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Conversion Interest

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Per EAA-PHEV_Maillist:2006./7./5 or Y!

Request for Show of Interest in Converting Prius to PHEV Instructions

Calcars, in collaboration with the EAA’s Eaa-Phev SIG, intends to assemble and publish public domain instructions for converting a 2004-5 Prius* into a PRIUS+ plug-in hybrid. The first versions of this conversion will use 20 BB Battery EVP20-12B1 AGM PbA batteries (, along with the OEM hybrid battery, because of their cost and easy availability. They will provide a 10+ mile pure electric range in EV-only mode (at up to 34 mph), or assist the ICE for the first 20+ miles of mixed driving at around double the Prius’ normal mileage. This battery pack costs about $800 and can be expected to deliver 200-400 full (80% DOD) cycles. The added weight is around 300 lbs, which existing conversions have been handling without upgraded springs. We aren’t removing the OEM battery or changing the HSD system, but the spare tire space will not be accessible.

A li-ion and/or NiMH battery pack may become available within a year, probably capable of 20-25 EV-only miles, weighing 100-200 lb, lasting 2000-4000 cycles, and selling for in the ballpark of $6000. Such a pack should be retrofitable into either version (below) of the conversion.

Two versions, both with similar performance and using Hybrid Interfaces’ CAN-View device (, are anticipated:

  • The first version is the cheapest, using contactors and a homemade wirewound power resistor for control of the current between the OEM and PHEV battery, and a Delta-Q ( overnight charger.
  • In the second, more sophisticated, version, the Delta-Q charger is replaced with a high-power (PFC-40) charger from Manzanita Micro (, which doubles as a DC:DC converter, used, in place of the resistor, to more carefully control current into and out of the PHEV battery while driving.

The total cost in materials (all commonly available), including batteries, is around $3,000 for the resistor/Delta-Q version or $4500 for the Manzanita Micro version. The time for two skilled people to assemble and install it (not including gathering parts) is probably at least 80 hours, though this is bound to decrease if and when kits become available with all parts included and some of the pieces pre-cut and drilled. A printed circuit board is anticipated becoming available either for a modest donation to Calcars or from a third party. That board is being designed to be useable with various battery packs and modifications. EXPERIENCE IN SAFELY WORKING WITH HIGH VOLTAGES IS MANDATORY. If you do not have this experience yourself, hire an electrician.

These changes will void Toyota’s warranty on affected components (no doubt the OEM hybrid battery and battery management computer; anything else is debatable). A modified Prius will operate as before the conversion when the PHEV battery pack is depleted. Slightly higher mileage than before is even possible despite the extra weight, due to the decreased internal resistance and larger capacity of the then-paralleled battery packs.

It will be valuable for us to learn how many DIYers actually intend to convert a Prius into a PRIUS+ using the plans we are currently working on. Therefore, please send me an email if you do intend to do so. Include answers to the following questions:

  1. Do you already have a 2004-5 Prius available to convert?
  2. If not, do you have or expect to get 2006 Prius that you would like to convert?
  3. If a conversion kit -- probably costing a little more than the retail parts costs but saving much parts-gathering and some assembly time -- becomes available, would you buy it?
  4. When do you plan to begin your conversion, once the plans or a kit is available?
  5. If an NiMH or li-ion battery pack, as described above, were to become available, how much would you be willing to pay for it (in place of the $800 PbA pack)?

To respond, PLEASE DO NOT USE YOUR “Reply” BUTTON OR LINK, which will direct your response to the whole internet group! Instead, please mail to:

* The 2006 Prius’ multi-function display is incompatible with the
current CAN-View electronics that is used in this conversion.  This
limitation may be eliminated in the future.  If you are interested in
converting a 2006 Prius, please let us know so we can asses this demand.