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Prius PHEV

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This is an Initial Page for the EAA-PHEV Projects. Which will focus on the Prius Plus Conversions for the moment. There are other PHEV projects on the Conversion Hybrids page and PHEV Projects page. CalCars has a great PHEV talking points to bring you up to speed regarding Plug-in Hybrids.
"If we knew what it was we were doing, it would not be called research, would it?" -- Albert Einstein


The maillist discussions contain essential data not covered on this page or in the documentation linked from this page. They are a must-see for anyone contemplating doing a conversion!

See the EAA-PHEV_Maillist: for past messages and instructions for joining the list. Another perhaps more familure way to read the discussion is at the archives: . Those who join the group are reminded to observe standard email etiquette and be respectful of each other. Once you've joined you can send messages to The information is provided as a public service and the EAA assumes no liability for its use. Anyone modifying a Prius or other vehicle does so at their own risk and may void their manufacturer's warranty.

You do not need to register on this wiki site to join the maillist. However if you intend to contribute to these pages you should create an account and login when you make changes. Although even this is not strictly required as anyone can edit pages at the moment, it does help keep track of who's changing what, and you also get to create a page of your own!


  • Use this information at your own risk, there is no warranty expressed nor implied. Even should you perform these modifications to the letter you could still cause damage to any number of components in your Prius and cause it to no longer function. In performing some of these modifications you may also void your warrenty with Toyota.
  • HV warning: The Prius Traction Battery is a HV (High Voltage) Battery and could cause serious injury or death if proper precautions are not taken while working with it.

See also: Local - for dismantle and safety warnings


  • EAA = Electric Auto Association. -
  • PHEV = Plug-in Hybrid Electric Vehicle.
  • MFD = Multi-Function Display, or the Touch Screen which is standard on all Prii.
  • CAN = Controller Area Network used within the Prius which all the computer systems use to communicate.
  • SOC = State Of Charge, similar to a gas gauge, indicates how Full or Empty a battery is.


Enabling EV-Mode Button

EV-only mode is auto canceled outside of the following limits:

  • >=180V
  • >=45% SOC (mode cannot be entered unles SOC is >=50%)
  • Accelerator pedal position <=120 (out of 255), corresponds to approx. 120A or 24kW
  • Vehicle speed <34 mph (55 kph)
  • Defrost not selected (and possibly some other control selections)


  • The ICE will start 7 seconds after vehicle startup unless EV-only mode is entered during that time.
  • Once the ICE starts the first time during a trip, it will not stop until it has warmed up.
  • The A/C compressor is electric, and the A/C will work while in EV-only mode.
  • When the EV-only mode is entered, the hybrid model screen will show. A beep will be heard only if that screen wasn't already showing.
  • Three beeps are heard whenever EV-only mode is requested but not allowed, and when it is auto canceled.
    One must wait around five seconds before a new EV-only request is acknowledged.

Regenerative braking is limited by all of the following:

  • 120A
  • ~30A if SOC is >=80%
  • HV 250-270V (progressively greater limitation)

None of the above voltage limits have been tested for temperature-based variations.

SOC Spoofing

Note: More information is now available in PriusPlus (.doc) and the EAA-PHEV_Maillist:.

The Prius' battery management computer (BMS, called the Battery ECU) communicates to the main hybrid computers via the CAN bus. It indicates battery voltage, current, temperature, and its estimates of state-of-charge (SOC), and maximum allowable charge and discharge current.

The BMS' estimate of SOC is critical, as the hybrid controller keeps SOC within 40-80% (the lower and upper limits of the (nonlinear) display graph), and tries to keep it around 60%. When the SOC is above 60%, the hybrid controller works to discharge the battery by using battery power (and less gasoline) even during normal cruise. This increases to around 30A (~6kW) at 70% and above.

When the SOC is below 60%, the hybrid controller works to charge the battery by making the ICE work extra hard even during normal cruise. Below 40% SOC, stranger things happen and it is difficult to get the engine to put out much power at all.

For a PHEV, the object of SOC spoofing is to keep the BMS's indicated SOC at 80% or above until the battery is discharged enough to accept significant regenerative braking current; then between 70-80% -- to force less gasoline use even during non-EV-only mode -- until the battery's real state-of-charge has come near its lower limit. At that point, the BMS's indicated SOC should hover around 60% to keep the battery's real state-of-charge from trending further downward (bad for the battery) or upward (thereby wasting gasoline).

Dan Kroushl did some experiments with higher voltage batteries that proved that the BMS's indicated SOC can be spoofed (Thanks, Dan!). That led me to do enough further experimentation to discover that it is definitely possible to do what I indicated in the above paragraph, and generally how to do it. However, the circuitry and programming to do so is still in development. It generally involves, as needed, providing a higher voltage to the BMS than the actual battery voltage.

Toyota's BMS also checks the voltage of 13 taps on the OEM battery. These voltages must be equal to each other or the BMS will indicate a fault. Since few PHEV battery packs, unlike the OEM pack, are divisible into 14 equal subpacks, these tap voltages must be spoofed, too. Fortunately, it has been found that a fairly simple voltage divider can accomplish this.

Because of all of the above, CalCars' overall PRIUS+ circuit diagram is still in flux, and definitive answers about it are as yet unavailable.

SOC management

Note: More information is now available in PriusPlus (.doc) and the EAA-PHEV_Maillist:.

Here is the minimum needed in terms of a computer for spoofing the Prius' built-in BMS:

  • CAN message reading and parsing (CAN bus writing is NOT necessary or even desirable)
  • The ability to separately close and open two reed switch contacts based on CAN information
    • one to set EV-only mode, based on it not already being set, speed <34 mph, power request <120 (out of 200), SOC >49%, and a few other parameters.
    • one to set a voltage boost (to be explained later) to keep perceived SOC within a given range until the battery is sufficiently depleted
  • Amp-hour integration and display from the appropriate CAN bus messages
  • HV battery voltage and current display (both analog and digital desirable) from the appropriate CAN bus messages
  • Display of trip info (since reset): # of CAN errors (important for debugging), odometer, milligallons of gasoline used, Amp-hr and/or kWh used, trip milligallon/mi, Wh/mi, and mpg, highest peak charge and discharge currents, highest and lowest HV battery voltages, and the battery's internal resistance (beginning, current, and end)

Additional displays, desirable but not necessary:

  • Not strictly necessary, but SUPER desirable: storage of CAN trip running data on a removable medium (like a CompactFlash card) for later analysis.
  • Small graphical engine tachometer (but see rectangular suggestion below)
  • Gasoline use rate (milligallon/min and/or milligallon/mi (inverse of mpg), or just a binary for gasoline being used
  • Tiny graphical brake cylinder pressure (sum of that for each wheel), to indicate amount of non-regenerative braking being used

A very cool display would be two rectangular graphs indicating engine and electric power:

  • Engine power (e.g. blue for combustion): vertical: RPM, horizontal: torque
  • Electric (e.g. red for discharge, green for charge): vertical: HV battery voltage; horizontal: HV battery current (absolute value)

The areas can be calibrated so that they show the relative power being produced by the electric motor vs. the engine. The same pair of rectangles could display and compare the power going into regenerative braking vs. that being wasted in the friction brakes.


Any battery which meets the minimum requirements could be used to replace the Stock Prius NiMH battery pack. The stock battery does not necessarily need to be removed, but should be disconnected while in PHEV mode due to serious problems that can occur from paralleling this sort of battery. Adding more than 300 lb to the vehicle's weight is not recommended, at least without beefing up the suspension; removing the OEM battery pack removes around 80 lb.

This application is on the edge of the capabilities of lead-acid (PbA) batteries. CalCars' PbA conversion works, but just barely. The battery pack's internal resistance, though O.K. at high states-of-charge (SOC) is twice as high at low SOC. As this is what ultimately limits discharge, it also means that ordinary hybrid operation is compromised, too, once this low SOC is reached. Additionally, and possibly most important, low temperature operation is very compromised. Range is reduced at least 20% at 50 vs. 70+ degrees F, and at freezing temperatures CalCars' PRIUS+ wouldn't drive at all on the PbA pack!

  • CalCars has used 18 EVP20-12B B&B 20Ah 12v SLA PbA batteries from (~270 lb and 12Ah at 2C rate) for a total of 3.1 kWh. These PbA batteries have exceptional high power capabilities, cost $750 total, and deliver 10 miles of pure EV driving or 20 miles of doubled gasoline mileage in mixed driving per charge. CalCars' first set lasted 200 cycles or $0.35/mile -- but see RonG's further information under Charging below.
  • Another PbA possibility is the Odssey PC625 by EnerSys Inc. They are slightly lighter for the same high rate capacity and may have longer cycle life than the EVP20-12B. I believe these are what used to be the much-touted Hawker brand. More info on these batteries, where to get them, and for how much will be forthcoming -- and, again, see RonG's further information under Charging below.
  • CalCars has nearly completed a project with Electro Energy Inc. using a custom pack of their specialized NiMH batteries. Though more expensive, it will weigh the same as the EVP20-12's but provide more than double the range, better performance, probable low temperature operation, and much longer expected lifetime. We are hoping that Electro Energy will make these available for conversions later this year (2006). Stay tuned.
  • EDrive plans on using 18 exotic 12V U-1 U-charge 40Ah @C/5 Li-ion batteries from Valence for a total of 9.9 kWh.
  • Useful files by Ron Gremban:
  • You may want to use some Anderson high power connectors for the DC connections.

Minimum Requirements

We will focus on the Voltage requirements, as the AmpHour capacity of the pack may vary depending on who's implementing it and for what type of useage for various ranges from 5 to 50 or more miles. While a PbA pack may only deliver about 1/2 its nameplate capacity (due to Peukert's Law, which does not affect other chemistries) a Lithium pack may be able to deliver all of it's nameplate capacity. It all depends on how the battery's capacity is rated. The Prius consumes about 1 Ah or 300 Wh per mile.

  • The 6.5Ah Stock NiMH pack consists of 168 cells (28 modules) in series.
    • 60% nominal to 45% minimum SOC => 15% of 6.5 Ah is 1 Ah for 1 mile EV-only range.
    • 80% maximum to 45% minimum SOC => 35% of 6.5 Ah is 2.2 Ah for > 2 miles EV-only range.
    • Nominal Voltage is 201.6V, ranging from 180V to 270V durring use.
  • The 20Ah CalCars PbA pack (12Ah at 2C rate)
    • Delivers ~10 miles EV-only or 20 miles Mixed-mode driving PHEV range.
    • Nominal Voltage is 216V, still ranging form 180V to 270V durring use (range set by Toyota's hybrid system).
  • The 40Ah EDrive Li-ion pack
    • Delivers ~30 miles EV-only or 60 miles Mixed-mode driving PHEV range.
    • Nominal Voltage is 230V.

CalCars' published chemistry-neutral PRIUS+ battery spec is reproduced below. We have found 18 12V PbA modules or 180 NiMH cells in series to be near optimum.

Note: our PRIUS+ gets approx. 1 mile of pure EV range (or 2 miles of Mixed-mode driving range, with double normal gasoline mileage) per Amp-hr expended from the battery. Therefore, a 30Ah battery, used to 70% depth-of-discharge (DOD) would provide 24 miles of EV-only range or 48 miles of Mixed-mode driving PHEV range.

  • California Cars Initiative PRIUS+ Project - Preliminary Public Chemistry-neutral Battery Pack Specification
  • Subject to Change 6/17/05 By Ron Gremban
    • Voltages:
      • Maximum (during regenerative braking, 120A or max for SOC): 260V
      • Minimum (during 120A discharge at minimum normal SOC): 180V
      • Nominal: 210-230V (depends on the chemistry)
    • Amp-hr at 60A rate: 30-50 (5.5-10.0 kWh)
    • Max intermittent discharge rate: 200A
    • Max intermittent charge rate (when SOC is low enough to accept this full rate): 120A
    • DC internal resistance*, over normal range of SOC: Less than 0.25 ohm (0.20 ohm desired)
    • Charge equalization: Please indicate requirements and provisions for charge equalization
    • Thermal management: Please indicate requirements and provisions for thermal management
    • Min cycle life*: At least 1000 cycles between normal SOC limits; 2000 cycles or more highly desired
    • Max battery pack weight*: 300 lb (136 kg); 170 lb (77 kg) desired (replaces 70 lb hybrid battery)
    • Max price*: $800/kWh or less to CalCars, including charge balancing, cooling, thermal management, and SOC monitoring
    • Projected high-volume auto manufacturer's cost in two years*: $0.40 or less per kilowatt-hr lifetime throughput
  • *The values of these parameters should be maximized (or minimized, as appropriate), given a reasonable cost of doing so

Battery analysis by Richard

per EAA-PHEV_Maillist:2005./12./2:
I have completed my analysis for the battery pack and am attaching it. The first attachment explains the analysis and the second is a spread sheet of some of the data I ran.


  • CalCars uses a Brusa NLG5 charger from This is a $4000, 4kW charger capable of being reprogramed for any chemistry and any voltage output up to 520V. It is invaluable for our experimentation, especially since, for our world's first PRIUS+, we didn't know what would be necessary. A less flexible, lower power, less expensive charger may be more suitable for subsequent conversions.
  • CalCars believes that most PHEVs need only a 1-1.5 kW charger, able to plug into ordinary 120V, 15A GFI circuits. Since gasoline is available as a range extender, charging need be no faster than overnight.
  • Delta-q is taking a strong interest in PHEVs. They are the charger supplier for EDrive Systems' conversions and have begun working with CalCars on creating chargers for both our PbA and Electro Energy NiMH conversions.
  • PFC-20 Running off Prius HV Pack [1] Charging a BEV. This charger could run in both directions.
    • charging from the grid to the additional pack
    • charging from the additional pack to other EV's
    • charging from the Prius ICE/Gas to other EV's (up to 10kW)

RonG: Important new PbA charging information from Tony Goulet at West Coast Batteries Inc. re. Odyssey (and all PbA) batteries (after nearly 40 years of working with PbA batteries off and on, I am still learning more about them):

Sulfation of PbA plates does not only happen at low states-of-charge (SOC) as I used to believe. Sulfur is deposited on the battery's negative plates corresponding to its depth of discharge (DOD). After three days or so this sulfur becomes crystalized and can no longer be removed from the plate by charging. This means that any portion of a PbA battery's capacity that is left in an uncharged state for three days is permanently lost, and the battery now has less capacity! This can happen repeatedly, causing increasing loss of capacity. (Of course, as I already knew, if a PbA battery is left in a very low SOC for long, the sulfur may grow crystal trees that often permanently short out a battery right through its separators.)

The second piece of this tale is that unless a PbA battery has been held at a constant-voltage absorbtion charging stage for 8-12 hours, SOC is less than 100%. If e.g. 97% charge is achieved and the battery is not charged more completely for three days, the uncharged 3% of the capacity will probably be permanently lost to sulfation! It is therefore very important to charge a PbA battery to 100% at least every other day -- and less than a quite long absorbtion charge period (including ~3% overcharge on new batteries, or 10-20% overcharge on older ones) will not suffice!

User Interfaces

CAN-View Scanner

One of the graph screens
CAN-View local gallery
See the Official Website

This is a MFD Integrated CAN Scanner, named CAN-View.
The Official Website is

Additional accessories
These additional features are supported by the 2nd version of the CAN-View, some require additional hardware.

My CAN Project

Vass Software on the left

OK, not "My" Can Project, but some very nice software none the less.

May Required Linux and all that that entails
Open Source courtesy of Attila Vass!

Other CAN scanners

Schematics and Documentation

Click show for a short list of the current PHEV conversion and kit options for the Toyota Prius.

For Prius conversion details see the Prius PHEV article and comparisons table.

  1. Ron's Original PriusPlus History and current Contactor Based PriusPlus documentation for DIY projects.
  2. Ryan's PriusBlue is the testbed for DC-DC PFC Based PiPrius kits and documentation for DIY projects.
  3. Toyota OEM Prius PHEV and Prime Could use some work on this page
  4. Enginer China. But the rest of these appear to all be defunct as of 2020?
  5. |~- Hybrids-Plus USA/Colorado/Li -~|~- EnergyCS USA -~|~- Hymotion USA/Canada -~|~- Amberjac UK. -~|~- EDrive USA -~| Peter mentioned Plug-In Conversions |~-

--={ Project Overview }={ 2007 Maker Faire }={ Theory }={ Instructions }={ Parts List }={ RawData }={ Latest News }=--
--={ Doc Process }={ Mechanical }={ Electrical }={ Electronic }={ Interest }={ Talk:PriusPlus Main Discussion }=--

--={ Historic }={ Battery }={ Schematics }={ PseudoCode }={ Photos }=--

Team Photo from the PriusPlus conversion of Sven's Prius from Nov 2006.

This is the home of the PRIUS+ PHEV DIY (Do-it-Yourself) documentation. These pages are currently anonymously editable, which may change in the future. Please feel free to use the Discussion page for general discussion and commentary on the main article. If you would like to add to an existing section use the "edit" link near that topic's heading. Don't forget to use the Summary field to describe your changes. While editing use the "Show Preview" button to make sure your changes look like you expect them to, before you click "Save Page".

  This item is out of date but kept for posterity sake,  
  please refer to the current PriusPlus project page.  


The Combined "Map" image below may be out of date and is best for a general overview.
See the PriusPlus History (.doc) and main Prius PHEV page.
Click any of the three schematics to jump to its current version.

Navigation Image

SOC & Control Schematic 2f.
(full size, right click here).
Battery Tap Emulator 2g.
(full size, right click here).
High Power Schematic 2d.
(full size, right click here).

Note: It might be good to have various types of schematic sets. For example a higher level version might include an automatic switch over from the stock to the added battery pack, adding a DPDT contactor on the HV mains and a complex battery tap coupler relay board. An example of a lower level version might exclude the various interlocks, center pack relay, and simple isolation relays instead of opto-isolators. Though neither might prove particularly practical they would help by offering various potential design layouts which might be more adaptable to platforms other than the 2004-2005 Prius.

Management Pseudo Code

This is the management pseudo code which determines how the Prius PHEV functions.

It may be incomplete and is intended to be general enough to be implamented on various hardware.

CAN Parameters

Thess are CAN Parameters of particular interest used in the pseudo code:

  • SOC - The reported SOC is used to determine when to enable the SOC Spoofing circuit which raises the perceived or reported SOC.
  • Speed - Used to determine when to re-enter EV-mode after leaving it due to high speed or power demand.
  • Voltage - The reported Voltage is that measured by the Prius which may or may not be the Real Voltage.
  • Amperage - The reported Amperage is the amount of current being removed from or added to the battery.
  • Temperature - Battery temp used for temperature conpensation.
  • Spoof Voltage - Determined by measuring the difference in reported Voltage which occurs when the SOC Spoofing circuit is enabled.
  • Real Voltage - Used to determine the replacement HV batteries real SOC.
    • Real Voltage is reported Voltage if the SOC Spoof circuit is disabled.
    • Real Voltage is Spoof Voltage subtracted from reported Voltage if SOC Spoof circuit is enabled.
  • Real SOC - Could be determined in a number of ways using Real Voltage and reported Amperage.
    This aspect had not been fully resolved, so more details are sure to come at a later time
    • Zero Crossing SOC - Used to determine the real SOC by measuring Real Voltage as Amperage crosses zero.
    • Under Load SOC - Used by combining Real Voltage and reported Amperage in a manner to determine the batteries internal resistance, which is an indication the batteries SOC.
    • Watt Hour SOC - By counting Watt Hours removed from and added to the battery one can determine how much energy is left in a battery given that you knew how much you had to begin with, the capacity of the battery, and it's puckert characteristics. Unfortunantly WH counting is unlikely to be accurate based on the amperage, voltage, and power levels reported on the CAN buss as these are pre-filtered. Additional hardware such as a dedicated WH meter would be nesicarily to implement this type of SOC indication, however it may not be a bad idea use such a device even if not integrated as a control device.

User Settings

  • Note: Below within the code logic _XX_ are user settings, the listed value is a best guess default for that setting. These may need to be changed in order to allow for various battery chemestries and user preferances.
    • _30_seconds - Denied Timeout
    • _82_% - Spoof circuit enable SOC level
    • _20_% - Real SOC lower limit
    • _40_% - Real SOC PHEV entry lower limit
    • _33_MPH - EV-Mode re-entry speed

Pseudo Code Logic

  • Initialization and Discovery:
    • Upon startup the current state of the system is unknow, has the battery been left discharged or is it fully charged?
      • Determine current Real SOC, using Temperature compensated no-load or zero-crossing Real Voltage.
    • Depending on the user settings and the CAN management device PHEV mode might be entered immediatly given that Real SOC is high enough. Temperature might also be considered if it is very low, sub-freezing?. Defroster settings might also disable PHEV-Mode from the Prius side of the system.
      • Enter EV-Mode, Verify EV-Mode was entered
        • If EV-Mode denied, timeout re-attempts for _30_ seconds
    • If SOC < _82_%
      • Then enable SOC Spoof circuit
        • Measure voltage differance to aquire Spoof Voltage. Cycling of the SOC Spoofing circuit for multiple samples may provide more accurate readings. This setting might be saved by the CAN management device as it should rarely change and is manually set.
      • Else disable SOC Spoof circuit
        • still enable momentarily to determine Spoof Voltage.
  • PHEV-Mode Loop Start
    • If SOC < _82_%
      • Then enable SOC Spoof circuit
      • Else disable SOC Spoof circuit
    • If Speed < _33_ MPH and EV-Mode is not enabled
      • Re-enable EV-mode
        • If EV-mode denied, timeout re-attempts for _30_ seconds
    • If Real SOC < _30_%
      • Then leave PHEV mode and return to normal HEV mode, End PHEV-Mode Loop, Enter HEV-Mode Loop.
    • Return to PHEV-Mode Loop Start
  • HEV-Mode Loop Start
    • If Real SOC > _40_%
      • Then re-enter PHEV-Mode Loop
  • Return to HEV-Mode Loop Start

Installation Photos

Thought we might put a collection of photos that relate to the physical installation to help visualize just what is involved.

Ron Gremban

These are pictures of Ron Gremban's/CalCars 2004 Prius+ with it's B&B batteries, as seen here. Most are of the batteries and electronics in the tool tray space under the cargo area. I believe These are from the first conversion of Ron's Prius with it's initial battery pack which has since been replaces, it also may not reflect the current state of the Prius+ schematics.

From Yahoo