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Imagine you drove your (pure) electric car to a very high mountain resort. It took 3 full battery charges to get up here, but by golly, you did it. And you plugged in, so battery is topped up now. Due to your altitude, your car also has a lot of potential energy.

Going home, you start down the mountain. Two-lane road, twisty, 25-45 mph, 8-20% grades, 5000' vertical drop. Continuous braking all the way down, you can't get going fast enough for aerodynamic drag to slow you appreciably. You are a mountain-skilled driver, and know better than to ride the friction brakes - they would burn up in the first 500 vertical feet!

Normally in a gas car, you do classic downshifting, and spin the engine faster to achieve braking action. The heat is blown out the radiator, and you can do this all day. What happens in an electric?

  • Regenerative braking is no use; the battery is full.
  • As discussed, friction brakes would fry very quickly.

Now, locomotives regenerate into big resistor banks, and they can do that all day. Are electric cars required to have an ability like that? Are they required to have enormous, actively-cooled friction brakes that can run continuously? Or do they simply ignore this scenario?

  • Just to be clear, you are talking a purely electric car like a Tesla or Chevy Bolt, and not a hybrid like a Prius, correct? – Pᴀᴜʟsᴛᴇʀ2 Sep 16 '17 at 10:17
  • @Pᴀᴜʟsᴛᴇʀ2 as it said "it took 3 full battery charges to get up there" I assumed it was 100% electric... – Solar Mike Sep 16 '17 at 10:22
  • @SolarMike - You know what happens when you assume something right? You make an ass/u/me. Hybrids have batteries, too. I'm just making sure. – Pᴀᴜʟsᴛᴇʀ2 Sep 16 '17 at 10:25
  • Yes and many hybids are designed to get the tax rebate while only adding acceleration and not being designed to be really "greener"... Also, there was no mention of engine braking so you must have been adding a lot of info into the original question that was not there... – Solar Mike Sep 16 '17 at 10:27
  • deleting my answer as it does not seem relevant... – Solar Mike Sep 16 '17 at 10:28
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For most electrics, no. They would need to use the usual friction brakes in your scenario. So once those burn up they are SOL. It would be possible to build an all-electric to use a dump load. But I don't know of any that actually do that. Maybe turn on all the lights and have the AC at max as a stand-in for a dump load. Then it would be able to regenerative brake that small amount. Still probably SOL.

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This is a really bad scenario for a purely electric car. Regenerative braking cannot be used to store the heat in the chemical energy of the battery, if the battery is already full. You need some component to dissipate the heat, and I don't think there's any component to do so other than the brakes.

My understanding is that most electric motors do not have active cooling, and therefore, are not up to the task of dissipating the power. Inverters, on the other hand, may have active cooling, but the cooling system's capacity probably isn't sized for dissipating all of the energy, only the waste heat in normal operation. You could perhaps dissipate some of the heat in the inverters, but I don't really believe this is a full solution to the problem.

This scenario would benefit in a major way from a plug-in hybrid electric vehicle that has an internal combustion engine that could dissipate the heat.

My 2016 Toyota RAV4 hybrid has an emulated manual gear change mode, where you can with emulated gear changes adjust the rotational speed of the internal combustion engine, and thus, adjust the amount of engine braking. This is intended mainly for mountainous regions where you may need the ability to engine-brake when going downhill.

  • So, not using the electric side to do the braking then and as a hybrid does not really answer the question. – Solar Mike Sep 16 '17 at 10:17
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Most pure electric cars have a battery large enough to drive at least 200 km. That's enough to climb a 20 km high mountain at a constant grade of 10%. So your hypothetical resort is 60 km up.

Anyway, this seems a rather theoretical scenario to me. On real-world roads, you have a few km of downhill followed by an uphill section. After the first uphill section, your battery has expended enough energy for regenerative braking on the next downhill section.
So on the first downhill section you would have to use the friction brakes. After that, you can use regenerative braking.

If you do get into a situation where the battery is full while you're going downhill:

On a Tesla (any model), a warning light appears on the display, because the driver needs to know that the feel of the brakes is going to be different.

Luckily, the Tesla Model S allows you to charge up to different levels, so just charge to 70% or 80%, whatever is needed to keep the regen active (I am not sure what charge level it cuts off, and I think it is gradual).

  • "Most pure electric cars have a battery large enough to drive at least 200 km." Errm... are you a traveler from the future? :D – AnoE Dec 11 '17 at 16:38
  • Every single Tesla sold has far more range than that. A 200-km range is far less theoretical than a 60 km-high mountain. – Hobbes Dec 11 '17 at 16:45
  • en.wikipedia.org/wiki/List_of_electric_cars_currently_available lists 43 models, 16 of which are advertised > 200 km. That's 37%, hardly "most". The OP has picked this scenario obviously to make a point. I certainly know routes in the alps that go constantly down for a long time. They usually do have off-ramps designed for trucks with failing brakes, but those are certainly not something you would want to use routinely with your car (although they would solve the problem, I guess... unless it's a scorching hot summer day and the cooling would just not happen...). – AnoE Dec 11 '17 at 17:08

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