Hymotion Toyota Prius

728 miles at 99.9+ MPG

728 miles at 99.9+ MPG

In October 2008 I upgraded my Toyota Prius with the Hymotion battery system from A123 Systems. This is a 5 kwh lithium battery that enables the Toyota Prius to be plugged in to any standard 110/120 volt outlet. The extra battery basically provides the Toyota Prius Hybrid Synergy Drive all of the electricity it could ever want.

If you have experience driving a Toyota Prius, you know already that you can move the car short distances in electric mode. The stock Toyota battery quickly loses energy doing this, so you can only move a few hundred meters or perhaps 1 mile if you are if you are in “stealth mode” at 34 mph or less.

Once a Hymotion battery is added, it is a brand new ballgame. In my own testing I have been able to go 22.5 miles in pure electric mode.

The key is to keep your speed under 34 mph. You can coast along at higher speeds or use hills to accelerate faster. But if you want to maintain just the pure EV mode, 34 mph or less is the general rule. Like this…

At highway speeds of 55 mph and using cruise control, you can expect 40 to 50 miles of extra battery assistance. You will be using gasoline at 55 mph, but it will be massively assisted by electricity from the Hymotion battery and your average gasoline consumption will be over 99.9 mpg. Once the Hymotion battery is depleted, then you are driving a stock Toyota Prius that gets about 48 to 55 mpg on the highway, depending on how you drive and the terrain.

At my local rate for electricity, it costs 8 cents per kwh. It is a 5 kwh battery, so it costs about 40 cents to recharge the battery. Depending on the driving style, the distance and the terrain, most users will get 30-40 miles from the Hymotion battery. So it is about 1 penny per mile in electricity costs. That cost will vary based upon whatever your local electricity costs. Even if you are in an expensive area for electricity, it is still cheaper per mile than gasoline.

There are a few different companies doing Plug-In conversions for the Toyota Prius. “Hybrids Plus” in Boulder CO is one of them. But I think that Hymotion/A123 Systems has everyone beat. They have done a few things to set the Hymotion battery above every other conversion.

1) They have sent the Hymotion upgraded Prius back through full crash testing to make sure there are no problems with adding 180 lbs of batteries where the spare tire used to be.

2) Installing the Hymotion battery does not void your Toyota warranty.

3) 3 year warranty on the Hymotion battery system from A123 Systems.

4) It is professionally installed by a trained local technician. In the Seattle region it is The Green Car Company.



  1. Great Comments from you on the Hymotion Plug-in Hybrid Prius. ( I read your initial blog too for the first 3 months of ownership & now feel much better about making our purchase). We’re just about ready to purchase one from The Green Car Company here in Seattle area and are very excited about “walkiing the walk” for reducing National Security Risky Behavior, reducing our carbon footprint, reducing the acceleration of climate change and spreading the word about how others can do the same. (By allowing The Green Car Company to put logo on the car with “100 mpg+” next to it they give us free oil changes for life & we get to spread the good word about plug-in hybrid cars!

  2. Lucky you! In Australia we don’t have this availability.

  3. At 65 miles per hour with cruise control, I get between 24 and 30 miles of operation per charge. Overall miles divided by total gallons for the past year since the L5 upgrade comes out to 69 miles per gallon on my 2008 Prius. If I could charge my car at work, the average might have come out more like 90 mpg. I can’t remember really enjoying a car so much. 🙂

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