What do you do when B doesn't slow you down enough? On a 6-mile 6% grade in the Northwest recently, I was continually accelerating well beyond the speed limit in B, and the battery was showing all green bars. Where does the Prius dump the additional energy? I finally adopted the tactic of coasting in Neutral, and stabbing the brakes (now only friction brakes)(?)) when I got 5mph over the limit to get me back to the limit. This seemed to work well, and the brakes didn't catch fire, but what SHOULD I have done? Next time I'll order the drag chute option.
Assuming B was warranted, it was a long downhill: if you're still speeding up to much, no harm in using the brakes too.
Geez. The car will take care of itself with regards to regeneration and NOT overcharge/harm the HV Battery. Unless you experience true runaway vehicle acceleration (as in the gas pedal is being depressed by a ghost), then going to N, would be appropriate. What you did was not a good thing. If you feel B did a poor job of engine braking, traverse that same road, but this time stay in D. You will find yourself using the physical brakes more frequently and with greater force, to maintain a safe speed, possibly overheating the brakes to the point of failure. Report back with your observations of B vs D on this road. You will probably not second guess B again.
P is not an engine brake, the engine is unencumbered in P, the transmission is locked so that Motor/Generator 2 cannot rotate by a pawl in the gears. B alters the programming so that when deaccerating the engine rotates as an air pump, wasting energy.
But won't using the brakes in that situation continue to charge the battery even more? When do the friction brakes come in?
Using the brakes in B will charge the battery slower than in D. Why might be twofold: 1. Because the brakes are working in concert with engine braking. 2. Because the computers are reducing the regen portion of braking, relying on actual friction braking more. #2 I'm not that clear on, but bottom line: it'll take longer to completely charge the battery, and get you to the bottom of the hill with cooler brakes, using B and braking, compared to using D and braking. What you're trying to do is preemptively reduce battery charging, to forestall a situation where the battery is fully charged, and the computers have no option but to brake 100% via friction brakes. The closer to the bottom this happens, the better. Again, these tactics are only needed on long downhill runs, coming down mountains. Around town I always leave it in D, brake as needed. The car has a large capacity to charge, I've never seen it top up. Even coming down Mount Seymour (local ski mountain), when I do use B, it's still never got to that point.
In general when driving the Prius on downhills, it's best to use the brake pedal with a steady force to engage regenerative breaking until you get up to 7 or 8 bars of charge. Then use B mode whenever the car accelerates faster than desired when you're fully off the gas pedal. If you need additional stopping power while in B mode, use the brakes. However as Mendel has pointed out (and we've discussed in other threads), on very long downhills where you would quickly fully charge the battery if doing the above behavior, it's best to switch into B at the top of the hill before the battery is fully charged and then it will slowly and steadily charge the battery as you descend. It's better on the battery to slowly charge it, and you'll reduce wear on the normal brakes somewhat. When the Prius detects the battery "fully" charged (80% SOC, indicated by 8 bars), it disengages regenerative braking, and then only normal brakes are available (applies to both D and B). However the brakes are sized in consideration of working with regenerative braking, therefore on long downhills to reduce the chance of overheating and reduce brake wear, B mode should be used if more than gentle braking is required. I use B mode coming down long downhills driving I-70 and other mountain roads in Colorado. Otherwise I never use it. If you don't want to think too much about it, just use B mode like you would use "3" or "S" in an automatic transmission car. Another trick you can use is if B mode is doing too much braking, you can gently push down the gas pedal until the car shows no energy arrows with the "Energy Monitor" screen displayed (you have to be fairly precise). This is effectively neutral, and the engine will spin at it's minimum possible rpm (e.g. 960 rpm between 40 and 72mph) without injecting any fuel (confirmed via my scanguage reporting 9999mpg). You can control the amount of engine braking with the gas pedal in this way. However if you push the gas pedal down far enough, it will rev the engine and inject fuel similar to if you were in drive mode (I haven't played much with this - I switch back to D, if I need engine power). The only useful times for N mode I've found is for automatic carwashes, and to keep the electric motors from using energy by "creeping" when going downhill in traffic below 7mph. Otherwise it's better in D mode to just push down the gas pedal until no arrows are shown.
The friction brakes came in the moment you switched to N, or the moment the HV battery filled up, or the moment you exceeded about 25kW of regenerative braking power (before the battery filled). At highway speed, the later happens with very light braking, around 0.05 g. On a 6% downgrade, if your car slows down at all, you are exceeding the regenerative braking limit and going into friction. Neutral was the wrong answer. It is unsafe (mostly in older vintage cars when brakes were not as good), illegal, and maximizes the risk of brake overheating. D would give cooler brakes, and B the coolest brakes.
Thanks to all, especially Kevin. I bought the car in Florida in 2005, and just last week drove to WA via Denver, so I've puzzled about this only recently, first in a downhill run from Cotopaxi to south of Denver. Even in B, the car (heavily loaded) wanted to go a lot faster than I did.
You should have left it in "B" and used the brakes as necessary to control the speed. If you had been more patient you would have experienced what the Prius will do once the HV battery is Full (80%) at 8 bars. Referring to the chart below: You will notice that the 6th & 7th bars are a much bigger range than the others, so it takes proportionately longer to go through these two bars to the final bar. Also notice that the 8th bar will light at 75-77% so when it illuminates the HV battery is not 80% (full). Even though the 8th bar has a fairly small percentage range, it still takes a lot longer than you'd think to achieve 80%. When it does reach 80%, you will hear an alarming rise in engine RPM as the system swaps out regen resistance and swaps in a further amount of engine braking resistance. But fear not, the wheels are not falling off, it is just the engine being used as an air pump to create resistance and waste energy in the form of heat to replace the resistance that was provided by regen braking. This point has not as yet been mentioned in any of the previous posts in this discussion. This change in configuration is far more severe than what you hear as you put the car in "B". As mentioned by a couple of others, B mode does a similar thing – replaces some regen with a greater amount of engine braking to end up with a lower charge rate into the HV battery and a higher braking resistance. Hope this explanation helps you to trust your car more.
@dolj Thanks for that! Would you mind if I post a pdf conversion of it? Addendum: PDF format of dolj's graph attached.
