I talked to a couple of knowledgeable co-workers and did some additional research and can at least shed a little more light on what
other cars do in the "full battery downhill" situation.
Nissan Leaf
As a full EV, these will let you charge the battery to true 100%, and can also be programmed to stop at 80%. My co-worker who lives at the top of a long hill and owns a Leaf informs me that if you charge to 100% then go down a long hill the regen will just quit after a bit, when the battery is in danger of overcharging, and you're left to use mechanical braking. You can feel it at that point. Telling the car to stop charging at 80% or cranking up the heater to burn excess energy are ways of manually avoiding or mitigating.
So that one makes perfect sense--as a pure EV there's nowehere else to put the energy other than the heater or brakes.
Plug-in Prius
These have a full transmission, and according to my knowledgeable co-worker they will start the engine when the battery is
really full, and use compressive engine braking at that point to augment the mechanical brakes. Makes sense because of how the transmission is configured.
Chevrolet Volt
The Volt's ICE is physically capable of being linked to the wheels, but is only designed to at speeds over 70mph (
full drivetrain description here). Per Chevrolet themselves, what the Volt does is
really interesting. The car has a smaller 55kW motor-generator whose primary purpose is generation from the ICE, but is connected to the wheels above 70mph with an appropriate-ratio gear in order to increase EV efficiency at high speeds. With a full battery, the car connects
both electric motors to the wheels and dumps energy being generated from the main motor into the secondary one. Since the motors are
actively cooled (they apparently have their own radiator), it can dump about 12kW of energy into the electric motors as a means of bleeding off energy to augment the mechanical brakes without ever needing to bring the ICE online.
A
forum thread elsewhere quotes a response from someone at Chevrolet explaining this.
This post in that same thread is someone who actually tested this and verified it with the car's computer. The specific message:
You have come across a feature of the vehicle which protects the battery from overcharging. Under certain circumstances, the electric motors will resist one another to provide braking in addition to the friction brakes on the vehicle. In order to meet emission requirements, the Volt does not spin the engine, but uses clutch 2 in the drive unit to link both motors. When the vehicle is at low speeds, clutch 2 requires that the resultant planetary gearset speeds increase to compensate. You will hear the electric motors at higher speeds, which is certainly a change from their normally silent operation.
The Volt was validated using the steepest, longest descent in the nation, Pike's Peak. With a full battery, the volt can descend Pike's Peak without issue with a combination of friction brakes and the electric motors. This is part of the Voltec propulsion system which has many more delighting features waiting for you to discover.
Volt Advisor Trevor
Chevrolet Volt Advisor Team
(877) 486-5846
[email protected]
Clarity
This one's the question mark at this point. Since it has a mechanical transmision, it could theoretically do what the Prius does. Since the drivetrain as I understand it is somewhat similar to the Volt's, it's also theoretically possible to do what the Volt does. Or it could do the wacky third option and use the generator to put load on the ICE. Or it could just force you to rely on the brake pads like a pure EV. Which of those it actually does is anybody's guess until someone supplies more concrete info,
but...
...the
real question is whether it's inappropriately using the ICE to actually
provide energy--rather than just waste it--in some circumstances, which is what some of the posts in this thread seem to be describing. If so,
that's a bug, and should be fixed. One acceptable way for a PHEV to provide supplementary braking on a full battery is to use compressive braking in the ICE, but inappropriately generating power with the ICE is wrong behavior.
Since I don't have anywhere to charge at the top of a hill I can't really test this myself or experiment. I did try putting it in HV Charge mode with a full battery and driving up a hill, goosing it to encourage ICE use, but the car fairly aggressively used battery energy until the SOC was about 90%, and I only had enough hill to charge it back to about 95%.
Tbh, it really doesn’t matter how the Clarity, or any EV works. [...] If it is a design flaw by Honda, it needs to be fixed. It’s that simple.
That's kind of the point of getting to the bottom of it--depending on exactly what the car is doing, it might
not be a design flaw, just not the behavior people are expecting. Conversely, it might be doing something logical, but doing it wrong, so it
is a design flaw. I've never seen it happen, personally, but I'm curious what's going on and also figure it's useful to know what, exactly, is happening, in order to distinguish between unexpected behavior and an actual car software problem.
I'm going to post a poll to try and gather a bit more data.
