Plug-in Prius

2009 Prius Conversion  – A Berkeley, CA company, 3ProngPower, Inc.  (, converted my standard 2009 Prius into a PHEV (plug-in hybrid electric vehicle). You might note that the rear-end is sitting higher as a result of the suspension upgrade to accommodate the extra 300 lbs of lithium cells, adding a rechargeable powersupply of 10KWh. The manufacturing company, Plug-In Supply has been VERY accommodating with upgrades and fine-tuning.

The lithium cells fit into a steel enclosure that fits in the rear compartment under the storage panel over what was once access to the mini-spare tire. The charger unit is on the right with 110 plug on the bumper.

The lithium cells are individually monitored and managed by an Elithion (  Battery Management System (BMS).

Here’s a picture of consumption for a combination of city and freeway driving. Sixty-eight miles at 99.9 mpg.

Finally had time today to do a meaningful comparison of my Prius’ mileage before and after the 10KWh conversion. I wanted to see how far I could use the conversion before the big battery (green bars begin to drop to blue) was no longer aiding the ride. This was just over 100mi, over my usual commute (90% highway) with the PHEV switch engaged the whole time, only briefly in pure EV mode (<32mph), over a hilly course that my prior mileage was 44mpg….My mileage was 76.3 over the 100mi course.

I will be doing gas-consumption testing as I have time over the next few tanks.

I have yet to test the fully charged battery in “ICE-kill” mode (pure EV with 52mph limit), just need to become more comfortable with the changes before I try it.

Plug-in Prius with solar & wind power

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19 Responses to Plug-in Prius

  1. Zack Simkover says:

    Is 99mpg the max it will read (100% electric) or is that an actual mpg of gas burned?

  2. ss says:

    Thats part of the Prius software that has a max of 99.9mpg…poor design. The real-world mileage will be based on miles/gallon at the pumps…Will wait till I have several tanks for a meaningful average.

  3. Toutizes says:

    Looks great. The contacts seem a bit exposed. What if you have a large liquid spill in the trunk?

  4. ss says:

    Thanks for the “heads-up”…seems like something to be conscious of avoiding….the steel cover seems like it would protect against spills of less than 1 gallon, but there may also be vulnerability in the wiring/plugs behind the battery pack and in the recharger.

  5. Buzz says:

    Nice. Looking forward to seeing your average mileage numbers. Wonder f they get much better when the weather warms up? Should reduce both the battery and the gas energy-rate.

    Did you qualify for the Fed tax credit on this?
    Whose LiFePO cells do they use for the 10kWh?
    Have you noticed much of a handling difference with the suspension
    change and the different (better?) weight distribution. Of course, we’re
    not talking about a Porsche here so perhaps “handling” is an overstatement. :-)

  6. ss says:

    Opinions vary but I am certainly going to try to get the tax credit. “Sky” cells from China Aviation. I would say that the handling has suffered, mildly…can feel the extra weight. My commutes do involve some hills and sharp turns….Average mileage this early in (ie less than 300mi) are pretty stable around 80mpg. I do think this will improve both as temperatures warm and as I become (inevitable for any Prius driver) a better hyper-miler…I am beginning to use the ICE-kill button more, but this requires a bit of familiarity as the only way to reset (have the hybrid mode available again) is to pull over and do a restart.

  7. Buzz says:

    Any estimate of the wall-to-wheel energy use? My ancient EV (Ford Ranger) has poor driving efficiency of about 300 Wh/mi. But, when the weather is cold the charger runs longer because it warms up the batteries to charge and runs a cooling fan for the internal charger. So, the winter months yield closer to 400Wh/mi. Wondering if you’re set up to monitor this and if so maybe post.

  8. ss says:

    I have been waiting for both a warmer day and more time to take the car out, fully charged to do a pure EV test.
    3Prong has allowed a very high DOD. I am aware that if I let the battery go down <25% I will see fewer cycles in the life of the pack. My usual commute (36mi round-trip) in the 75-95mpg range seems to be fairly consistent requiring about a 5KWh recharge. On this commute I would estimate my pure EV miles to be about 12…the other 24mi of the commute are done in “mixed-mode” with the battery pack boosting the ICE.

  9. jim spar says:

    the 3prongpower 10kWh unit is from plugin supply in marin county calif, right? when i spoke with robb protheroe, the owner of plugin supply, he was just coming out of a bad partnership experience with some folks in southern calif. evidence of their disagreement is still seen on both his and their websites. i sense he is still in development mode with the 10kwh system, saying on the phone that he was trying this new bms unit or that new cell. i really want a 10kWh lithium pack, but i am worried about spending ten thousand for a system that may need upgrades or fixes or other tweeks in the future, and whether robb will be around to do them. to what extent does 3prong stand behind robb’s system, or is the customer dealing with robb directly for fixes, etc.?
    thanks scott

  10. ss says:

    You are correct…Robb P has supported my unit well. It is not perfect and hardware and software updates are still coming. As far as when the best time to jump in on a PHEV conversion goes, that is something you need to answer for yourself. Considering the pure EV’s that are on the market, I still think the PHEV converted Prius is pretty high up there on bang for the buck….no regrets here.

  11. jim spar says:

    okay Scott, thanks.
    it sounds like 3prongpower was only the installer and that you work directly with Robb P for maintenance or upgrades. If Robb were to disappear, do you have a secondary source for maintenance in nor calif or are you totally dependent on him?
    what types of problems have you encountered that required his fixes or upgrades? Are these what a new customer would likely encounter or once a fix is made, it doesnt present a problem for future units?
    do you drive much in the electric only mode? if so, what is your real-world range say in summer weather?
    thanks again Scott

  12. ss says:

    Correct, and Robb P has been good about keeping me up-to-date…First major upgrade was better cables that greatly affected electric range.

    I have just had the latest software (new board) that is probably designed more for someone using mixed mode to the max, unlike me, who likes to optimize my commute with a combination of EV mode and mixed mode.

    Another modification I made that seems to positively affect handling is to install a stiffer stabilizing plate underneath the chasis. Bought it here: Seems to handle the extra weight better and “track” straighter.

    Basically I use pure EV mode for neighborhood trips, going to town when I am not in a hurry…As previously stated I can get a pure EV range now of close to 50mi (on the flat 35-40mph average). I am sure a better hyper-miler could milk it for more.

  13. jim spar says:

    thanks for your reply Scott. Does Robb provide those system upgrades as part of the service or do you pay for each when installed? If so, are the costly?
    My mechanic, who works on Prius’, cautioned me against adding 300 lbs to the rear even with a spring upgrade. I told him I drive like a grandma, but he still recommends a lighter alternative, which means a smaller range. What has been your experience re handling of your converted Prius?

  14. jim spar says:

    thanks for your reply Scott. Does Robb provide those system upgrades as part of the service or do you pay for each when installed? If so, are they costly?
    My mechanic, who works on Prius’, cautioned me against adding 300 lbs to the rear even with a spring upgrade. I told him I drive like a grandma, but he still recommends a lighter alternative. What has been your experience re the handling of your converted Prius?

  15. ss says:

    Again Jim, not sure if you have seen this, but adding a 300lb battery-pack, does seriously affect handling, the bigger springs help, but I also found that the chassis brace, discussed here: is a major improvement…Almost normal.

  16. Looks great. The contacts seem a bit exposed. What if you have a large liquid spill in the trunk?

  17. ss says:

    So, the aluminum cover would protect from a liquid spill of under one gallon pretty good from the cells. There is an exposed board and numerous plugs in the rear at the edge where the hatchback closes, just under the rug….just another thing to be concious of.

  18. AN says:

    Hello Scott.

    What a wonderful conversion you have. Too expensive for me for the time being. I was wondering about two things:
    1) It may not be your major concern, but have you done an ROI calculation (with or without the solar panels) and figured when will the upgrade would pay off?
    2) Does the the 10kWh get recharged from regenerative braking like the original battery?

  19. ss says:

    It is a very comfortable conversion and a great place to wait for EV’s will better range and more reasonable price tags.

    I am sure it would take >10 years for the upgrade to pay-off. The equation is just too complex, so why bother. I converted for more than pure economics…It feels great to use the energy I make like this.

    The next software (actually a control board) should have both a 72mph pure EV upgrade and also regeneration to the large pack.

    BTW, if you are anywhere near the Bay Area, I am planning to have the car on display at the Green Drive Expo, Richmond CA, September 17th.


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