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".
We'd like discussion of this page to happen here on this page’s Discussion tab and general discussion of the conversion to occur on the eaa-phev maillist.
 About the Open Source PriusPlus Project
The PriusPlus project is an open source project set up to develop a method to convert a '04-'07 Toyota Prius into a Plug-in Hybrid. The project is a combination of hardware and software development to allow qualified individuals or individuals working with a qualified electrician to convert their own car to a PHEV.
 Current Status
While the documentation is not yet as thorough as would be desired, the necessary information is available to complete a conversion. Please use the links above to navigate to the various documentation pages. Many photos are available on the RawData page.
See this 2007 08 01 maillist post for the Latest News. If you are interested in helping improve the documentation, please contact Chris Ewert (chris at infolaunch.com)
Another avid Prius enthusiast named Cheap! is currently in the process of his own conversion using CalCars PriusPlus processes and documenting his progress at priuschat.
Be sure to heed all warnings and read the Safety section below.
The PriusPlus conversion retains the OEM hybrid battery and its management computer while adding a lead-acid pack consisting of 20 BB Battery EVP20-12B 12V, 20 Amp-hour sealed AGM PbA batteries. PbA battery chemistry is very inexpensive but leads to significant limitations (see below). However, with this relatively inexpensive conversion (as little as $4000 parts cost, including the battery) you can be the first in your community to actually own and drive a plug-in hybrid, and you can achieve 100+ mpg (plus electricity) for 15-20 miles/day!
News flash (Sept '07): Reasonably-priced conversion options using Nilar NiMH packs of 3 sizes and possibly PSI Li-ion phosphate packs are now becoming available, as well as pre-built battery boxes and wiring harnesses. Stay tuned for details.
Though we are using the best and most cost-effective PbA modules we could find, a PbA PHEV is neither economically nor operationally up to par. But as of now, it is the only relatively inexpensive way to own and drive a real PHEV. More advanced batteries are on the way and may well be available by the time (1-2 years from now) the PbA battery needs replacement (see below for specifics.)
Below is a fairly comprehensive list of our public domain PbA Prius conversion’s advantages, operating and mechanical characteristics, and limitations:
- Detailed instructions for easy installation by anyone trained in high-voltage safety
- Pure electric propulsion at up to 34 mph for up to 10-12 miles per charge
- Lower gasoline use at all speeds until the PHEV battery is depleted
- Displacement of gasoline:
- at an equivalent energy cost of less than $1.00/gallon (at up to $0.09/kWh), possibly using low-cost nighttime electricity if your utility provides optional time-of-use metering
- with renewable electricity, if you have solar panels on your home or specifically buy electricity from renewable sources
- No modification of the Prius’ hybrid propulsion system, for uncompromised vehicle reliability
- Several selectable text and graphics display screens for real-time observation of hybrid system and PHEV parameters
- A pull-out power cord that plugs into any standard U.S. 15A, 120VAC outlet or a mounted bumper inlet
- An interlock to prevent driving away while the battery charger is still plugged into a live electrical outlet
 Prius-forced limitations
- This conversion will no doubt void parts of Toyota’s warranty. U.S. law prohibits aftermarket modifications from voiding any part of a vehicle’s warranty except for problems specifically caused by the modification. If repair of the OEM battery, transaxle, or other parts of the hybrid system becomes necessary, Toyota may argue that the PHEV modification has caused the problem. The outcome is uncertain, see also our Warranty article.
- EV-only mode (which inhibits ICE operation) works only up to 34 mph, 120A of power (modest acceleration), and a few more obscure limits
- The conversion’s data display can share the vehicle’s multifunction display screen only in 2004-5 Prii; later models require an additional display screen (optional on 2004-5 Prii).
- When the Prius starts the ICE for the first time after being turned on, the ICE must warm up for several minutes before EV mode can be re-entered. This process burns more fuel than normal.
- The charge / discharge limits are affected by the temperature of the OEM battery.
 PbA limitations
- The conversion adds 300+ lbs to the vehicle’s weight to provide 10 miles (16 km) of electric range per charge (16.7 usable Wh/kg)
- Though I have safely driven 17,000 miles in my 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 heavier-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.
- By Ronald Gremban: On March 5, 2013, a fire that started in my Prius, in my garage, burnt up my home. No one was hurt, but my partner Lynne just escaped, and a wonderful cat that lived with us died. Though the DIY conversion described here was no longer in the car, the cause of the fire is as yet unknown, and many thousands of pure gasoline cars burn up every year killing many people, I mention this as a cautionary tale of a worst-case scenario that can happen despite care. And note that the designs presented here have not been tested to be safe against all possible failure modes.
- Potentially lethal voltages are involved. It is important for the high-voltage wiring to be done by an electrician or an engineer experienced with high-voltage safety.
- Once the conversion is complete, all high voltages are inside screw- or bolt-secured areas, but these areas are exposed during parts of the conversion process, during battery replacement and other servicing, and possibly after a crash.
- In keeping with hybrid automotive standards, high voltage cabling is labeled with orange (as #4 gauge and larger orange wire is not readily available, we specify the addition of orange shrink-wrap at each end)
- This design should be adequate to contain the batteries in any normal driving conditions, but extreme or off-road maneuvers could damage the installation, potentially causing a hazard.
- More importantly, though we believe the parts are well-secured, we are not automotive design engineers, the design has not been crash-tested, and its characteristics during and after a crash are unknown. In particular, it is uncertain whether the battery box would remain intact and in place during a roll-over incident.
- PbA modules could tear out of their brackets and fly around the passenger compartment
- Though these AGM PbA modules are not flooded, they could leak acid if crushed.
- Short circuits could arise, causing sparks, hot, molten metal, and possibly igniting a fire.
- The battery pack may modify the characteristics of the vehicle’s rear crush zone.
 Operational characteristics
- Entry into EV-only mode is automated. Manual entry is also available. Due to a complex interaction with Toyota’s battery management computer, EV-only mode is unavailable for a short distance (usually less than ½ block) after slowing down from highway speeds.
- The paralleling of the PHEV battery to the OEM battery is switched on and off as needed. Occupants will hear the contactor periodically doing this, but will not directly feel any effects from it.
- When the PHEV battery is fully charged, EV-only mode can be sustained up a steep, extended hill at maximum allowable EV-only power (100-120A). At lower states of charge, the voltage may fall, causing the engine to start.
- When going down a long hill at low PHEV battery SOC, some regenerative current does flow back into the PHEV battery. This effect, however, is not as large as would be desirable.
- The Prius’ hybrid system allows EV-only mode only when the OEM battery temperature is below 107 deg F, but non-EV-only maximum battery temperature is enough higher that Toyota’s battery cooling system does not work hard to avoid exceeding 107 degrees. The conversion modifies this system so that the fan comes on at full speed whenever OEM battery temperature is above approximately 90 deg F. This almost always keeps OEM battery temperature within EV-only range, but is somewhat noisy.
 Other characteristics
- It will probably be helpful at various points to refer to specific Prius Service Manual pages, available for download by paying Toyota $10/day for access (only one day is needed).
- The mechanical design is sturdy and simple. It should be easy for anyone mechanically inclined to build; even more so if and when various prefabricated parts become available.
- This conversion includes a simple and inexpensive insulated battery box that is both air-cooled when necessary and heated as needed, but only during charging.
- The thermostatically-controlled cooling fans need only run at high ambient temperatures, to keep battery temperatures below 120 deg F. However, they are also run during non-heated charging to ensure any venting of the modules is flushed to outside the passenger compartment.
- PbA batteries have increasingly poor operating characteristics at low temperatures, starting around 55 deg F. We thermally insulate and heat them to retain their usefulness down to lower ambient temperatures. An advantage of retaining the OEM battery for normal hybrid operation is that poor PbA cold-weather performance affects only PHEV, not normal hybrid, operation.
- Since we have not come up with a scheme to heat the battery pack from waste engine heat, and electric heating from the battery would be unproductive, we heat electrically only during charging. This should keep the battery pack sufficiently warm for effective charging and for several hours thereafter in most climates. The system has not been sufficiently tested, however, due to very moderate California weather during development.
- Heating is accomplished via ordinary drugstore electric heating pads, thermostatically controlled via a thermistor and powered by the AC line during charging. They consume about 100 watts when in use, so overnight heating can add 1 kWh, or around 25%, to cold-weather electricity consumption.
 Planned Enhancements/Options
- A new version of the logic board has been designed, as much as possible, to accomodate a DC:DC converter such as the Manzanita Micro PFC-40, various battery chemistries, various separate PHEV battery management systems (BMS), and digital data acquisition.
- Manzanita Micro is designing a PiPrius PHEV conversion kit for sale, using a lower-voltage pack of larger PbA batteries and their PFC-40 charger as both a charger and DC:DC converter between batteries. It is our intention to develop a version of this conversion that uses the PFC-40 in the same ways. It will probably require the next version of our logic board, and will have the following relative characteristics:
- Lower peak currents in both batteries, due to a 40A current limit between them
- Increased regenerative braking back into the PHEV battery, mainly valuable during extended downhill grades
- Possibly slightly longer EV range, due to the above advantages
- A range of PHEV battery voltages can be accommodated.
- Various battery chemistries can be used and accurately kept within their specifications by the required battery management system (BMS).
- Because the PHEV battery must have a BMS, it is possible to more carefully control its minimum state-of-charge (SOC), thereby more carefully trading off useful capacity vs. cycle life.
- During charge, the battery pack is not isolated from input power. This means extra care must be taken to electrically insulate the battery pack from the vehicle chassis, the hybrid system, and all possible human contact points.
- The batteries are not kept in parallel during post-PHEV hybrid operations, possibly limiting hybrid efficiency to that of an unmodified Prius. More complex PFC-40 switching may minimize or eliminate this effect.
- Unless voltage minima and maxima turn out to be sufficient cues, a battery management system (BMS), including reasonably accurate state-of-charge (SOC) measurement, will be required to maintain SOC limits.
 Advanced batteries
- 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):
| Yr daily|
|kWh||$||EV mi|| Wt,|
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 Wikipedia:Peukert's Law).
 Parts availability
All parts are off-the-shelf with the following exceptions:
- the logic board
- Once we know demand, we will order a bunch fabricated. Post-fabrication rework is also required due to major architectural changes since its inception.
- A reworked, stuffed, and fully-tested form of this board may be made available at some point. If so, it will probably cost hundreds of dollars and save 20-40 hours of parts purchase, identification, stuffing, soldering, and testing.
- We do plan to redesign this board in the future, using IC logic, a PLA, and/or a microcontroller. The new version will probably be required for eventual upgrade of a conversion to use a more advanced battery pack.
- a version of CAN-View with relays to control the conversion. 2006+ Prii require a version capable of driving an add-on touch-screen; this should be available by the time anyone needs it.
- a charger designed for the PbA battery pack. Three options are planned:
- a Delta-q charger (http://www.delta-q.com) 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 http://www.metricmind.com/index1.htm 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 http://manzanitamicro.com 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.
- mechanical parts that require fabrication from off-the-shelf pieces such as angle aluminum and sheets of ABS, plexiglass, and/or lexan (polycarbonate)
- Some of these parts may eventually become available prefabricated.
- we, or some other supplier or affiliate, may offer various components to speed and simplify the conversion. We’re interested (see the Conversion Interest page) in knowing how much interest there is for pre-assembled (or at least pre-cut) components, at a higher cost than fabrication via you own free labor but no doubt lower than what you would have to pay a fabricator.
The table below outlines approximate component costs, as well as estimated labor costs for component fabrication or assembly. A range is given, as the number of components made and where they are done has a great bearing on the final cost. We are not fabricators, professional assemblers, or product retailers, so these are guesses as to what a for-profit company or craftsman would charge. If a great number of kits or components are desired, offshore fabricators may beat these estimates by a lot.
Please answer the new questions for our information-gathering at the Conversion Interest page.
Estimated Fabrication Costs
Assembled and tested circuit board $250-500 Battery tray (4 needed) $150-250 Battery box top $150-200 Battery box foundation $150-200 Electronics tray, assembled and wired $500-1K Set of pre-built battery cables $150-200 Pre-built low-power wiring harness $150-300 Total (including 4 trays) $1950-2500
These are estimates for the labor only; approximate components costs are below.
Est. Component Costs Min Max Battery set (20 + 2 spares) 900 1100 Battery wire & lugs 100 150 Heating pads & insulation 100 100 CAN-View 600 600 Display (opt for 2004-5 Prii) 0 200 Charger (Delta-q or Brusa) 800 2500 Cord reel & base, brackets 100 100 Contactors (3) 240 330 Fuses & holders 60 A (2) 100 150 Fans (3) 60 120 All metal & plastic 200 300 Circuit board 100 100 Circuit board components 200 300 Connectors 200 300 Misc. electronics 150 200 Total 3850 6550
For reference and posterity sake, the previous incarnation of Ron's original conversion at PriusPlus History.
- ^ awaiting url to Prius Service Manual website.