Prior to 2010 model, EV button does not come with the Prius. If someone installs it and use it (with care or abuse), it can affect the life of the battery. NiMH loves small charge/discharge cycles to hyper-extend the life of the pack. EV button would potentially increase the number of "deeper" (80% to 40% SOC) cycle usage. This will shorten the battery life. I have the button installed and I only use it very lightly, only when it makes sense to do so.
