General Disclaimer:   (HV) (DC) injury or death hazard,   use at your own risk,   may void warranty.


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Future home of the PriusPlus PHEV DIY (Do-it-Yourself) documentation...

Preliminary Information

Original Documentation (.doc)

To would-be Prius-to-PHEV converters from Ron Gremban and Tom Driscoll at CalCars, October 4, 2006:

We are still at work on the plans and instructions for a public domain do-it-yourself (DIY) conversion of 2004+ Prii into plug-in hybrids (PHEVs) using (for now) lead-acid (PbA) batteries. The conversion we are documenting has been running very well for many weeks since its last improvements. We are shooting for documentation availability by November 1. It will be published here at

Our 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!

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, and especially 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 vacuum-cleaner-like pull-out power cord that plugs into any standard U.S. 15A, 120VAC outlet
  • An interlock to prevent driving away while the battery charger is still plugged into a live electrical outlet

Prius-forced limitations

  • High voltages are involved that can be lethal if not treated with sufficient respect.
    • 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 guage and larger orange wire is not readily available, we specify the addition of orange shrink-wrap at each end)
  • 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.
  • 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).

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 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.
    • 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
  • 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.

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.
  • 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.
  • 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

  • Documentation will include all necessary schematics, wiring diagrams, mechanical assembly drawings, and instructions. We will include pictures and possibly some video sequences. 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.
  • However, as 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.
  • 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.
  • 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 as low temperatures, starting around 55 deg F. Heating them can retain their advantages down to low 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 energy consumption.

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-specific electronics. Any new battery’s enclosure, mounting, and thermal management system will no doubt be very different.

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

Example pack

Chemistry Uvable
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).

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 ( capable of working with an external display, as required for 2006+ Prii. This should be available by the time anyone needs it.
  • a charger designed for the PbA battery pack. Two options are planned:
    • a Delta-q charger ( designed for the PbA battery pack, at a projected price of $800. However, even pre-production units may not be available until some time 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.
  • 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 suppler or affiliate, may offer various components to speed and simplify the conversion. We’re interested (see below) in knowing how much interest there is for pre-assembled (or at least pre-cut) components, at a cost higher 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.

Any response is not a commitment to buy anything, or an order, but a way to help us make it easy for you by helping determine how far to go in having components made. We want to know from the converters the following:

1.What fabricated components or assemblies are you interested in buying, at the following estimated prices?

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.

2.Is anyone in the audience interested in becoming a fabricator and/or kit supplier? This question applies to for-profit businesses (we know of one or two who may be interested) as well as to individuals.
a.Are you interested in fabricating subassemblies? If so, which ones?
b.Are you interested in assembling components into full or partial kits?
c.Would you be interested in organizing the orders? We need a volunteer or for-profit business to organize this process of deciding what the interest is, what can be economically be built given that interest, who will do it; then how to commission, order, pay for, collect, store, and deliver the parts and subassemblies.
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