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Yesterday when I was driving home from New Hampshire, I went down a long steep grade for about 5 minutes in “B” after about 3 minutes a noise started, a buzzing or humming sound from the engine compartment. There were no warning lights and the only thing I notice was that the battery was fully charged, is this normal when the battery is fully charged? And where does the excess energy get dissipated?
 

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Well, my wife accidentally drove home from work in 'B' one day last week, (didn't notice until she parked that she was in 'B',) and mentioned a funny buzzing noise. I checked, and the battery wasn't full, so I doubt it's related to battery status. Probably either some kind of "You're in engine braking mode!!!" warning, or at least something related to engine braking that causes it.

I haven't left it in 'B' for more than one or two minutes, so I haven't heard the noise she's talking about.
 

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full batteries

I noticed last week when driving down a long, steep grade from a nearby mountain lake in "B" that once the batteries were fully charged the engine sound changed and the revs went higher. I guess once the batteries are totally full, and there is no regeneration available to slow the vehicle, it has to use up that energy the old fashioned way. Too bad they do not provide a deep discharge button to tell the CPU to take the batteries lower than normal so there is more capacity for recapturing the energy on the way back down.

Todd
 

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Diana_04Prius said:
Yesterday when I was driving home from New Hampshire, I went down a long steep grade for about 5 minutes in “B” after about 3 minutes a noise started, a buzzing or humming sound from the engine compartment. There were no warning lights and the only thing I notice was that the battery was fully charged, is this normal when the battery is fully charged? And where does the excess energy get dissipated?
Diana,
I suspect what you were hearing was the internal combustion engine (ICE) spinning off the excess energy that your full (or nearly full) battery was no longer accepting. This is a normal function. The first time I heard that noise I asked about it, too, and the following is the excellent response which Robert Snyder provided to my question:

robert snyder said:
B mode, aka engine braking mode, simulates the effects of putting a conventional automatic in 2 or 1 without any gas. It opposes the forward motion of the Prius with the larger motor/generator (MG2). This generates electric power which is sent to the battery to the extent that it can accept it. The rest of the electric power is sent to the smaller motor/generator (MG1) to spin the engine (without gas). As the battery becomes less able to accept energy, more must be dumped into the engine (while MG1 has additional capacity) or MG2 must reduce its braking effect.

A good rule of thumb with the Prius is to ignore engine sounds. They don't typically track the behavior of other cars. Once you've become more enlightened to the energy flows of the Prius you can once again listen to the engine and have it make sense. Engine sound correlates closely with the amount of power the engine is providing (or absorbing in the case of B mode). Engine sound is only very loosely correlated to vehicle speed and only moderately correlated to total power use. In other cars engine sound is highly correlated to vehicle speed and total power use.
Just in case you don't know, the Prius has one ICE and two electric motor/generators, which are known as MG1 and MG2.

Drive happy,
Moo :)
 
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