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[edit] Theory Overview

Todo This needs to be polished, but its just a quick overview for someone looking to do a conversion of how the system functions.

The fundamental basis of this conversion is the reported state of charge (SOC) of the stock NiMH battery in the Prius and keeping that reported state of charge where we want it to encourage the hybrid synergy drive (HSD) to use as much electric power as possible (at the right times) to offset gasoline usage. During different driving profiles, it is better to use electricity at different times. However, putting that aside for now, generally, to allow all EV driving, the SOC needs to be kept in a certain range (typically around 60-63%). When the reported SOC drops below the lower threshold, the PHEV battery and the OEM battery need to be paralleled. It has been found that EV mode can cause the OEM battery voltage to drop below 200 volts while accelerating. The algorithm for determining when to parallel the OEM battery and the PHEV battery needs to parallel the batteries when the voltage drops below 200 volts to make sure the car doesn't cancel EV mode because the OEM battery voltage is too low. To get the Prius to use electricity in highway driving, the reported SOC needs to be brought up to over 70% (typically 72-73%, however, never exceeding 80%.) The charge current limit (CCL or ACL in CAN-View) must be monitored to make sure the OEM battery is not being overcharged or overheated. The Prius will then enter a "get rid of charge any way possible" and be encouraged to use more electricity (up to about 6kW.) When the batteries are paralleled, it causes a voltage rise (because the PHEV pack is a higher nominal voltage than the stock battery). When the voltage hits a certain point, it causes a state of charge drift, which, once started, very rapidly increases the reported SOC.

In the current revision of the system, CAN-View (a computer sitting on the CAN bus monitoring status) is responsible for controlling the contactors. See below for more information on specific relays on CAN-View. The output from the 6 CAN-View relays is fed into a logic statement (currently just relay 3 OR 5 OR 6) to determine when to parallel the two packs. Another relay (#4) is a special relay turned on when the system is enabled, and off when the system is disabled.

There are 4 known ways to get the Prius to use electricity in place of gas:

EV Mode (0-34mph) The Japanese version of the Prius has a button on the dash which allows the car to enter an all electric mode at speeds up to 34mph. The button is not present on the North American version, however the functionality is still present and can be enabled by tapping a wire in the dash and grounding it to enter EV mode. EV mode is available when a few criteria are met. While this mode may seem like the only option for good mileage, similar mileage can actually be obtained without using EV mode with a high SOC (see below)

Stealth (0-42mph) Stealth mode is when the car shuts off the gas engine while coasting. With a PHEV conversion, instead of being able to enter stealth for just a mile or two, you can instead stay in stealth mode much longer. Stealth will use the gas engine for acceleration (when it is most efficient) and then shut off the gas engine for coasting (when it is otherwise least efficient.) For really long trips, this is the most efficient place to use electricity.

ICEspin (42+ mph) ICEspin is difficult to enter, but will provide up to 3kW of electric drive at high speeds. It operates much like stealth, except the gas engine is still physically spinning, but the fuel injectors are off. Like stealth, a PHEV conversion allows significantly more range than the stock Prius.

ICErun + High SOC While the ICE is running, the Prius evaluates the OEM battery's reported state of charge (SOC). If the SOC is below about 60%, the Prius works to charge the battery from the ICE when the ICE is running. In a PHEV conversion, this is the opposite of what is desired (unless the PHEV battery is depleted.) If the SOC is higher than about 60%, it actually uses electricity to offset gasoline usage, (reason being that otherwise that electricity would just get wasted because regenerative braking would have nowhere to put the new power and burn it up in the brake pads.) For this reason, if the reported state of charge is either altered on the CAN bus or the OEM battery really has that much charge, the Prius will use up to about 30-40 amps (~6-8kW) to assist the gas engine. With a higher SOC, it is much easier to enter stealth and ICEspin as well. The ideal SOC, in terms of maximum electricity used, seems to be about 74%.

[edit] The PHEV Battery Pack

The PHEV pack consists of twenty 12 volt 20 amp hour sealed lead acid batteries connected in series. The batteries themselves sit in an aluminum box and are mounted above the spare tire well, but below the false floor in the trunk. The pack has a nominal voltage of 240 volts and has a total energy storage of about 4.8 kWh (not all usable.) In this design, the PHEV battery pack has a higher nominal voltage than the stock NiMH battery and is used to charge the stock NiMH battery. Contactors (large relays) are used to connect and disconnect the PHEV battery pack from the stock battery when charging is needed. The higher voltage pack cannot always be connected to the stock pack, because that would overcharge the batteries. NiMH battery packs also cannot easily be charged in parallel, so simply adding a second NiMH battery pack is not simple. The current from the battery pack is less than 60 amps, and therefore the pack is fused with 60 amp 300VDC (or higher) fuses. The batteries must be connected using 8 AWG wire or larger (smaller AWG number) to handle the amount of current.

The PHEV battery does not have its own battery management computer. As the PHEV battery’s state-of-charge (SOC) decreases, it is put in parallel with the OEM battery more and more continuously. Charge-sustaining operation at the PHEV battery’s minimum intended SOC occurs when the PHEV battery’s voltage matches the voltage of the OEM battery’s 60% SOC voltage well enough that average PHEV battery current becomes zero. This is a soft limit that depends upon driving conditions, temperatures, PHEV battery condition, and the state of the moon; and PHEV operation slowly morphs into hybrid operation rather than changing abruptly. Ordinarily, around 10-13 Amp-hr is removed from the PHEV battery before electric assist is exhausted. The depth-of-discharge (DOD = 100% - SOC) that this corresponds to is anyone’s guess, as due to Peukert’s Law (PbA batteries have lower capacity at high discharge rates) and high, variable discharge rates, the battery pack’s capacity is diminished by a large, unknown amount.

[edit] Current PbA limitations

  • The conversion adds 300+ lbs to the vehicle’s weight to provide 10 miles of electric range per charge (16.7 usable Wh/kg)
    • Though Ron has safely driven 17,000 miles in his converted Prius, the added weight could possibly cause vehicle instability during driving, and the battery may modify the effectiveness of the vehicle’s rear crush zone.
    • Existing conversions sit 1-2 inches low in the rear. Air shocks or heaver-duty rear springs would be nice, but have not yet been developed.
    • Though there are indications that improved hybrid efficiency due to a lower combined internal resistance of the two-battery combination at least partially compensates for the added weight, city gasoline mileage is otherwise reduced by up to 10%.
  • Operating costs are high due to an expected cycle life of only 300-400 deep cycles, providing only one to two years of daily driving (at 400 cycles, 10 electric miles per 2.1 kWh cycle, and $800/pack, battery cost is $0.95/kWh throughput or $0.20/electric-mile (in addition to the cost of electricity, usually 2-4 cents/mile depending on utility rates).
  • For decent battery life, the battery must always be charged within a day of discharge, making charging a required rather than optional operation (if planning to drive to somewhere without access to electricity, temporarily turn off PHEV operation).
  • PbA batteries perform very poorly in cold weather. Though our design includes a thermally insulated battery pack, heated during charging, this feature has been insufficiently tested due to moderate California temperatures during development.

[edit] Possible Future Battery Options

More advanced batteries may be retrofittable to the conversion. This will probably require upgrading to CalCars’ not-yet-designed next version of logic board, and will also probably require additional battery management electronics. Any new battery’s enclosure, mounting, and thermal management system will no doubt also be very different.

Possible future batteries and their likely characteristics (incl. low-volume pricing):

Example pack

Chemistry Usable
Yr daily
kWh $ EV mi Wt,
16 400 1.1 $380 $0.95 20.0 2.1 $ 798 10 289
NiMH worst 36 2000 5.5 $1,200 $0.60 12.6 4.2 $5,040 20 257
NiMH best 36 4000 11.0 $800 $0.20 4.2 4.2 $3,360 20 257
Li-ion worst 56 1000 2.7 $1,200 $1.20 25.2 4.2 $5,040 20 165
Li-ion best 100 4000 11.0 $800 $0.20 4.2 6.3 $5,040 30 139
NiZn worst 36 500 1.4 $500 $1.00 21.0 4.2 $2,100 20 257
NiZn best 36 2000 5.5 $350 $0.18 3.7 4.2 $1,470 20 257
Firefly PbA worst 36 1000 2.7 $350 $0.35 7.4 4.2 $1,470 20 257
Firefly PbA best 45 4000 11.0 $250 $0.06 1.3 5.25 $1,313 25 257

Note that figures are for usable, not total, capacity in kWh (usually 80%, but much less for the current PbA pack (4.8 kWh total capacity), due to Peukert’s Law).

[edit] The Charger

The charger runs on standard 120v (or 240v) AC power and is used to recharge the PHEV pack. Three options are planned:

  • a Delta-q charger ( designed for the PbA battery pack, at a projected price of $800. We are in discussions with the company and will soon know if/when pre-production units will be available; UL-approved units are likely to be available in 2007.
  • the Brusa NLG503 charger, available through for $2650 retail including cables (a group rate is possible). Users can reprogram this charger for other voltages and battery chemistries, so it would be a good purchase for developers anticipating an eventual high-tech replacement battery.
  • (eventually) the Manzanita Micro PFC-40 charger, available through for around $2000. This charger has programmable but less sophisticated charging algorithms, but can also double as a high-power DC:DC converter between the battery packs. Its output is not line isolated. Its incorporation will require modifications/enhancements of this conversion, and control circuitry and algorithms that have not yet been developed.

Useful information on charging lead-acid batteries can be found at

[edit] The CAN-View

The CAN-View is a computer which monitors the CAN bus (the bus which the different microprocessors in the Prius use to communicate with each other) and both displays information to the driver on a display as well as control the extra plug-in systems. The Can-View computer can be programmed to turn on and off a series of relays which are used to control the PHEV operations. There are currently 2 versions of CAN-View available. Version 3 requires an '04 or '05 Prius and makes use of the built in display (or MFD) while Version 4 works with an 04-07 Prius but requires an external touchscreen (since the built in touchscreens were changed in the '06 model and are no longer compatible.) CAN-View is simple to install and installation typically requires between a half hour to one and a half hours. For more information, see CAN-View.

Version 3 of CAN-View must be ordered with the PHEV relay board option to be used in this conversion. Version 4 comes standard with the PHEV relays. CAN-View has 6 relays. Relays RL1 and RL4 are special relays which are try-EV mode and PHEV/orig. RL4 is triggered by pressing "orig/PHEV" on the CAN-View screen.

RL2, RL3, RL5 and RL6 are programmable.

[edit] EV Mode Button/Wire

The Prius can be put into "EV" mode which essentially turns the car into an electric car for speeds under 34mph. While Prius's come standard with a button in the dash in some countries, the button is not on the North American model, however the software is still present. EV mode can be entered by momentarily grounding pin 27 on plug H16 on the HV ECU. If the car exceeds 34mph or a host of other conditions are not met (such as the current charge limit, OEM battery temperature, low SOC, throttle, etc), the Prius will automatically exit EV mode and resume ICE use. For more detailed information on EV mode, see Prius EV Mode Button.

[edit] Control Circuit Board

A circuit board is needed which contains the logic to control the added heaters, fans, contactors, etc. The board is roughly 5" by 6" and is mounted in the electronics tray, between the PHEV battery and the stock battery.

Sub parts of this board are;

  • Power management - takes input from CAN-View and controls the contactors connecting the PHEV battery with the stock battery.
  • Battery Heating & Cooling - senses and controls the fans and heaters to keep battery temperatures within defined ranges.
  • System diagnostics - A simple LED board interface to monitor the technical operation of the system for debugging.
  • Charge interlock - stops the car from being driven away while plugged into a live outlet.

[edit] PHEV Battery Heating & Cooling

Lead acid batteries do not function as well when they are either hot or cold. The pack is heated and cooled as necessary by three standard 12" x 15" heating pads and three fans. The circuit board is responsible for controlling the heaters and fans. Depending on the layout, vents are provided either throught he bottom of the tire well or through the stock vent behind the storage bin on the rear drivers side of the car.

[edit] OEM Battery Fan Controls

The OEM controller connects the light green wire to the OEM fan to +14V when the OEM battery temperature reaches around 89-96 deg F. This control leaves that connection in place but taps it and runs it to J8F pin 1. Once the fan is so energized, proportional control is affected by varying the current to the violet line to the negative terminal of the OEM fan. The voltage of this line is monitored, and a DTC (system error) is asserted if the fan has become an open circuit. Control is effected by removing the violet line from the OEM fan and running it instead to J8F pin 3. J8F pin 5 is then run to the OEM fan. A diode (actually 3 in-line 3A diodes in parallel to handle 5A) between pin 3 and pin 5 allows the OEM control to operate the fan normally when the control board is unplugged (J8F plugs into J8M on the control board).

The control board has a 2.2K resistor between pins 1 and 3 of J8, a controlled pullup from pin 1 to +14V, and a controlled pulldown from pin 5 to chassis ground. When not in PHEV mode, the control board does nothing. When in PHEV mode, pin 5 is always pulled down to ground. This causes the fan to run at full bore whenever there is power on pin 1 (normally when the OEM battery temperature is above 89-96 deg F). The 2.2K resistor provides just enough load to the OEM proportional control circuit that it doesn't think the fan is an open-circuit and declare a DTC (hybrid system error). At the cost of some passenger compartment noise, this keeps the OEM battery temperature below 100 deg F instead of around 114 deg F, thereby keeping it cool enough for EV mode, which will not work when the OEM battery's temperature is above 104 deg F, to continue to be enabled. The lower temperature also prevents significant temperature-related reductions in CCL. It would no doubt be possible to proportionately control the fan to this lower temperature, too, but this system is not yet that sophisticated.

The pullup on J8 pin 1 is to allow the PHEV system to force the OEM fan ON when desired and the OEM battery temperature is below 89-96 deg F. A recommended use of this feature is to force the fan ON when the OEM battery temperature is so low that discharge current limit (DCL) is below 100A, thereby (sometimes severely) limiting EV mode current. If the driver, as would be expected, is using the cabin heater, the fan can speed up the heating of the OEM battery by blowing cabin air over it, thereby enabling EV mode and increasing DCL to useful values sooner than without forcing the fan ON.

[edit] Electronics Tray

[edit] HVD1

The purpose of the diode is to allow energy from the regen braking into the PHEV battery pack. This functionality is off when CAN-View's ORIG/PHEV relay is set for ORIG (which turns off HVRL2.) The heatsink can dissipate >200 watts for short braking or around 100 watts for several minutes during downhill braking, and must be well connected to the heatsink and in the path of a fan when is on whenever the car is in ready mode. The diode will only heat when braking or when the battery is being charged by the ICE. The diode requires a DO-5 mounting kit to insulate the diode from the heatsink electrically, but still allow thermal conductivity.

[edit] Contactors

HVRL1 is responsible for paralleling the PHEV battery pack and the OEM Prius battery. HVRL2 is responsible for enabling / disabling the system. HVRL3 is used for the optional power resistor. HVRL1 and HVRL2 both have snubbers across the terminals to reduce arcing and extend the life of the contactors.

[edit] References

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