Downhill Dharging With Regenerative Braking

Downhill Dharging With Regenerative Braking

Just a little thought experiment for you guys.

What if you are at the top of a mountain with your Tesla and almost no battery. You can fill the trunk with rocks and also a trailer (if you have one) and then go downhill and charge the battery as much as you can with regenerative braking. You will charge a lot more than if you had no rocks or extra wieght in the car because of the extra potential energy from the rocks. When you are at the bottom of the mountain could you leave the rocks, go back uphill only with your car and empty trailer and repeat the process? It seems like a great way to get "free" energy since you are only limited by the amount of rocks you have available.

Sorry if my english is bad, it´s not my first language.

juanmacastillo96 | 29/05/2018

I meant Charging

lilbean | 29/05/2018

But it's so much easier to just drive downhill only.

SUN 2 DRV | 29/05/2018

Who's going to move the rocks back up hill for the next guy to use?

lilbean | 29/05/2018


Uncle Paul | 29/05/2018

There is actually a huge mining truck that works on this principal.

It starts at the bottom of the hill, and drives empty to the top. Loads up with tons of ore and recharges all the way to the bottom. Regenerative braking hands the descent, so the brakes are not needed all the way down.

That gives it enough juice to make the trip back up the mountain empty.

The savings in fuel and brakes are incredible. Air pollution is also dramatically reduced.

Perfect operating environment for a work truck.

For normal EV owner it will be lots more trouble than it is worth. So much easier to simply plug in your car than carrying rocks.

SCCRENDO | 29/05/2018

@OP. To complete your experiment perhaps you need to do the math and see whether you would indeed have enough energy to get back to the top of the hill. Remember regenerative braking is not 100% efficient. And I guess you would need to explain the purpose of driving up and down the hill. Unless it is to transport the rocks.

lilbean | 29/05/2018

I wouldn't have the energy to load the rocks. Not to mention, the rocks will jack up my trunk and scratch it. I may hurt my back. It will take too much time to load the rocks. Dumping is illegal in LA. There wouldn't be room for the rocks because of the windmill mount for my car and the hamster cages for the wheels are in the trunk and also the huge backup battery I also store in there. It just isn't feasible for me.

SCCRENDO | 29/05/2018

@lilbean. It looks like Tesla has attracted a unique crowd of owners and wannabee owners. Elon is a success. It looks like he could sell ice to Alaskans in winter.

lilbean | 30/05/2018


Rocky_H | 30/05/2018

Here's an article on that dump truck that @Uncle Paul was referring to. It carries a full load downhill, and then drives back uphill empty many times a day.

Uncle Paul | 30/05/2018

Thanks Rocky_H

Clever truck.

Tesla-David | 30/05/2018

I did not realize there was a double post of this topic. Here is what I posted at other link:

I can relate a good example of that, from our M3 yesterday. We drove uphill to the local Costco Store using 0.46 kWh of energy for the 1.45 mile trip, and used exactly 0 kWh of energy getting back home, coasting with regenerative charging. Regenerative charging works great on downhills, but those pesky uphill's and flats always use energy. We did not even have to add rocks to get the recharge. ;0)

reed_lewis | 30/05/2018

This is why Tesla is designing their own NAV system. They are going to make all trips go downhill! :)

science-isbetter | 31/05/2018

Juan Ma Castillo. About rocks, downhill and Tesla.

Warning....this has not been peer reviewed.

Let's say you put a ton (literally 2,000 pounds) of rocks in your Tesla. Let's say the vertical drop of your downhill is 1,000 feet (more than a very tall building). Then, assuming 100% efficiency, (ignoring rolling friction and other losses in the process...a very generous assumption) you will have generated 2,000,000 foot pounds of energy.

2,000,000 foot pounds of energy is 753 watt-hours On a good, warm day, people are getting about 250 watt hours per mile. Therefore, on a good, warm day, your downhill rock run will get you about three miles of energy.

But WAIT. Now you have to go back up the hill with an empty load. We can ignore the lifting energy used to go back up because we didn't count the weight of the car when calculating the energy gained in the downhill.

BUT...WAIT some more. You still have to go back up the hill. Let's say the hill has the maximum grade of a US Interstate highway. That is 7%. So, to go back up the 1,000 feet you need to travel 14,285 feet. Three miles is 15,840 feet.

Consequently, you have gained about 1,600 feet for your trouble. About a little less than a quarter mile. Remember, I am assuming 100 % I'm not counting the battery discharge of the horizontal component to go downhill on the 14,285 horizontal distance. That doesn't come for free.

Therefore, I don't recommend that you try it. Only people with rocks in their heads and not in their Tesla's trunk or frunk should try it. But not you.

WARNING...this has not been peer reviewed. I am open to comments.

Rocky_H | 31/05/2018

@science-is-better, Your units were killing me, because I think of foot pounds as a torque measurement, so I thought you had something really wrong in getting energy from it, but I did the calculation differently and also came up with 753 watt hours of energy.

PE = mgh
Potential energy = mass X acceleration of gravity X height
2,000 pounds = 907kg
1,000 feet = 305 meters
PE = (907kg) (9.8) (305)
PE = 2,711,023 Joules
2,711,023 Joules = 0.753 kWh

I thought it would have been more than that.

Rocky_H | 31/05/2018

Thinking through your units, it does make sense now. A change in energy is Work. Work = force times distance. And F = ma, so Force = the mass times acceleration of gravity. So those do work out to the same things as the mgh variables from the potential energy equation.

science-is_better | 31/05/2018

Thanks for the review Rocky! I'm sure you know...weight, unlike mass, is a force. And you're right...foot-pounds is also a unit of torque.

lilbean | 31/05/2018

I think it would be easier to stuff the car with people instead of rocks. They could lift themselves out and walk up the hill by themselves too. Wow, I'm so smart!

SCCRENDO | 31/05/2018

@lilbean. You would be useless. We need to load the car with obese people. And if they continue walking up the hill they would lose weight and then we could sign up another group. Thinking about it, what a great business idea.

lilbean | 31/05/2018

Hahaha! I'm crying! Workers comp premiums would be through the roof!

ragtopday | 31/05/2018

@ science-is_better

the correct unit is pound feet, not foot pound. Most people get it wrong

science-is_better | 01/06/2018

@ragtop. Correct. Foot-pound for energy and pound-feet for torque. As long as it's not pound sand, thank you.

sbeggs | 01/06/2018

Is this like Dharma?

lilbean | 01/06/2018

Haha, @sbeggs!
OP hasn’t dome badk to dhange the title. Maybe he doesn’t know that he dan.

Silver2K | 01/06/2018

Step 1:
at the top of the hill, eat a whole lot of bk burgers, fries and non-diet soda (2 for $6 bk burgers should save some money)

Step 2:
During descent, eat lots of sugar free gummy bears while regening to induce a diarrhea effect

Step 3:
Relieve self for about an hr

Step 4:
drive back to the top of the hill using less energy

Rocky_H | 01/06/2018

Wait a second. There can't be any difference in pound feet or foot pounds.

lilbean | 01/06/2018


Tarla's Driver | 01/06/2018

If you search, you can find the reports from going up and down Mount Washington in New Hampshire. I forget the exact numbers, but I seem to recall that they got back 50% of the miles that they lost going up the mountain. I think this was a RWD S, and AWD might get better regen--I think they still had to use the brakes a fair bit.

science-is_better | 01/06/2018

@Rocky. It's just a naming convention. There's a cute cartoon you can find on Put "batman foot pounds" in the search box.

SCCRENDO | 01/06/2018

@Tarla's Driver. I guess we all appreciate the benefit of regeneration. However my cynical question relates to the purpose of driving up and down the same hill losing a significant amount of miles each time.
@Silver. The binge and purge strategy will likely not cause significant wight change to impact the car's weight to a significant degree. We need to get the passengers to get out of the car and walk up. This will also help the passengers lose weight. The good news is that when they lose sufficient weight we can sign up the next group.

Rocky_H | 01/06/2018

@science-is_better, OK, I just read a couple of articles about it, and it seems to just come down to: "The fork on the outside should be used for the salad, but the fork on the inside should be used for the main course."

Yodrak. | 05/06/2018

""The fork on the outside should be used for the salad, but the fork on the inside should be used for the main course.""

Of course, you should always work from the outside in.

lilbean | 05/06/2018

But what if you use chopsticks?

Rocky_H | 05/06/2018

You can't put a lot of pound feet of torque on your chopsticks.

vp09 | 07/06/2018

That's a good article. But how do they feed the 10 megawatt hours per trip back into the Swiss grid?

vp09 | 07/06/2018

Sorry extra 10 kWh. 20 times a day. So 200 kWh produced per day from regen braking.

Remnant | 22/06/2018

@juanmacastillo96 (May 29, 2018)

<< I meant Charging >>

OK, then correct it in the title of the thread (only its author can do that).