Go down a hill after a full charge. There's no regen (according to the power meter) at all so you can feel what the friction brakes feel like unassisted. In fact, that's why I avoid giving my Clarity a full charge. For several miles, there's little to no regen so you're just wearing out your brakes (and the Li battery) prematurely.
There may be no regenerated electricity sent to the battery, but that doesn't mean only the brakes are being used to slow the car. This is a tired topic that I keep bringing up, but I'm still looking for the final answer so I'll bring it up again. It's all about the pseudo engine-braking that the Clarity needs to perform to go down a mountain without overusing its mechanical brakes.
Page 434 of the Clarity Owners Manual says:
Applying constant pressure to the brakes while going down a long hill can cause the brakes to heat up, resulting in a loss of stopping power. Therefore, when descending a long hill, release the accelerator pedal and allow regenerative braking to slow the vehicle down. To adjust the rate of deceleration, use the deceleration paddle selector. When the high voltage battery is recharging, regenerative braking may not be possible.
So Honda's engineers know that even when the battery is fully charged, there must always be a way to slow the car down without staying on the brakes all the way down a mountain. However, the last sentence about when the high voltage battery is recharging doesn't make sense to me. The main purpose for regenerative braking is to recharge the high voltage battery! The secondary purpose is to provide braking that doesn't involve the mechanical brakes.
When its battery is fully charged, the Accord Hybrid performs pseudo engine-braking by using the starter motor/generator to spin the engine, which for this purpose receives no fuel and the valves are kept closed. The engine provides the resistance to use up the torque the starter motor/generator is producing from the electricity regenerated by the traction motor serving as a generator.
However when the Clarity PHEV's battery is fully charged and the driver begins to slow the car, it often
starts its engine. The engine does use up a little power to get started, but not much and not for long.
So I can see only one possibility that explains why the Clarity, unlike the Accord Hybrid, runs its engine to produce pseudo engine-braking. My big unconfirmed theory is that the Clarity uses the electricity coming from the traction motor to
run the starter motor/generator in reverse. In this scheme, the engine is providing even more resistance to the starter motor/generator than if it was simply spinning without fuel and with its valves closed.
If anyone has any better theories about how the Clarity performs pseudo engine-braking (or if AnthonyW can find a service bulletin with the actual answer), I'm all ears (er, eyes).
