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Prius EV Mode Button

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Enabling the EV Mode Button, The secret missing feature.

The EV Mode Button is a standard part of the Japan and EU Prius but was not installed in the US versions, perhaps because of extended warranties in Ca due to AT-PZEV regulations and qualifications criteria. The US Prius is still entirely aware of the EV Mode though it can never get the signal or command to enter or exit the mode because the wire and button are missing. EV Mode is a very enlightening feature as it encourages the Prius to stay in its electric mode for far longer than it normally would. It will normally drive 1 to 2 miles on a fully charged battery, after which it must restart the engine using gasoline to recharge the battery. It is not a good idea to fully cycle the battery often as this decreases any battery's life and is not helpful for mileage if used improperly. If used correctly is can be advantageous for times when you know you won't need to start the engine. It's also fun for showing off and offers a small glimpse of the potential of electric vehicles.

The EV Mode function itself is activated and deactivated by momentarily grounding the H14 #27 pin on the HV ECU located behind the glove box(s). (ph) has a 3-wire EV Mode Button kit with easy instructions which allows you to turn on and off the EV Mode through the Cruise control stalk. prius-evbutton-install.pdf <ref>Media:Prius-evbutton-install.pdf from</ref>from and EV-button-Y.html from have more information for "do it yourself" type people. This factoryEV.pdf <ref>Media:FactoryEV.pdf from</ref> by ScubaX at PriusChat talks about the OEM EV-Mode button and dimmer switchs white/black wire which is ground(-) and the grey wire which is illumination control, there doesn't appear to be any OEM EV-Mode status indication. The recirculation button on the steering pad might also make a good EV Mode button but it's not a simple switch and is instead a variable resistance control connected to the climate control system. You can also take a look at the PiPrius conversion process#CAN-View_Version3 page which discusses one particular PHEV conversion but includes some steps regarding the EV Mode button installation. The OEM "EV Select Switch + Housing can be acquired at though any momentary switch can be used.

How the EV button works

Things that cancel or prevent EV Mode use:

  • Pushing the EV-Mode button cycles into and out of EV-Mode, if allowed.
  • On cold days, heat and defrost will both prevent EV mode if the engine is not warm enough (usually > 150F)
  • The battery voltage must remain above 180v.
  • The CCL (Charge Current Limit) must remain above 50A.
  • The battery must remain above 45% SOC (2 out of 8 bars).
    • The battery must be above 50% SOC (3 out of 8 bars) to engage EV mode.
    • The battery must remain below 80% SOC (8 out of 8 bars).
  • The traction battery must remain below 120°F
    • The traction battery must be below 113°F to engage EV mode.
    • (low traction battery temperatures below 40°F will likely result in a CCL which will prevent EV mode, but a low battery temperature itself will not cancel EV mode.)
  • The gas engine (ICE) must be above 32°F (0°C) to engage EV mode.
  • The speed must remain below 34mph (55 km/h) (27mph or 44 km/h on the European model).
  • The accelerator pedal must not be depressed more than about half to 3/4 of the way down (<=120 out of 255, corresponds to approx. 120A or 24kW).
    • Other EV modes such as "Stealth" (up to 42mph) and "Warp-Stealth"/"ICE Spin" (above 42mph with the ICE at ~950 RPMs) regularly occur when there is ample battery energy and low power demand without the use of the EV-Mode Button.

Other Notes:

  • The ICE will start 7 seconds after vehicle startup unless EV-only mode is entered during that time.
    • Once the ICE starts the first time during a trip, it will not stop or allow EV-Mode until the catalytic converter has warmed up. During this time, the car is in an EV dominate mode, where the car prefers to use the electric motor instead of the ICE to allow the ICE to slowly warm up.
    • Once the catalytic converter has warmed up, in cold weather, the ICE will continue to warm to about 100°F. During this time, while the car will not go into stealth mode, even if stopped, EV mode is available as long as heaters are off.
  • The A/C compressor is electric, and the A/C will work while in EV-only mode.
  • When the EV-only mode is entered, the hybrid model screen will show. A beep will be heard only if that screen wasn't already showing.
  • Three beeps are heard whenever EV-only mode is requested but not allowed, and when it is auto canceled.
  • One must wait around five seconds before a new EV-only request is acknowledged.

Things you need to add the feature.

  • 22 gage wire, about 6 feet.
  • Standard Phillips screwdriver.
  • Small Jeweler's screwdriver.
  • Soldering may be required.
  • 1/2 to 3 hours of time.
  • optional components:
    • momentary switch, extra batteries, drop cord?

Parallel Packs

I'm aware of a few other people who have added additional battery capacity to Prii, although they did not grid-recharge, those projects are rather close to becoming PHEV projects.

EV-Mode and your Warranty

Regarding your warranty