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| − | History. I was the technical lead for CalCars 2004 PRIUS+ modification of
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| − | my 2004 Prius into a pluggable hybrid (PHEV). Felix Kramer, CalCars
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| − | founder, enlisted me in mid-2004, shortly after I first contacted him. The
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| − | project had a lot of internet collaboration as well as physical volunteer
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| − | help, largely from Electric Automobile Association (EAA) members (thanks to
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| − | everyone). You can view much of the history of the project at
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| − | http://autos.groups.yahoo.com/group/priusplus/messages.
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| − | Greg Hanson offered EnergyCS help at low cost. They had a CAN bus
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| − | controller that they turned into a battery management computer to replace
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| − | Toyotas Battery ECU by emulating that OEM computers CAN messages to the
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| − | rest of the hybrid system. This system, though proprietary, got our PRIUS+
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| − | working quickly; first smoke-tested in late September, it was working well
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| − | by early November 2004.
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| − | Greg and others at EnergyCS then got excited, built their own Prius
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| − | conversion using Valence Li-ion batteries, and with CleanTech, a company
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| − | to which Felix introduced them started EDrive Systems to commercialize
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| − | Prius PHEV conversions. This is an exciting development, which may provide,
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| − | for most people, the first opportunity to own a PHEV.
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| − |
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| − | However, there is also a community of experimenters, schools, and others
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| − | interested in doing such modifications themselves, and the EnergyCS
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| − | controller is not available for this purpose. Fortunately, Dan Kroushl
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| − | discovered and shared the information that a higher-voltage battery is
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| − | capable of fooling Toyotas Battery ECU into giving a higher SOC reading. I
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| − | started experimenting with higher voltages of PbA batteries and came up with
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| − | an algorithm for fooling the Battery ECU in an organized way. It has so far
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| − | been tried only with PbA batteries, and only with manual control, but I hope
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| − | and expect it to work more generally. It will be described in detail below.
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| − | Battery pack. I specified a BB Battery PbA battery pack (18 EVP20-12B1
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| − | modules from http://www.electricrider.com/batteries.htm) for our first
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| − | conversion attempt, in order to go through our learning curve with
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| − | inexpensive, fairly indestructible batteries. This turned out to be an
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| − | excellent decision, as, though it required replacement after 200 cycles and
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| − | just under a year, this pack has sustained our PRIUS+ through over a year of
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| − | work on finding a more advanced battery pack. The BB electric bicycle
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| − | batteries, though very marginal for this application, are actually quite
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| − | remarkable among PbA batteries. When fully charged, these 20Ah (12Ah @ ½
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| − | hour rate) batteries have a lower internal resistance (~250 milliohms) than
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| − | the OEM battery (300-350 milliohms) and are capable of supplying the Priuss
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| − | maximum draw of 200A a 10C or 17C rate, depending on which capacity you
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| − | calculate it against. At low SOC, however, the internal resistance is >500
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| − | milliohms, too high for full hybrid functionality. My PRIUS+s performance
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| − | has been documented in CalCars PRIUS+ Fact Sheet, available at
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| − | http://www.CalCars.org. In short, the BB Batteries weigh about 260 lb and
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| − | provide for approximately double the normal Prius gasoline mileage for
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| − | around 20 miles of mixed (town and highway) driving on each charge.
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| − |
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| − | Next, we planned on installing a NiMH pack, largely to prove that the
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| − | chemistry that is already in all of todays mass produced hybrids is also
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| − | capable of powering electrically and economically effective PHEVs; thereby
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| − | removing one more auto company excuse for not building them. After that, we
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| − | would move on to Li-ion, which we expect to require additional engineering
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| − | work to ensure proper cell-by-cell charge and discharge control as well as
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| − | to engineer out any likelihood of thermal runaway problems, to which this
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| − | chemistry is prone.
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| − |
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| − |
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| − | I also looked at many Li-ion possibilities. The least expensive option is
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| − | currently 18650 cells, produced in huge volumes for laptop computers.
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| − | However, these, like the sub-C NiMH cells, are very small (2-2.8Ah). Also,
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| − | they are typically designed for 2-5 hour discharge rates and lack high power
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| − | capabilities. One pays more for high power, low susceptibility to thermal
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| − | runaway, and long cycle life and you still have to parallel many cells to
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| − | reach the necessary capacity. A typical PRIUS+ pack would use at least 1200
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| − | cells, providing plenty of failure points. While looking only at
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| − | technologies that are in production, if only in the early stages, CalCars
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| − | has begun discussions with more than one specialty Li-ion manufacturer with
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| − | lines of high-power-capable Li-ion cells.
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| − | CalCars and Electro Energy, Inc, in Danbury, Connecticut, are now in the
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| − | late stages of a joint project to adapt their unique form of NiMH packs to
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| − | the PRIUS+. As part of the project, I have designed control circuitry that
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| − | is specific to the characteristics of this battery as well as Toyotas BMS.
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| − | When complete (expected yet this first quarter of 2006), the pack should
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| − | weigh about the same as the BB Battery pack but last for 40-50 miles of
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| − | mixed driving. Internal resistance figures we have seen so far are in the
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| − | 200 milliohms range. It is possible that Electro Energy will elect to sell
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| − | this pack to experimenters, so stay tuned.
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