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This question is based on Toyota Hybrid flyers / experience but I suppose it's meant to be general for hybrids?

The concept is to use electric motor for braking (therefore often the power of electric engine exceeds the power that can be drawn from battery) and charge battery in that way. But what happens if the battery is full? What does the hybrid do with the surplus energy? Or then is the gasoline engine used for braking?

I've noticed engine working loud when using downhill assistant at the end of a long downhill when the battery was full, but it could be because the slope was more steep at the end.

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  • There are many hybrid systems from plug ins where the engine rarely needs to be used (Chevy Volt) to mild ones like Honda's IMA cars). And also various brake regen systems. Please specify.
    – geoO
    Jun 1 '18 at 15:12
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I live in a hilly area - there's one road I regularly use that's 600m ascent and then the same amount to descend on the other side. About half-way down (in "D" position), the battery becomes full, and the Prius uses engine compression for the remainder of the descent to dissipate the remaining energy (as if it were in "B" position). It doesn't use the disc brakes for this (and you don't suffer brake fade).

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  • Are you using the extra engine braking position, or does Prius do it automatically? My RAV4 hybrid has "S" mode which allows adjusting the level of engine braking; my understanding is that the Prius just has "D" and "B". I've never driven RAV4 down a hill that is so long it requires engine braking.
    – juhist
    Jun 1 '18 at 12:48
  • Prius has just "D" and "B" as you say. In this case, it's in D (but when the battery becomes full it acts the same as in B). If I put it in B at the top, then it engine-brakes the whole descent (and charges more slowly - taking most of the hill to fill the battery, instead of just half).
    – Toby Speight
    Jun 1 '18 at 12:52
  • Ok, that's a good clarification -- you got my upvote!
    – juhist
    Jun 1 '18 at 12:54
  • I now have a Plug-in version - its battery is big enough that it only fills to about ⅓ on the same descent. P.S. I edited to clarify - thanks for the comment!
    – Toby Speight
    Jun 1 '18 at 12:54
  • I have a Hyundai Ioniq which behaves like this too — on long descents, it will fill its battery, then turn the combustion engine on for engin compression. All this controlled by the brake pedal (unless I need more braking than can be provided in this manner) or by cruise control, and the brake lights don’t come on unless the car’s speed is dropping significantly enough.
    – Stephen Kitt
    May 7 '19 at 12:06
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You're missing part of how the system works. You have the electric motor and the gasoline engine. Both are used in conjunction to motivate the vehicle when needed (high acceleration). They assist each other. When the battery is full, the gasoline engine can shut down and allow the electric motor to do all of the work. When the battery gets down to a certain level, the gasoline engine can be restarted and help to motivate the vehicle. As the brake is pressed, the electric engine doubles as a generator, charging the batteries until full. If more braking is needed, there are regular rotors/calipers which do the job through friction. If the batteries are completely full during braking, there is some wasted energy ... but I understand this doesn't happen very frequently. Yes, downhill could be one of those situations.

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  • But how exactly the energy is wasted? AFAIK electricity generator can be damaged if the energy is not taken away.
    – Danubian Sailor
    Jun 25 '17 at 10:28
  • @DanubianSailor - The energy can be wasted through the normal braking process (ie: friction/heat). The system will protect itself from over charging. Once the batteries are full, the electric motor will stopped being used as a braking source and the regular brakes will do all the work.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jun 26 '17 at 12:57
  • A Prius doesn't ride the discs - it uses engine compression, and will only resort to friction brakes if you brake sharply (rather than maintaining steady speed downhill).
    – Toby Speight
    Jun 1 '18 at 11:13
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TL;DR: hybrids not only do regenerative braking, they also have an electric boost

Actually, the battery is never full. My understanding is that it is always kept between 40% and 80% of charge to prolong its lifetime. There is probably room for small amounts of regenerative braking when the battery is 80% full. Also, the battery is connected to two (or three in case of AWD) motor-generators. If the battery is close to 80% full, the motor-generators supplement the power of the internal combustion engine. If the battery is so full that the control computer does not want to put more energy to it, the system switches to using disk brakes. You can hear the switch to disk brakes when braking to a full stop. When fully stopped, you hear the brake pads move when the system switches from regenerative braking to disk brake pads.

Some Toyota hybrids have an engine braking position. On my 2016 Toyota RAV4 hybrid, it is actually a manual shifting position which has 6 emulated gears. When you select a lower gear, it means that there is more engine braking and also more acceleration if you need it suddenly. Handy for driving in mountainous regions and joining a freeway. Not so handy for driving down small hills, as this engine braking position does not do regenerative braking.

If you have driven a Toyota hybrid, you have noticed the "EV" light. It means the car can use only electricity to propel it when the power demand is low. For example, on my 2016 Toyota RAV4 hybrid, the battery can provide 42 hp, whereas the total combined output of the hybrid system is 197 hp. So, only 21% of the power can be provided by electricity alone. This means in practice that when you accelerate or drive at high speeds, only part of the energy is provided by electricity and part of the energy is provided by the internal combustion engine.

What is the idea of a hybrid, then? The idea is that gasoline engines work poorly at part load. Let's assume that the optimal energy efficiency of the internal combustion engine is obtained at 70 hp power demand (the electric CVT automatically selects the optimal RPM based on power demand). If you demand 50 hp, it cannot be provided by the battery (that is capable only below 42 hp). So, the engine is run at 70 hp, and the excess 70 hp - 50 hp = 20 hp goes to the battery (assuming the battery is not full). If you demand 30 hp, it is alternately provided by the battery and alternately the battery is charged at 70 hp - 30 hp = 40 hp. So, by alternately using the internal combustion engine and battery, the internal combustion engine is run at higher efficiency. Also, electric boost on demand allows using more efficient Atkinson cycle engines because the electric boost overcomes their drawbacks.

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  • Yes, I understand that principles, but the weak point is the limited battery capacity. So once the battery is full, electric engine is simply not used for braking?
    – Danubian Sailor
    Jun 25 '17 at 10:30
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    Well, the battery is never full, but when it is about 80% charged, the system smoothly switches to disk brake pads.
    – juhist
    Jun 25 '17 at 12:26
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When the battery is charged to its "fully charged" amount it will stop using regenerative braking. It will use the usual brakes like a non-hybrid. You can feel the difference.

Extra info: There may be a dashboard indication of it. The Nissan Leaf dashboard has this (not a hybrid though). Don't know of any Toyota, Honda, or BMW that show it. Reduced regenerative braking indicator. There are four circles to the left of the white dot in the upper left of the image. Three are single, one is double. This indicates that 1/4 of the regenerative braking capacity is available. When fully charged it will be zero and not do regenerative braking.

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  • That's just wrong - it uses engine compression, not the brakes. See my answer.
    – Toby Speight
    Jun 1 '18 at 11:06

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