Calcars' Technical Analysis of Toyota's PHEV Prototype by Ron Gremban

Calcars' Technical Analysis of Toyota's PHEV Prototype by Ron Gremban
Jul 27, 2007 (From the CalCars-News archive)

CalCars-News

This posting originally appeared at CalCars-News, our newsletter of breaking CalCars and plug-in hybrid news. View the original posting here.

We've had a good bit of technical discussion about the new "Toyota Plug-in HV" over at http://www.eaa-phev.org the Electric Auto Association Plug-in Hybrid Project, where our Open Source conversions programs are developed and discussed. We thought we'd share some preliminary findings with others besides the few hundred subscribers to that list. (The email archive is most easily viewable at http://groups.yahoo.com/group/eaa-phev. To sign up to post to the list, go to http://www.eaa-phev.org/wiki/Maillist

Here's CalCars Technology Lead Ron Gremban analyzing the system (which of course he's not seen). (We've added only explanations for acronyms; even if you don't have an engineering background, you can still benefit by skimming this!) Disclaimer/caution: Ron is not trying to draw conclusions, or predict what Toyota will eventually build. Clearly this was in the tradition of the conversions: what could the company do quickly, with its existing components and few modifications of the software.

Below are my specific comments about the Prius-based Toyota test PHEVs. We at CalCars believe that, although auto companies parrot the line that only Li-ion is capable and it's not ready, both NiMH and Li-ion are excellent chemistries for PHEVs, along with several other candidates (e.g. Firefly carbon-foam-PbA) that may become commercially available in the near future. There are CalCars-style PHEV conversions running on two different brands of NiMH batteries -- Electro Energy and Nilar -- and we expect battery availability and DIY plans for one or both of them in the next few months.

Looking at the specs, it appears that they are using a Prius (same physical/mechanical specs, now confirmed) with a second Prius battery pack. I think they are probably adding the second battery pack with its own copy of the existing Battery Engine Control Unit (ECU), then tweaking the hybrid control software (which nobody else can do) to allow more EV mode speed, power, and other relaxed limitations.

Another experimenter calculated that at 62 mph (100 kph, the stated top EV speed) and the Internal Combustion Engine (ICE) stopped, Motor Generator 1 (MG1) would be turning at essentially 10,000 rpm, its rated top speed, which makes sense of that raised limitation. My calculations are as follows:

My driving tests (city driving up to 33 mph, 3-4 stops/mile) in EV mode indicate a usage of 200-210 W-hr/mile from the battery. Highway mileage, due to a lack of starts and stops, is likely to be similar.

I assume 15% (not 30%) of Original Equipment Manufacturer (OEM) battery capacity normally available to a normal Prius for EV mode: from around 60% State of Charge (SOC) that the battery is normally kept at (not the 80% maximum, which is seldom available) to the 45% that causes a forceful exit to EV mode. This provides 1.3 kWh •0.15 = 195 W-hr, or enough for just less than one mile, which matches empirical data.

8 miles at 200 W-hr/mi = 1.6 kWh required = just over 60% of the 2.6 kWh capacity of two OEM Prius batteries, a reasonable amount to use. This means that the two batteries could be charged up to 100%, then discharged to 40% SOC (60% Depth of Discharge (DOD)), or they could be charged to only 90% (thus avoiding end-of-charge issues) and discharged to 30% SOC. In either case, the limits necessary to reach 8 miles of EV range using a pair of OEM Prius batteries are only slightly wider than those already used for production Prii. This could be what Toyota meant by their reference to "two oversize packs of nickel-metal hydride batteries".

However, once the full EV range is used, I would expect the system to drift the SOC back up to a value higher than 30% or 40% instead of continuing charge-sustaining operations at that minimum SOC. Doing so would use gasoline-produced energy, thus defeating part of the value of the (already short) EV range!

I see the real value of such a minimum-range Prius-based PHEV from Toyota, if it should ever be released to the public, as three-fold:

1. It could be the first PHEV ever from a major auto manufacturer, thus further validating the concept, allowing consumers to speak with their pocketbooks, and hopefully starting a race.

2. It would furthermore validate the fact that useful PHEVs can be produced with today's batteries and with NiMH batteries.

3. Because it would have EV mode enabled up to 62 mph and, presumably, up to MG2's full 50 kW -- two things that Prius converters have been unable to do, because it would entail reverse engineering Toyota's whole hybrid control code -- it would be a much better platform for PHEV enhancements via the addition of a larger battery pack. Presumably Toyota will have relaxed or removed several other EV mode limitations the current Prius has, too.

By the way, if you haven't yet read about it, it is worth checking out A123's exciting, game-changing proposal to the California Air Resources Board to encourage PHEV conversions by giving the original manufacturer clean air credits for certified aftermarket conversions of their vehicles http://www.calcars.org/calcars-news/803.html.

/ron