Turning the Prius into a Plug-in Hybrid
14 December 2004
The California Cars Initiative (CalCars), a non-profit startup dedicated to jump-starting the market for plug-in hybrids (PHEV), is building a prototype Prius (the Prius+) capable of functioning as a plug-in hybrid and running in full EV (electric vehicle) mode for longer distances than possible with the original Toyota equipment.
As designed and programmed, the Prius has a driver-selectable EV (electric vehicle) drive mode. When toggled, EV mode turns off the combustion engine and the Prius functions as an all electric vehicle—particularly good for starting up and neighborhood travel. The default EV driving mode can be used for about one mile with a maximum speed of about 34 mph. EV mode is not currently activated for Priuses bound for the US, although it is on models in Japan and Asia.
Not being activated is different than not being enabled. The EV mode program logic is still in the ECU—it’s only the button and the wires that are missing. Shortly after the Prius’ arrival in the US, then, enterprising technically-oriented owners began hacking or modifying the Prius to enable EV mode. What started out as a customized D-I-Y (Do-It-Yourself) hack (example here) has become a more formalized aftermarket product (example here).
In EV mode, the software will automatically restart the engine when the battery needs to be recharged, if acceleration is required, or if the vehicle speed exceeds a threshold of 35 mph. In any of those conditions, EV mode switches off and the Prius operates as usual.
As an EV, however, the Prius is not very satisfying. The battery pack wasn’t selected with extended EV operation in mind, and, more critically, the vehicle can’t plug in to recharge the batteries. Staying far away from a plug-in was definitely part of Toyota’s planning. The company went to great lengths during the iinitialroll out of this version of the Prius to point out that it did NOT plug in.
This was clearly a conscious product marketing and design decision. There are a number of hybrids—such as DaimlerChrysler’s new Sprinter vans (earlier post)—that do offer the plug-in option.
Enter CalCars. CalCars decided to use the Prius as a high-visibility platform to demonstrate the fuel economy benefits of a grid-pluggable hybrid that offers an extended EV range.
The CalCars team is adding a different battery pack and grid-charging capabilities. The group has started with a prototype using lead-acid batteries that delivers less than 10 miles of EV-only range at low speeds. They hope to upgrade to a custom-built NiMH pack for an expected range of some 20 miles. CalCars would like to build a second prototype using a Li-Ion battery and hope for a 30+ mile range.
There are still limitations to the Prius as an EV even with those modifications—the Toyota software still rules the ECU, so EV mode will automatically terminate if acceleration is less than gentle, or when the car tops 35 mph. So rather than concentrating solely on the EV results, CalCars is focusing on the increased mileage enabled by the conversion.
As we’d fervently hoped, the batteries contribute significantly to improved efficiency even at higher speeds: Ron showed 83 mpg for a 10-mile mostly-highway run. (Remember that this initial battery pack weighs about 300 pounds -- appropriate for our first tests but not for an acceptable vehicle, and we expect even better performance ahead.) This validates our approach of emphasizing the fuel economy benefits rather than the amount of miles driven in EV-only mode.—Felix Kramer, Founder California Cars Initiative, Message 226
CalCars tracks its progress and developments via the PRIUS+ Plug-In Hybrid Conversion Group, from which the snippet above is taken. It’s worthwhile taking a look—it’s a good window into what’s involved.
CalCars is also looking for volunteers and donors. Details are available at its website.
(Thanks to Mike C. for the pointer!)
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