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2018 PHEV Touring Atlanta, GA
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The only way to avoid this is to disconnect the transmission from the engine by putting in the clutch or shifting to neutral.
In the Clarity you can shift to neutral fairly easily by pressing and holding the neutral button for a second. Apparently that's not quite as illegal as is often believed. But what you can also do to coast is to lightly press the accelerator pedal and hold the power needle at 0, i.e. no regen and no power produced. Sort of a pain, but I do it when I can. I think maybe Tesla or some other car there's a setting for how much regen you get when you take your foot off the accelerator pedal. One-pedal aficionados will set it for maximum regen, but you can also specify 0 regen. To get any regen with that setting you have to either press the brake pedal or use the paddles.
 

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2020 Clarity Touring
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You're missing a use case for all vehicles and a specific use case for EVs. On downhills all internal combustion engines have drag from the compression combustion chambers (cylinders), which causes the car to slow down faster than just coasting. The specific use case for EVs is to imitate this behavior by applying resistance to one or more of the electric motors, which causes power to flow backwards towards the battery. The only way to avoid this is to disconnect the transmission from the engine by putting in the clutch or shifting to neutral.

In my Volt this results in the car decelerating at about the same rate as any other automatic transmission ICEV while in Drive. But if I put it in L the computer increases the the regenerative braking to slow down fast enough the car is required by the FMVSS to turn on the brake lights. On a long descent on cruise control my Volt in D will slowly accelerate (it doesn't have ACC), but L will maintain speed by modulating the regenerative braking system.

When you let off the throttle in the Clarity the same thing happens - the car is programmed to slow down like it's a regular automatic transmission ICEV. The regen paddles increase and decrease the regenerative resistance. When using ACC, the Clarity adjusts the regeneration level to maintain downhill speed. (I've seen reports the 2020s don't do this.)

Regenerative braking is somewhat of a misnomer - it can occur anytime the car is slowing down relative to the Earth's gravity well or road speed. Like all new ICEVs, PHEVs first turn off the fuel injectors to save gas. Power consumption is zero at this point. Unlike ICEVs PHEVs can then apply resistance to the electric motor for power regeneration to slow down relative to the road or the gravity well. As an example I set the ACC in my wife's Clarity to the posted 65 MPH coming down the south side of the Grand Mesa in Colorado. By the time I reached Delta, CO, which is 3,600 ft lower elevation, the car's regenerative braking system had recharged almost half the battery capacity. This was at a steady state road speed.

Be aware that at speeds above 60-70 MPH the power needed to overcome air resistance reduces the amount of power that can be fed back into the battery so at high speeds you may not see regenerative braking during descents through rolling hills, but you will see it coming down the high passes on I-70 in Colorado and the long steep grades through Pennsylvania and West Virginia.
I am well aware of everything you mentioned. I have two PHEVs and have driven them both over hilly terrain. If you absolutely must slow down then, yes, regen braking is the way to go. However, during normal highway driving where you are not descending steep grades I still maintain that it gives very little benefit. About the only thing you can definitively say about regen braking is that it’s always more efficient than friction braking. So if you must brake it’s better to do it with regen braking. The emulation of drag by ICE is only there to make the car “feel” like a regular car. There’s no other good reason for your car to be putting energy back into the battery unless you absolutely must slow down or stop. If I put the car on cruise control on a fairly flat stretch of road and it constantly oscillates between 64 mph and 66 mph to maintain a speed of 65 mph then the times when it is ‘decelerating’ and putting energy back into the battery are just wasted. The car is better off allowing itself to slow slightly by coasting. Or, it is better to drive slightly under the speed limit, put the car in “N” and coast when you go down a hill slightly gaining speed, than it is to drag the car down by regeneration to maintain a precise speed.
 

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In the Clarity you can shift to neutral fairly easily by pressing and holding the neutral button for a second. Apparently that's not quite as illegal as is often believed. But what you can also do to coast is to lightly press the accelerator pedal and hold the power needle at 0, i.e. no regen and no power produced. Sort of a pain, but I do it when I can. I think maybe Tesla or some other car there's a setting for how much regen you get when you take your foot off the accelerator pedal. One-pedal aficionados will set it for maximum regen, but you can also specify 0 regen. To get any regen with that setting you have to either press the brake pedal or use the paddles.
It would be great if the Clarity had a setting for zero regen when you lift off of the accelerator.
 
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