2017 prius prime premium 120k. Just started noticing when the battery is fully charged for its 25 mile trip the engine is kicking in either driving or at idle. EV mode is selected with the green circle around it. Not sure why this is happening? I’ve tried different modes to see if there is a difference. Does the car use the conventional heater core to produce heat? Or is it a electric heater. It’s cold outside now and the only thing I can think of is the engine needs to run to make heat.
When the Prime is cold it behaves different. It's complicated and difficult to explain because there are so many things the Prime monitors. So many different combinations of internal and external sensor readings and any one or two sensor readings can trip an engine on situation when a threshold or two is exceeded. When the Prime is cold, if the driver expects the heat pump to provide all the heat needed for the occupants to be comfortable while in EV mode, that could be one reason. The heat pump gets less efficient as the temps get lower and the traction battery also can get held back from providing full output until it reaches a threshold temp itself. This is just one instance where a pair of threshold setting will switch the engine on automatically. Another set of threshold setting can be tripped when using the Defroster. Switching on the defroster is one way many owners use to manually start the engine. It's doesn't always work, but it's the easiest way to get the engine to switch on 98% or so of the time and it also typically works while the Prime is in Park. Asking the Prime for more EV power than the computers parameters have set for the Traction Battery to provide, due to it's being cold enough for the computer to throttle its output, will also switch on the engine. There are at least one or two others that some owners have experienced without understanding why, but generally there are only a few situations that trigger the Gen 4 Prime engine on situation consistently, most of them related to cold ambient air and traction battery temps combined with warmer temp settings of the HAVC heating system(s). Same can be true in the opposite direction when the engine is running during very cold weather. As the HAVC settings get set higher the engine will stay running longer to keep the coolant warm enough to provide that temp output from the heater. And when only the seat and or steering wheel heaters are used and the cabin heater is kept off, while in HV mode, there are speed thresholds that can be observed that also trigger the engine on. Of course that is not related to engine on in EV mode question, but illustrates other thresholds the computers are keeping track of.
What vvillovv said - it's complicated - but there are approximate hand-waving temperatures at which the behaviour shifts happen. Heat pump gives up at -10°C, so engine will start for cabin heat below that. Battery is liable to not be up to full output when below around 0°C, so you might get engine starts from full-throttle EV below freezing (unless you've had the battery heater on). Windscreen defroster will start the engine below +15°C.