Where does extra energy go?

Heat from engine braking

 

If you are in D, and braking, and the battery has no more capacity, the system will just transition to using the friction brakes. This will convert the remaining energy to heat radiating away from the rotors and pads.

If you are in B, and braking, the system will already be sending some of the energy to MG1 and using it to spin the engine, with the fuel injection turned off. This creates a sound and a feel very much like downshifting in a conventional car, and through friction and pumping action it converts the energy into heat that is radiated away from the engine cooling system and carried away in the exhaust.

B mode starts doing this proactively even while the battery still has capacity, on the assumption that you've selected B because you know you'll be braking for a long stretch. When the battery does reach capacity, it just does more of this. And of course if you are braking harder than MG1's capacity to spin the engine, it brings in the friction brakes to handle the rest.

-Chap

 

Heres another thread that discusses it.
Fully Charged HV Battery==No Regenerative Braking? | PriusChat

 

If you had taken your literature seriously at school and read James Thurber's "My Life and Hard Times" you would know that the excess electricity leaks all over the road, which causes havoc to pedestrians in the winter.

 

If you're in D, and not braking, the whole car is designed to lose as little of your energy as possible. The only energy you're losing in that case is the unavoidable amount through drivetrain and tire friction, and air resistance, kept to a very low value by the weird shape of the car.

If you're in B and not braking, the car is programmed to do a bit more aggressive slowing when your foot is off the go pedal, again to mimic the feel of a downshifted conventional car. It handles that the same way it would handle very light braking in B mode (only without lighting the brake lights); some battery charging and some engine spin, changing to all engine spin when the battery can't take any more.

As in any car, if you're on a downhill and picking up more energy than these methods dissipate, the excess energy doesn't "go" anywhere; the car picks up speed. If it picks up more speed than you want, you'll use the brake.

The cruise control is interesting. In Gen 1, you could engage B mode with the cruise on, and if you did, the cruise would control your speed on downhills as well as uphill and flat. It would never apply the friction brakes on its own, but it would bring in regen and/or engine braking generously to control speed on downhills.

I notice my Gen 3 cruise control will not do that in B (it is actually programmed to cancel cruise if you shift to B), but it will actually do some of that (a little less aggressively) in normal D mode, which the Gen 1 cruise did not. Again, if your downhill incline exceeds the modest amount of regen/engine-braking the cruise control will use on its own, then you just pick up speed, and you apply the brake if that's a problem.

-Chap

 

Heat, through engine pumping and frictional losses, as the engine must run in an inefficient mode of operation. (This heat is lost to the exhaust and to the coolant.)

This is basically how a non-hybrid will run at light loads - below the minimum efficient power output.

The hybrid system will avoid this as much as possible, of course, but there's some times where it's unavoidable.

 

Or to summarize, when the battery is "full," any "extra energy" from downhilling goes to the same destinations it would in a non-hybrid---to friction brakes and to spinning the engine (especially if you downshift the conventional car or select "B" in a Prius). If you're "not braking, but driving," there's no "extra energy" to be disposed of, aside from normal unavoidable waste.

 

CR94: priusmatty's question was a specific scenario - battery at maximum state of charge, ICE running.

To expand... in any scenario EXCEPT that, if the engine is running, it's producing at least minimum efficient power (for a certain value of "efficient"). The electric side of the system is powerful enough to support loads below that, and the engine is shut off.

There's two scenarios I can think of, where the battery at maximum SoC would result in ICE running.

The first of those would be at high vehicle speed, such that MG1 would be overrevving to keep the ICE at zero RPM, so the ICE is always running. In that case, the engine may fall below its most thermally efficient modes of operation (less fuel's injected, but the engine is less efficient at converting fuel to work at those light loads, meaning more of the fuel that is injected is turned into heat, rather than motion). When the throttle is lifted entirely, the ICE may be in overrun, which means that it's not injecting any fuel, but the engine's internal friction still produces heat. This, as I said earlier, is how a non-hybrid works.

The other situation is if the battery reaches maximum SoC, in some cases, it may actually start spinning the engine to burn off energy. (This would usually happen under braking, especially in B on the shifter.) Friction in the engine is turned into heat, but no fuel's injected.

 

First of all, the meter/gauge you are looking out isn't 100% accurate. It represents a percentage, in other words when it shows fully charged, it really isn't, and when it is nearly empty, it isn't as empty as it is showing either.

Secondly, at least on my Prius the system DOES allow for more than an 80% charge. I'm assuming you're not using a tech stream, or other tool?
At least with my Prius, all I have to do is go down a big hill in "B" and when I reach the bottom the battery will be maxed out (again according to the meter/gauge).

I have heard, that Toyota has designed the system to embrace "The Middle Way" as far as battery usage goes. That is keeping it as much as possible operating in the 80% range.
But I wouldn't necessarily connect that operational reality with what you are seeing on your dashboard.

 

If you're really at around 80% (i.e., if six or seven of the eight bars in the battery's energy display are lit) then when you use regenerative braking the energy goes into the battery just like it always does. The Prius C normally keeps the battery a little below it's maximum capacity for just that reason - it gives it some "headroom" to recover the energy of your momentum when you stop.

I live in hilly British Columbia in Canada, and it's not uncommon at all to see my battery gauge start at 6 bars and climb up to a full 8 bars when using regenerative braking down a long descending grade.

Now of course even when all 8 bars are lit the battery still isn't really "full" because the battery management software keeps even more hidden "headroom" in the interests of battery longevity. But it never uses that hidden headroom - if the battery gauge shows a full 8 bars and you press the brake pedal then the friction brakes engage and you use them to stop just like in a conventional car. In that case the energy is going into heating up the brake pads and drums/rotors.

The other interesting scenario is when you're going down a long grade with the car shifted into "B" ("braking") mode. In this mode the car kicks in a lot of regeneration even when you don't have your foot on the brake pedal. It's supposed to keep your speed down when descending a hill much like the low gears on conventional transmissions do. But of course you can fill up the battery to it's full 8 bar level when doing this too, and the car still has to continue to resist speeding up. So when that happens the CVT shifts the engine into a high speed band so that it can provide compression braking, which is in fact exactly what happens in a conventional car in low gear. In that mode you can think of the energy going into the work needed to compress the air in the cylinders (it's actually the opposite of that, since in a gas engine "compression" braking is actually the work required to create a vacuum in the cylinders).

 

Don't know if it's relevant, but I've hit full bars on my battery meter on the dash a few times while going down long hills. It's few and far between though, maybe 8x in the last 2 years. 70% of the time I drive I have one bar short of full per the meter.

 

Compared to the actual % state-of-charge shown on a ScanGauge, etc., the dash "battery meter" shows a smaller range covering the car's normal usage of the battery. Full bars do not correspond to 100% SoC, and zero bars certainly don't mean 0%.

-Chap

 

Oops, and agreed. I missed the implication that matty was referring to state of charge, not dash meterage. I don't have a scanguage, but my ultraguage showed the same while playing with it.