Down the Mountain

Not normal at all, Check parking brake for dragging. On the normal flat surface do you get rolling and regen in drive? If not, very abnormal. On a steep downhill roll the HV will charge to green and one must use the brake position to slow it down or hit the footbrake occasionally. You could have a SERIOUS HV or associated problem, the feathering or stealth is not difficult to obtain. If you have any kind of warranty, get it to the dealer quick! Await more expert advice.

 

I agree, it sounds like you have a lot more friction than the couple of kw passive regen adds. Were you in 'B' gear by any chance ? Was the ICE on going downhill ? What is your usual MPG ?

 

If you're lucky you accidentally shifted it to "B", which is in fact what you want when coming down the mountain to avoid overheating the brakes. Be sure to shift back to "D" for non-mountain driving (otherwise fuel economy will be reduced).

Alternate explanations include a transmission or HV system problem. Brake problems seem unlikely if there's nothing funny at other times.

 

It's called engine braking and all cars do it. On the Prius it's only "simulated engine brake" from regeneration but it feels much the same as any other car. That's in "D" of course, if you're driving in "B" mode then yeah you get about twice as much "engine braking", more or less like in a conventional manual transmission shifted back to 3rd gear. That's by design.

 

The OP said driving down a hill in the mountains, so unless we are talking about the slow rolling hills of Kansas the car should be accelerating either in drive or neutral. Now for the terrain in Autralia, I cannot speak, but, we all have our own conceptions based on our local topography and experience.

 

Another thought, maybe he had a full battery, and the Prius was auto-discharging during the descent ?

 

No, not true. I've gone down many mountain grades in B mode and have had to keep my foot on the gas to keep up with traffic.

Tom

 

Well, on -real- mountain grades you have to keep using the brake, even in "B".

But yes, if you are in "B" on an average downhill grade (7% or less) you will have to feather the accelerator to keep up.

In "D" the Prius takes off down hills. Be warned! The drive has very little drag and the aerodynamics allow the car to coast very well. Pearl has been known to reach 120 km/hr (in an 80 zone) in less than a minute. "B", windows opened, and mild braking were necessary to control speed! Especially once the battery reaches full green bars! But that's on a 8-10% grade.

 

Most highway mountain grades are 6% or less. The combination of higher speed (more aerodynamic drag) and only 6% grade makes for something that B mode can handle pretty well.

I've gone down some steeper roads on climbing trips where it was B mode, brakes, and 12 mph all the way down. I would let the Prius go just fast enough to get out of friction braking. The engine sounded like a jet aircraft, but everything behaved exactly as advertised. As a bonus you have a really good SOC at the bottom.

Tom

 

There really wasn't enough info in the original post to know for sure if descents were all that steep or not. My point was simply that it may be more a matter of the OP having unrealistic expectations of how the car would coast rather than a real problem with the vehicle.

Honestly if you look at the majority of "problems" posted by new users here you'll find that more often than not they are misunderstandings or unrealistic expectations rather than real issues with the car. I just wanted to make sure the OP understood that the prius offers "engine braking" just like any other car, and even more so if in "B" mode.

 

It sounds like you had a over charged battery, and the car was discharging the battery going down hill. When all bars are green, the car does this automatically. I only experienced it once, and it was not long enough for me to notice what you reported.

 

When the HV battery hits the high limit, charging is curtailed. Regenerated electricity is instead used to drive the ICE like a big air pump for engine braking. This is the "auto B mode" of which you speak. If engine braking alone is insufficient, the Prius will also engage the friction brakes.

Tom

 

I think of the situation this way:
The HV battery is overcharged, Prius decides to use up some of the energy.
If the driver asks for energy, fine; otherwise the car will find some work to do but will not make the car move faster than the driver demanded. As Qbee wrote, this is usually engine spin, but can be friction braking in unusual cases.

I just feather the petrol pedal enough to start pulling some energy out of the battery, but not push down so hard that the ICE gets into the act on my behalf.

By the way, my personal experience in these battery overcharge situations is that the car just wants *some* battery energy demand. I have never seen auto discharge when the AC is running for example. I live in foothills on the side of a valley and experience full green batteries often, although I try to avoid really steep descents.

 

Just drive normally. If you know that you are facing this sort of descent, shift into B mode at the top. That will start engine braking and forestall charging to the high limit.

Tom