Talk:EAA-PHEV PRIUS Documentation

History. I was the technical lead for CalCars 2004 PRIUS+ modification of my 2004 Prius into a pluggable hybrid (PHEV). Felix Kramer, CalCars founder, enlisted me in mid-2004, shortly after I first contacted him. The project had a lot of internet collaboration as well as physical volunteer help, largely from Electric Automobile Association (EAA) members (thanks to everyone). You can view much of the history of the project at http://autos.groups.yahoo.com/group/priusplus/messages.

Greg Hanson offered EnergyCS help at low cost. They had a CAN bus controller that they turned into a battery management computer to replace Toyotas Battery ECU by emulating that OEM computers CAN messages to the rest of the hybrid system. This system, though proprietary, got our PRIUS+ working quickly; first smoke-tested in late September, it was working well by early November 2004.

Greg and others at EnergyCS then got excited, built their own Prius conversion using Valence Li-ion batteries, and with CleanTech, a company to which Felix introduced them started EDrive Systems to commercialize Prius PHEV conversions. This is an exciting development, which may provide, for most people, the first opportunity to own a PHEV.

However, there is also a community of experimenters, schools, and others interested in doing such modifications themselves, and the EnergyCS controller is not available for this purpose. Fortunately, Dan Kroushl discovered and shared the information that a higher-voltage battery is capable of fooling Toyotas Battery ECU into giving a higher SOC reading. I started experimenting with higher voltages of PbA batteries and came up with an algorithm for fooling the Battery ECU in an organized way. It has so far been tried only with PbA batteries, and only with manual control, but I hope and expect it to work more generally. It will be described in detail below.

Battery pack. I specified a BB Battery PbA battery pack (18 EVP20-12B1 modules from http://www.electricrider.com/batteries.htm) for our first conversion attempt, in order to go through our learning curve with inexpensive, fairly indestructible batteries. This turned out to be an excellent decision, as, though it required replacement after 200 cycles and just under a year, this pack has sustained our PRIUS+ through over a year of work on finding a more advanced battery pack. The BB electric bicycle batteries, though very marginal for this application, are actually quite remarkable among PbA batteries. When fully charged, these 20Ah (12Ah @ ½ hour rate) batteries have a lower internal resistance (~250 milliohms) than the OEM battery (300-350 milliohms) and are capable of supplying the Priuss maximum draw of 200A a 10C or 17C rate, depending on which capacity you calculate it against. At low SOC, however, the internal resistance is >500 milliohms, too high for full hybrid functionality. My PRIUS+s performance has been documented in CalCars PRIUS+ Fact Sheet, available at http://www.CalCars.org. In short, the BB Batteries weigh about 260 lb and provide for approximately double the normal Prius gasoline mileage for around 20 miles of mixed (town and highway) driving on each charge.

Next, we planned on installing a NiMH pack, largely to prove that the chemistry that is already in all of todays mass produced hybrids is also capable of powering electrically and economically effective PHEVs; thereby removing one more auto company excuse for not building them. After that, we would move on to Li-ion, which we expect to require additional engineering work to ensure proper cell-by-cell charge and discharge control as well as to engineer out any likelihood of thermal runaway problems, to which this chemistry is prone.

I also looked at many Li-ion possibilities. The least expensive option is currently 18650 cells, produced in huge volumes for laptop computers. However, these, like the sub-C NiMH cells, are very small (2-2.8Ah). Also, they are typically designed for 2-5 hour discharge rates and lack high power capabilities. One pays more for high power, low susceptibility to thermal runaway, and long cycle life and you still have to parallel many cells to reach the necessary capacity. A typical PRIUS+ pack would use at least 1200 cells, providing plenty of failure points. While looking only at technologies that are in production, if only in the early stages, CalCars has begun discussions with more than one specialty Li-ion manufacturer with lines of high-power-capable Li-ion cells.

CalCars and Electro Energy, Inc, in Danbury, Connecticut, are now in the late stages of a joint project to adapt their unique form of NiMH packs to the PRIUS+. As part of the project, I have designed control circuitry that is specific to the characteristics of this battery as well as Toyotas BMS. When complete (expected yet this first quarter of 2006), the pack should weigh about the same as the BB Battery pack but last for 40-50 miles of mixed driving. Internal resistance figures we have seen so far are in the 200 milliohms range. It is possible that Electro Energy will elect to sell this pack to experimenters, so stay tuned.