Are mountain descents of 4-5 minutes [getting 99+mpg] bad for the battery?

I believe the battery is protected, you will notice a change of sound when your battery is fully charged. That means it is no longer getting charged up, and so when you apply the brakes you are really using the brakes not the regen system. So that's where B mode is handy to use engine braking. I expereince this in PA at Rt 40 on the Uniontown hill.

 

It will stop charging the battery and use the engine for compression braking under those circumstances.

 

I just completed a trip through the NC mountains this past weekend and was monitoring the HV battery parameters with Engine Link on my iPad. When making descents once the battery was fully charged at 80% SOC it would transition to engine braking as stated above, where the engine acts like an air compressor.

Monitoring battery SOC, temps, etc. I did not see any problems. Actually the battery temps did not rise as much as I was expecting.

 

Is this an area where Gen2 is differert from Gen3?
I am thinking my car does not go into auto-engine-breaking, must use B mode.

 

Good News: Your battery will be just fine. The folks in Aichi planned on people using their Priuses in places (like Japan) that are topographically diverse.

Bad News: You were only getting 99+ MPG if you disregard the energy that it took you to get up the hill. Or.....A Hummer H1 will also get 99+MPG going down the same hill.


Good Luck!
Enjoy the fall scenery!

 

Since I have not had a Gen 2 I cannot answer. For my Gen 3 once the battery SOC hit 80% you could hear the engine rev up as it went in to engine braking mode without me changing to B mode. On the next descent I did change to B mode after the engine had already transitioned to engine braking and could not notice any obvious difference.

I do remember reading that B mode will limit regen to some degree if your battery is not already at 80%.

Maybe some others that have had the Gen 2 and now have the Gen 3 can answer your question.

 

It could be, but it would be a curious omission. Engine braking has always been an effective and free way of slowing an ICE vehicle on a descent. The car would have to intentionally decouple the wheels from the ICE in order for this not to happen. A simple test would be to lift off the gas on a descent with the battery full. Does the car slow? If so, you are seeing the effect of engine braking.

 

I think my Gen2 keeps going faster. But I am not the expert. There are many past threads on this I am sure, I am just too lazy to check back.

 

auto braking would SUCK. i hated all of my cars that that used auto braking while i had my cruise control on. Let my car build up all of the speed that he it wants.

 

While I still had room in the battery for regen the cruise control did a great job of maintaing speed. Once the battery was full and started engine braking, then it could no longer maintain set speed and started gaining speed to the point that I would have to use the friction brakes to keep from overspeeding.

 

Here you go. It does it just as expected. Automatic B-mode at 84% SOC | PriusChat

 

Actually the best I've seen with my Suburban on a downhill is 45 MPG. I doubt a Hummer would ever do much better than 60 MPG even when coasting down hill. That's the flaw with none hybrid autos, the ICE still consumes fuel, even on downhills and if you turn the ICE off you loose power brakes, power steering & the A/C.

As far as regen braking goes, Toyota, I believe, made a wise choice in avoiding full charging and full discharging of the traction battery. By stopping charging at 80% they avoid the heating affect that 100% charging causes. I suspect that cellphone & laptop batteries would last much longer if the chargers for those devices were setup more like the Prius. But then they wouldn't be able get you to by a new phone every 2 years, lol.

 

Most cars built in the last 25 years or so consume no fuel when off throttle with the engine spinning. Hybrid or not.

 

All the folks above are correct. Just a comment about the engine braking, either because you filled that battery or chose to use B mode.

The harder you brake, the faster and louder the engine sounds. This is to be expected.

 

I'm interested in hearing the source of your information. From everything I've seen, read & heard very few gas engines are setup to completely turn the fuel off when the engine is turning.

Update - I appear to have posted too soon. I did some google searching. looks like more vehicles do full fuel shut off during coasting than I expected.

 

I think that getting up to 80 mph, without your foot on the brake pedal, on a steep hill, with a full battery, would be expected since the ability of the design to simulate the behavior of a conventional car only goes so far. I go down long descents here, such that the energy history graph on the MFD shows all bars at 99mpg. It is the engine I worry about more than the batteries. I shift to B mode manually sooner than the car does, since I know the terrain. The regen current is limited when the battery is full, and especially if it is getting hot, so there is not much help there.

The engine does not provide all that much stopping power, and I don't feel good hearing it revving at 4000rpm frequently. I suspect I will not see the long brake pad life that most others report. I turn the AC up and roll down the windows for increased drag. A nice motorized spoiler would be helpful.

 

Come again? If the battery is full, and you don't manually put the car in neutral, it is engine braking.