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Comment by rnk | Hacker News Reader

Reducing weight is always useful alin improving efficiency and electric cars are really heavy compared to gas cars. But that's not going to make that much difference in range per batter efficiency. They should focus on the efficiency of the drivetrain, the cars aerodynamic properties. Tesla has a more efficient drive, train and better aerodynamics than most other vehicles, that leads to much better efficiency per battery power.

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I imagine that getting a heavier car up to speed in stop and go traffic would be more energy intensive. Also driving it uphill has to use up a lot more battery since it's so much heavier. The weight reduction should improve [1] tire longevity and reduce maintenance costs. Overall I think it could help with range and costs so it seems like a worthy thing to focus on.

[1] - https://arstechnica.com/cars/2022/12/heres-why-electric-vehi...

Regen breaking should in theory help mitigate that. A lot of that energy is recaptured when you slow down. Heavier vehicles have more energy to recapture, assuming the drive train is up to it.

Unless it's a steep grade you'll recoup a lot of that uphill energy going down hill. The energy used going up the hill is just stored potential kinetic energy.

Regen efficiency is nowhere near 100% though. You still lose a lot due to friction/conversion losses/etc.

You don't need to do regen to get that energy back. Your car gets significant air and rolling resistance going down the highway, its having to put in energy to keep the car going 40+mph. Using that stored kinetic energy of just going down the hill is extremely efficient so long as you're not having to do any braking, which unless you're rolling down a mountain you probably don't need to "ride the brakes". Hence my "unless its a steep grade".

The energy does not need to go back into the battery. It offsets the energy needed from the battery to keep the car going at speed.

Regen efficiency makes no difference in most hills, because you probably won't be using the regen for it.

Yeah weight also has all sorts of knock on effects on other components. Suspension, tires, brakes, frame all need to get beefed up for extra weight and/or wear quicker.

Reducing weight sure is the most effective way to improve efficiency.

The best Tesla model 3 uses 25 kWh / 100 miles and weighs two tons. A top of the line cargo bike like the Tern GSD is capable of carrying passengers and luggage using only ~1 kWh / 100 miles, because it weighs only 75 pounds.

Slightly reducing the weight of the battery pack in a full sized car is missing the forest for the trees when it comes to lowering transportation emissions. As long as we're supply constrained for lithium battery cells, those 50 kWh of batteries going into the Tesla would be better going into 50-100 e-cargo bikes that each take a car off the road.

It's not going as fast as the tesla. iirc the wind resistance goes up as the square of the speed (I need to find a ref for this, I read it all the time, I guess https://en.wikipedia.org/wiki/Drag_equation).

Anyway, you can't compare a bike going much slower than a vehicle. If we were riding bikes, at much lower speeds than 70, then no doubt we'd be getting vastly more distance for energy used. But that's a separate argument from the impact of weight on energy usage.

They’re both vehicles that can take you from point a to point b, so they can be compared.

That's true they they are both vehicles that move you, but a bike vs a car are in very different classes, unless your car only goes 30 mph in normal use, like in a congested city.

I'm surprised the article doesn't point out that a lighter battery (all else equal) makes a safer car.

Beyond the obvious stuff like reduced braking distance and improved handling, a lighter car means less energy to dissipate during a collision which allows safety products and features to work more effectively.

That's a great point, battery electric cars are heavier than gas cars and so hard to stop them. I can agree with that and I'm an EV proponent. Another thing that makes electric cars potentially more dangerous. They have way more instant acceleration AKA torque.

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