Full charge, coast downhill -> engine runs?

Something has been driving me nuts about my PiP lately: I am getting terrible mileage because it is constantly turning the engine on when it doesn't need to. I know running the heater causes the engine to kick on and kills mileage, so I have been doing experiments lately by religiously keeping the heater off to see if I can figure out what else will cause the engine to turn on, and besides putting the pedal down hard, I can't figure out any rhyme or reason to it. It's making me think the car is defective.

Here is my most clear-cut example: I live on the side of a steep hill. If I unplug my car (full charge), pull out of my driveway, point down the hill, and coast, gravity will easily get me up to well over the speed limit in less than a block (it's an 18% grade). And about halfway down the block, the internal combustion engine turns on. Wth is it doing? I don't even need any power yet! I'm heading to a destination less than 1 mile away that is completely down hill, I have a full battery, and yet i'm using the ICE.

The hardware store is one of these destinations I've been to repeatedly in this exact scenario (again: full charge, heater off), and here's how bad it is: it's a very short trip and it runs the ICE the entire time (except for the first half-block), and the result is a pathetic 24mpg despite the fact that it's all downhill! On the way back, I get 999mpg! Every time. My car gets better mileage driving up an 18% grade than going down it. Someone please explain this craziness to me.

 

And when I want to drive uphill or flat, I only get 75% of the range? That seems kind of ridiculous.

 

Browsing/searching around further, I see this is an oft-discussed topic. I guess I just don't see the logic of my car deciding "battery's too full, must slow down the car some other way" and deciding that burning gas is a better choice than using the brake pads. I can beat 24mpg going downhill in my pickup truck.

 

The problem with this strategy is that when I plug in the car, I don't know if next time I'm going to have to drive uphill or downhill. I'm not going to give up 75% of my EV range for all trips just because when I drive in one direction the car makes a dumb choice to burn gas instead of brake pads. I'll try going down the steep part of the hill (which is only 1 block long, so no risk of overheating the brakes) in N and see if that helps.

It sounds to me like the ideal solution would be to be able to tweak the "full battery" threshold such that when regen braking on a full charge, it's allowed to go slightly higher than 85% rather than jumping straight to panic-and-burn-dinosaurs mode. A little hysteresis goes a long way.

Edit: to answer the last question, I'm in Seattle, so it's chilly but not ridiculously cold. I think I saw somewhere that -10°C is the threshold for needing to turn the ICE on just because it's cold outside.

 

Personally, I'm happy to geek out and understand the counterintuitive nature of the system to come up with workarounds, but I'm not sure my wife is so excited about that. It's her car and the change in efficiency during the winter months (mostly for the heater) has really soured her on how much extra we paid for the plugin option, so she's less forgiving of quirks like this.