is also important to note that a max current test was ran (lead-acid fully charged, NiMH depleted) which led to the conclusion of max current flow being on average of 40 amps.4.3.1 Test SetupThe test setup consisted of 4 lead-acid batteries in parallel with 6-7 NiMH batteries. In between these sets of paralleled batteries was a relay and a diode. These were included to test the protection aspect of the design as well as a realistic voltage drop that the actual system would see. For data acquisition, volt meters were attached to both the lead-acids and the NiMHs. A current meter was placed in between the two sets of batteries in order to monitor the current. See figures 13, 14, and 15 for visual representations of the test setup.Figure 13: Test Setup for NiMH Battery Charging Scheme (Diagram)Figure 14: Test Setup for NiMH Battery Charging Scheme Figure 15: Close-Up on Relay and Lead-Acid Testing Configuration4.3.2 Test Procedure1. Float charge Lead acid batteries (settles to about 13 volts) and make sure each NiMH module is at nominal charge (7.2 volts)2. Create test circuit shown in figure 9 with relay open, 4 lead-acid batteries in series, and 6 NiMH batteries in series.3. Close relay and record Vlead-acid, VNiMH, and Itransfer every 30 seconds4. Repeat steps 1, 2, and 3 except replace the 6 NiMH with 7NiMH. Once this is complete, move to step 5.5. Take recorded values and average them to achieve 17 lead-acid battery to 28 NiMH battery ratio.4.3.3 Test ResultsFigure 16 shows that the lead-acid batteries are charging the NiMH batteries. This is apparent because as the NiMH voltage decays, the lead-acid matches the decay rate while keeping an average of 2 volts of potential difference. This was a 30 minute high load test.Figure 16: Close-Up on Relay and Lead-Acid Testing ConfigurationFigure 17 shows the difference of performance between having an added lead-acid battery pack and not having one. The blue data shows how only the NiMH batteries behave with a high load. The blue curve starts at full charge and within 2 minutes the test had to be stopped to avoid damaging the NiMH batteries. The red data depicts the NiMH batteries combined with the lead-acid battery pack. Clearly there is a major performance difference. With the added battery pack, the batteries were tested for a 30 minute length. This seems very impressive but it is actually more impressive if you take into account that the 32 volt mark is where these combined batteries must stop discharging to avoid damaging the lead-acid batteries. That means that this test could have been much longer. Both battery sets were tested at the same load. Figure 17: Close-Up on Relay and Lead-Acid Testing Configuration5. Mechanical SystemThis is an overview of the mechanical systems which includes the battery box layout, battery containment system, and the ventilation system. The current box is made out of plexiglass, angle iron, and steel rods. The box is fastened together by nuts and bolts. There are spacers mounted inside the box to ensure that batteries do not shift around in the box while the car is in motion and to keep the batteries desired spacing. These spacers are made out of plexiglass and are glued with epoxy to the plexiglass top and bottom of the box. Computer fans provide thermal management and venting. The overall weight of the entire system is roughly 550lb.Figure 18: Overall View of Battery Box5.1 Battery Box Layout2061210106045The white rectangles symbolize 1 battery; there are 17 in total. The gray rectangle symbolizes the area where the electronic boxes will be stored. There are separate electronic boxes in order to adhere to the NEC code, which states that there must be separation between low and high voltage components. The red circle shows where the fans are located.Figure 19: Battery Box Layout5.2 Battery Containment SystemThe figure below depicts our battery containment system. Each individual battery is held in its own cubby-hole on both the top and bottom of the box. Four bolts in each corner of the box sandwich the top and bottom of the box to ensure that the batteries are adequately held in position. The cubby-hole design also guarantees that the batteries have space to be thoroughly vented and cooled.Figure 20: Battery Containment System5.3 Ventilation SystemAir from the cargo area of the vehicle is pulled into the box and forced over the largest surface area of the batteries. Heat and sulfide gas is taken away from the batteries and forced outside the vehicle through an “exhaust pipe”.Figure 21: Ventilation System6. BUDGETBill of MaterialsItemQuantityPriceCostAdd on Board179.9979.99Microcontroller149.9949.99Current sensor19797Relay1107107Diodes42.379.48Charger(12V)14949Charger(48V)4542161.2M ohm Resistors10.0670.06720M ohm Resistors10.0670.0671.2k ohm Resistors20.0410.082NiMH battery cell840320Temp Sensor211.1422.28Heat Sink23.156.3Red LED20.0870.174Yellow LED20.0870.174Green LED20.1310.262Voltage Transducer LV 20-P142.4542.45TDK-Lambda CC6-1212DF-E115.815.8Weld on #4110.3710.373ft Slotted Angle Iron24.949.886ft Slotted Angle Iron28.6617.32Nut (box of 100)12.682.68Bolt (box of 100) 19.959.95Washer (box of 100)12.582.5812V DC Fan323.3470.02Acrylic (Plexiglass) 24" X 36" sheet, 1/4” thick 574.17370.85Acrylic (Plexiglass) 24" X 24" sheet, 1/4” thick 251.59103.18Acrylic (Plexiglass) 24" X 48" sheet, 1/16” thick 1242416" Bolts49.6738.68Thick-Wall Clear PVC Unthreaded Pipe (2ft)45.823.2General Purpose Low-Carbon Steel (6ft)104.4744.7Misc. Hardware (Paint, nuts/bolts, etc)150508 AWG wire501.0753.5Total Cost??1767.036Figure 22: Cost to Rebuild Entire System7. DESIGN FAILURE MODE EFFECT ANALYSIS (DFMEA)DFMEAELECTRICALSeverityOccurrenceDetectionScoreIndividual Battery Failure36236Relay102240Diode1025100Microcontroller1224Fan(s)53115MECHANICALVentilation (Feedback)875280Plexiglass Molding886384Box Frame104280Figure 23: DESIGN FAILURE MODE EFFECT ANALYSIS (DFMEA)The DFMEA shows the probability that a failure will occur for a particular component of the system. The highest score possible is 1000 which means catastrophic failure is unavoidable, therefore the lower the score the better. Our highest score occurred with the plexiglass molding. The reason for this is because the glued plexiglass has a tendency to break its bond if not glued 100% correctly. This is problematic because if the molding breaks, the batteries are now free to move about. With freely moving batteries, short circuiting and damages to the box and frame become very plausible. Based on its number of 384 out of 1000 this needs to be addressed in the next design.8. FUTURE WORK8.1 Electrical SystemThe electrical system has several different components right now that are not perfect. For instance the microcontroller is a prototyping board that can be purchased for $49.99. The problem with this is that the actual component needed can be purchased for around $3.00. When making several thousand of these products, reducing the cost of the microcontroller will be much more cost effective. The next item is the diodes. Right now a pack of diodes in parallel are used to reduce the cost of the prototype, however, if this were to be brought to market it could be better to reduce the components from multiple diodes to one diode. This will save space and money if bought in bulk. Also, the perforated circuit board that we used for our design for the sake of time should be translated to a printed circuit board (PCB). A PCB is more sturdy and reliable, and will save time and money over time due to how cheap they are to manufacture once designed. Lastly, instead of a visual energy gauge made out of LEDs, a microcontroller controlled LCD with Bluetooth would be the most marketable gauge scheme. This would enable the driver to constantly monitor the usable energy left in his/her lead-acid battery pack.8.2 Mechanical SystemThe current box designs have some issues that need to be addressed. First, the flat plexiglass pieces should be replaced with a stronger material such as high strength polycarbonate sheets. Second, steel slats should replace the current metal rods. With slats instead of rods, the overall box will be much stronger. Third, a standard molding for holding the batteries in place would be easier for manufacturing and constructing purposes. For a visual representation of what a future model would look like, see the figure below. Figure 24: Future Battery Box Design8.3 Safety327215555880To protect their batteries from inrush current, the Prius uses a system shown in the figure to the right. We would recommend a system similar to this with SMR 2 and 3 normally being open or closed to control the system however when the current becomes too high SMR 1 and 3 are closed instead to limit current. On the current design, the ventilation system allows back flow into the passenger area. This cannot be allowed for safety reasons; therefore, the future mechanical box should have a one-way venting system like in the figure below. For this design to work, the fans must have enough suction power to open the blinds. There also must some type of device, like a spring, to close the blinds when the fans are turned off.Figure 25: Future Battery Box Design9. APPENDICIESAppendix I: Lead Acid SpecificationsAerovironment Battery TestAppendix II: NiMH SpecificationsQuest Battery Appendix III: Electrical System16 VS 17 VS 18 Battery Voltage Profile DesignsPrius PHEV TechInfoDetermination of Lead-Acid Battery Capacity Via Mathematical Modeling TechniquesLM35 – Temperature SensorHoneywell, CSNR151 Current SensorOn Semiconductor, Zener Voltage RegulatorsVishay, 30A RectifierVishay, 60A DiodeVishay, Metal Film ResistorsLittle Rebel, Carbon Film ResistorsLEM HTR 200-SB Current TransducerPanasonic 120A RelayLEM Voltage Transducer LV 20-PCC-E Dual DC-DC Converter +/-12 to 15VT0-247 Diode Heat Sink Appendix IV: Mechanical SystemSparco Products, CartRubbermaid, 35-gallon boxWeld-on #4Construction Pictures of Final Product ................
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