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The Prius 'B' mode
Posted to Technical Theory Forum on 9/26/2010 1 Reply


I have hopefully learned to be careful about making dogmatic statements about complicated cars, like the Prius hybrid. But I was sitting in a brake class a couple of weeks ago and the instructor told the group that the Prius B mode increased regeneration to a higher level to add vehicle braking. I kept my mouth shut even though this ran counter to what I had observed previously, mostly because the information is not terribly significant when it comes to fixing a car. But, I decided to see if I could prove it. Now, I think I have.

What I did was flog my car up a fairly good grade to get the state of charge down to a level where I knew that the THS would be trying its best to recover to its comfortable levels of 50-63% SOC. I was able to punch the SOC down to 36% which is not easy to do without going way over the speed limit (I think I hit 79 mph going up hill in order to accomplish it.) I then did a U-turn and went back down the hill. I recorded a ten minute recording on the Techstream and then zoomed in on a flagged area where I had shifted into "B" mode on a pretty steep downhill. [2004 Toyota Prius, Engine/Propulsion Scan Data] As you can see, the battery SOC had already recovered up to 52% just by coasting. I think I saw as high as 60 amps of regen without applying the brake, which I presume was being supplied primarily by MG2, though since I didn't capture revolution and torque on the two motors I can't say for sure. But, beyond any doubt, the B mode actually is drawing massive amps (82) out of the battery pack in the process of reving up the ICE rpm to accomplish the B mode braking. It is definitely counter-indicated for fuel economy. As you can see here [2004 Toyota Prius, Engine/Propulsion Scan Data] after I have shifted back into drive (at the flag; there is a bit of a delay before it actually happens)we get a pretty good regen, once again only by coasting.

So, it is my conclusion that the B mode is using the electric motors to transfer more load back through the power splitter (planetary gear set) to the gas motor in order to slow the car down. It kind of makes sense if you realize that the battery pack and its life are really important to Toyota. Force feeding it high rates of amps for the purpose of slowing the car on a long hill is a bad idea. I tried to duplicate this test on Saturday, but I waited too long going down the hill and I heard my battery fan kick on high blow. Even though battery SOC was only at 65% (on a cool battery pack I've reached just over 81% on a long down grade) the hybrid system shut down regen entirely, no doubt to protect the battery, so I couldn't prove anything with the B mode.

It is dangerous to make dogmatic statements about the Toyota hybrid system and how it behaves. I did not know till yesterday that it may not be possible to push SOC to the max on a long hill if battery temp went too high. I just never tried it under ambient conditions of 105 degrees. So, we go on learning...

Phil from California

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