The "EV button" can be added to any Prius. It is standard equipment on the Japanese models.
Any pushbutton switch will work, but you can buy the JDM EV button for a totally stock look.
The button basically tells the Prius to not bother warming up the engine like it normally does when you first start the car. It does not increase the speed at which the Prius fires the ICE. I believe that speed is 41 MPH. Toyota is or was testing a few PHEV Prii in California that could stay in EV mode up to about 60 MPH. It is unknown whether this required a hardware change or simply a software (ECM) tweak. Toyota of course won't say.
There are a few companies that sell PHEV systems. Hymotion
(now owned by A123) is a big one. I have seen DIY jobs with gel cells as well. It isn't particularly complicated for someone with a DIY mentality.
I watched a Prius get converted last year. A high voltage/current connector (Anderson connector) is added where the stock battery connects. The additional battery is bolted into the cargo area. The EV button is added. Even without the EV button, the additional battery will improve mileage quite a bit because the Prius will use the electric more when the state of charge is high.
The relatively high cost of the battery (most use Lithium Ion) means the payback period is quite long. Of course this depends on the usage pattern.
If I am not mistaken, it is impossible to recharge the additional battery pack through Regenerative Braking
, only from the grid. So if you expend the additional battery on a long trip, you're driving an ordinary Prius with a bunch of extra weight until you can plug in again.
There are a couple or three PHEV Honda Insights in existence, and at least a couple more in the works that I am aware of. The MIMA
system is required to take full advantage of the additional battery capacity in the Insight.