As I understand it electric cars recharge themselves when you’re going down hill (as long as you’re using regenerative braking).

This made me wonder if in the following hypothetical situation you could get away with never having to recharge the car from the mains:

I live at the top of a big hill, and work at the bottom of it (say 500m elvation difference). My home water supply is fed from a spring, and is therefore free. I don’t need to use the boot of my electric car much, so can install a 200 litre tank in the bottom of the boot.

If I fill the tank in the boot with water at home every night and then let it drain away every day at work then the car will always weigh 200kg more when going downhill than going uphill.

Would the energy generated going downhill be sufficient to keep recharging the car so that I can keep doing this indefinitely without ever having to recharge from the mains?

if you do it right, you could even make money…

https://arstechnica.com/cars/2017/09/this-cement-quarry-dump-truck-will-be-the-worlds-biggest-electric-vehicle/

I think there’s only one way to find out…

Wonderful idea, but I doubt they’re even close to being that efficient, I’m sure someone who’s a Mathmagician can work out what energy is created by 200Kgs going down a slope or whatever.

I believe some funicular trains work by having two carriages connected by cables that are powered by the one at the top being filled with water from a stream or something, but can’t remember where.

They don’t generate an awful lot of charge in regeneration mode, perpetual motors aren’t here yet.

Would the energy generated going downhill be sufficient to keep recharging the car so that I can keep doing this indefinitely without ever having to recharge from the mains?

No.

IIRC the efficiency of energy regenerated is only about 25%.

The energy gained from lowering 200kg of water down a hill would be enough to raise 200kg back up the hill.  But with 25 efficiency, you’d only be able to raise 50kg back up the hill.  An electric car probably weighs at least 1000kg, so it won’t work.

Funicular railways work because the mass of water is WAY greater than the mass of the carriage and the people.

If only someone would invent a way of generating electricity by allowing water to be transported down a hill.

What a world that would be.

So, with a big enough trailer filled with water, it would work.

Sell spring water during your lunch hour.

Going to depend on regenerative and frictional losses I suppose.

You’re looking at adding 13% to the weight of a Leaf.

if you do it right, you could even make money…

“Some batteries start smoking, others burst into flames,” he said. “The crucial thing in this instance is to make sure the neighboring cells are not damaged by the fire and heat. Otherwise, there is the risk of a chain reaction.”

I’ll pass thanks.

IIRC the efficiency of energy regenerated is only about 25%.

It’s moved on a bit since the Prius days, modern electrical drive systems are between 40-60% efficient in regen depending on speed/charge strategy, battery chemistry, temperature, motor type and so on. Some of the stuff running on test benches is better, or at least, it hits the higher levels of efficiency at a wider speed range.

But still……

Would the energy generated going downhill be sufficient to keep recharging the car so that I can keep doing this indefinitely without ever having to recharge from the mains?

No.

It’s actually an issue in some places where people live at altitude then drive downhill to get to work. Battery is full fairly immediately after charging then driving away, therefore you can’t use the regen facilities. So to get a “normal” driving experience, rather than coasting (can be quite dangerous in some places), you have to brake all the way down the hill. Not a problem in the peak district, a major issue in say, Colorado….

“Some batteries start smoking, others burst into flames,” he said. “The crucial thing in this instance is to make sure the neighboring cells are not damaged by the fire and heat. Otherwise, there is the risk of a chain reaction.”

I’ll pass thanks.

You could use the water to keep it all cool in case there was a fire.

I’ll pass thanks.

Yup stick with petrol it’s much more stable.

So if my electric car was 40% efficient in regenerative braking that would mean I’d need it to weigh 2.5 times as much on the downhill leg as it did on the uphill leg?

So for a Leaf for example I’d need to carry 2,400kg of water for the downhill run. Don’t think that’s going to work!

It’s actually an issue in some places where people live at altitude then drive downhill to get to work. Battery is full fairly immediately after charging then driving away, therefore you can’t use the regen facilities. So to get a “normal” driving experience, rather than coasting (can be quite dangerous in some places), you have to brake all the way down the hill. Not a problem in the peak district, a major issue in say, Colorado….

My car connects the engine to the wheels when the battery gets full to provide some engine braking.  And of course it has a full set of normal brakes as I’m sure they all do.

So to get a “normal” driving experience, rather than coasting (can be quite dangerous in some places), you have to brake all the way down the hill. Not a problem in the peak district, a major issue in say, Colorado….

What are you on about? They have normal brakes it is not an issue in the slightest, it will engine brake regardless of charge

Only if its on a conveyor belt….

@drac –  no it won’t. At 100% charge my Leaf has no engine braking and it ramps up as charge dissapates.

Even my hybrid was a bit of a nightmare in the Alps when on long descents the battery would be full and there was zero engine braking or when there was some it felt like the engine was over revving. Mecahnical brakes alone on a big downhill at speed is a bit scary

I have an i3 and drive down a hill as soon as I set off for work. The regenerative braking only works once the batteries are below 95% (so you don’t overcharge), so my car does pretend regen braking when the battery is full, ie it applies the disc brakes to simulate the feel of regenerative.

One pedal regenerative braking is very good, I almost resent using the friction brakes on our other car now!

Drill a big mine shaft, install a pulley and attach a big long rope, one end goes on the bumper of your car, the other is connected to a weight thats 25kg more than your car.

put 50litres of water in the boot, this will pull you down hill.

empty water at work, return home, reconnect rope at bottom of hill, which will pull car up hill.

Or, attach a big long bungee cord to your car, roll down hill, stretching the cord as you go, then tie the end to a tree or something and go to work.

on the way home, stop at said tree, untie bungee cord and attach to car bumper, let the handbrake off and g ently coast back up hill, slowly recovering the energy you used in the morning.

you should try this, there’s no way it could go wrong eh?

My car connects the engine to the wheels when the battery gets full to provide some engine braking.  And of course it has a full set of normal brakes as I’m sure they all do.

No engine in an electric vehicle. Just motors. And the lack of engine drag torque, and a gear box (for downshifting), adds a shit load of heat to the brakes. Especially if you want it to behave like a normal car. By using things like pretend regen braking, As per CHBs example. Lots of dragging brakes is not good for them.

What are you on about?

Integrated system design for electric cars, why? Is it a subject you spend any time looking at? Or not?

Even my hybrid was a bit of a nightmare when on long descents the battery will be full and there was zero engine braking or when there was some it felt like the engine was over revving. Mecahnical brakes alone on a big downhill at speed is a bit scary

FIFM

Indeed, my hybrid has a “B” Brake mode for use when regen or engine braking is either an option or a requirement for topping up the battery. It’s extremely effective if the battery is less than 90% full, then you get a lot of regen/engine braking but once the battery is topped up it’s less effective and I have known the motor cooling fan come on to cool both batteries and electric motor.

The energy gained from lowering 200kg of water down a hill would be enough to raise 200kg back up the hill.  But with 25 efficiency, you’d only be able to raise 50kg back up the hill.  An electric car probably weighs at least 1000kg, so it won’t work.

Ah but you’re going downhill with (generously underestimating the mass of an EV) 1200kg and up with 1000, so by that metric you have enough energy to raise 240kg.

Or at 60% more, but still not enough – and that doesn’t consider the inefficiency of getting energy out of the battery into the wheels too, along with other inefficiencies (drag, rolling resistance etc).

So basically, it could be done if either your water tank (and regen, braking capability) was big enough, or your efficiency was high enough. What you wanna do is get a long rope and a pulley, and a mate who works your back shift…

Modern Permanent magnet driven EV’s with FoC inverters and Lithium batteries have a return trip efficiency of between 65 to 75% on average (end cases at very low or high load are less efficient).  That means you get back up to 75% of the energy used to accelerate the cars mass when you regen.  As a PM motor and FoC inverter are agnostic to current direction, they are able to drive or absorb without limitation as long as their is somewhere for the energy to go (ie the battery is not at 100% SoC).  If there is no where to go, the motor can act as a brake without regeneration, and the energy is lost as heat in the motor and inverter itself (there are obviously some thermal limitations to the power and duration for which this can be done)

Regarding the OPs original question, the best option would be to simply move house to the bottom of the hill, thereby saving energy, saving the planet, and reducing the amount of time you spend in a car……  (or ride to work and get REALLY fit on the way home 😉

It’s not perpetual, because when you get to the bottom of the hill, you’re done, no more regen to be had!

Modern EVs can regen as hard as they can accel, limited by battery SoC, vehicle dynamics and general driveability factors (most EVs don’t ever regen as hard as they could, because it feels odd to most drivers, if you come off the accel and effectively do a 1g deccel.  As drivers get used to EVs, then we are already seeing much more aggressive regen strategies and even “one pedal” driving capability (which Nissan have been trumpeting about on the new Leaf, but has been part of every i3 ever sold (since 2014…))

Our Kia soul does slightly better with the regen off. And better still on motorway runs. Not what I expected. The regen is only operative for a few moments.

It’s a nice package.

BMW have announcedrolled out wireless charging too. That would make it effortless if it worked properly.

Got the quick charger installed recently too.

I had one pedal driving on forklifts in the 80s, it was a safety feature in case the driver got bounced off due to a bump, it meant no runaway forklifts.

Modern EVs can regen as hard as they can accel,

Which isn’t hard enough to do an emergency stop.

No engine in an electric vehicle. Just motors.

My car’s a hybrid.  It has the B mode referred to above, but it does the same thing even in D if you get the battery full.

@p-jay. Linton or Lynmouth?

north Devon

If you have an electric or hybrid and live at the top of the hill, surely you’d tell it to stop charging at say 80 or 90%?

I think regen braking is a lot more common than we realise, lots of cars come with smart alternators that will charge the starter battery when slowing down. To do this, there needs to be spare capacity in the battery, so the computer only actively charges the battery to 12.4v (50% rather than the usual 100%) and the alternator will top it off to 15v whilst the car is decelerating.  The alternator can then freewheel during normal driving and therefore reduce drag on the engine.

Causes headaches for those with newer campervans as basic split charge systems cannot be relied upon to fully charge your leisure batteries, and you need specific battery to battery chargers that are compatible.

If you have an electric or hybrid and live at the top of the hill, surely you’d tell it to stop charging at say 80 or 90%?

You’d be amazed.

You’re all wrong.

Buy a 1000 litre bowser and fill it with spring water.

Attach said bowser to the towbar on your Subaru WRX/ Spec B or whatever.

Tow the bowser to work, sell spring water to morons, profit, free petrol money.

More efficient than fannying about with milk floats.

I believe some funicular trains work by having two carriages connected by cables that are powered by the one at the top being filled with water from a stream or something, but can’t remember where.

There’s certainly one at Lynmouth in Devon.

Funicular railways work because the mass of water is WAY greater than the mass of the carriage and the people

Not so. From Wikipedia, the Lynmouth Cliff Railway cars have a tank that holds 3182 litres, so that’s 3182 kg. They can carry 40 people, at (say) 75kg average, which is 3000kg. The mass of the car going up is the same as the one coming down, so ignore it. The mass difference is small, but it works because there’s no conversion of energy from electrical to mechanical and back, only potential to kinetic, and the only energy loss is friction in the pulleys and ropes and the wheel bearings of the cars.

There’s another in saltburn, in the north east, it’s fed by a stream.

–  (or ride to work and get REALLY fit on the way home

having encountered NewRetroTom, I can attest that he’s already at that stage..

– Callum