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0 pointsSomeone13y ago0 comments

Regenerative braking should not use the brake pads.

More importantly: battery chemistry is weird and differs widely between technologies (for example, should you completely empty lithium ion batteries every now and then, or is that disastrous for the batteries? How is that for NiMh batteries? Does the memory effect exist? Etc.).

That overnight loss likely was (mostly) not a loss at all, but due to the difference in mileage you can get out of a warm vs a cold battery pack, or maybe even out of one that had recently seen small charge cycles due to regenerative braking vs one that hadn't (IMO unlikely, but as I said, battery chemistry is weird)

If Tesla told me that regenerative braking would improve available range in this situation, I think I would take them for their word.

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lutusp13y ago

> If Tesla told me that regenerative braking would improve available range in this situation, I think I would take them for their word.

Think about what you're saying.

Thesis: periodically pressing on the brake pedal and allowing some of the car's energy to be captured by flywheel generation actually improves battery life and vehicle range compared to simply driving along at the same average speed. True or false?

In order for the above to be true, and given that acceleration takes battery energy and regenerative deceleration delivers battery energy, to argue that pressing on the brakes improves battery life is to argue that braking produces more energy for the battery than acceleration requires from it -- in other words, that the car is a perpetual motion machine, free of all natural constraints and scientific laws.

But the second law of thermodynamics -- http://en.wikipedia.org/wiki/Second_law_of_thermodynamics -- says one cannot get more energy out of a system than you have put into it, in fact, you always get back less than you put in.

Beyond the above-quoted thermodynamic law, there is the energy conservation law, to wit: energy cannot be created or destroyed, only changed in form (http://en.wikipedia.org/wiki/Law_of_energy_conservation). Therefore it is impossible for the car to deliver more energy by decelerating than it acquired by accelerating.

Therefore if someone at Tesla actually offered the advice that stop-and-go driving actually increases battery life and vehicle range, that person needs remedial physics education before being allowed to speak to customers again.

SomeoneOP13y ago

I don't think I said that; I certainly did not intend to.

Let me retry: the amount of energy you can get out of a battery will drop slowly, but we can ignore that.

Because of the properties of the battery, the amount of energy you can get out in a form that can drive the car may be a lot lower (extreme example: 1W of power may not be enough to even drive the electronics that control the starting of the engine. If so, a 1GWh battery that is full but delivers at most 1W will not take the car anywhere)

Secondly, the amount of power that you can get out and use to drive the car will depend on the environment (temperature, in particular) and, likely, on previous charge/discharge cycles. I know I am not an expert on this, but I know this isn't simple. http://en.wikipedia.org/wiki/Lithium-ion_battery#Conditionin... shows I am not alone in that. After scrapping the 'may's, 'belief's and 'debate's, little information, if any, is left.

Also, I understand the battery control software has safeguarding against full decharging, as that would be a costly affair. That software may have quirks. Given the age of the car's design and the number of hours all cars combined have been on the road, I think it is a safe bet that it does have quirks, especially for uncommon scenarios.

If the battery had more power than the software estimated, and that that power would be able to drive the car, once the battery warmed up, it all was a matter of convincing the software about that.

If the car's manufacturer suggests multiple small recharge-charge cycles (using regenerative braking) to do that, why would I distrust them? I know almost nothing about the chemistry of the batteries, and even less about their control software.

lutusp13y ago

> If the car's manufacturer suggests multiple small recharge-charge cycles (using regenerative braking) to do that, why would I distrust them?

The answer is simple -- they're wrong. Regenerative braking cannot recover more energy than was lost in getting the car to its present velocity, so the advice to engage in stop-and-go driving emanates from someone who doesn't understand physics.

Phase 1: Acceleration -- energy is provided by the battery to the car's electric motors. The battery energy required is greater than or equal to the car's final velocity as shown in m * v^2 / 2 (http://en.wikipedia.org/wiki/Kinetic_energy).

Phase 2: Braking (deceleration) -- energy may be recovered from the act of braking and delivered to the battery. There are two choices -- either turn the car's energy into heat with brake pads, or turn some of it into electrical energy that flows back into the battery using a method called "regenerative braking".

Can the energy recovered by Phase 2 equal the energy expended in Phase 1? No, this is not possible -- because of the second law of thermodynamics, one cannot recover all the energy, there are inevitable losses.

Therefore (read carefully) stop-and-go driving is always less efficient than driving at a fixed speed. Always.

> I know almost nothing about the chemistry of the batteries, and even less about their control software.

I'm not addressing what you may or may not know, only what the facts are. And if someone at Tesla actually offered the advice to engage in stop-and-go driving in order to increase the car's range, someone needs to go back to school.

Natsu13y ago

I would have thought the problems with the notion that you could recharge your car by driving it funny to be obvious: that would make it into a perpetual motion machine. Such things do not exist, nor can they.

You lack knowledge of physics, not chemistry. The implications of entropy are what you do not seem to realize. These things are independent of the car's inner workings.

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lutusp13y ago

> If Tesla told me that regenerative braking would improve available range in this situation, I think I would take them for their word.

Think about what you're saying.

SomeoneOP13y ago

I don't think I said that; I certainly did not intend to.

Let me retry: the amount of energy you can get out of a battery will drop slowly, but we can ignore that.

If the battery had more power than the software estimated, and that that power would be able to drive the car, once the battery warmed up, it all was a matter of convincing the software about that.

lutusp13y ago

> If the car's manufacturer suggests multiple small recharge-charge cycles (using regenerative braking) to do that, why would I distrust them?

Therefore (read carefully) stop-and-go driving is always less efficient than driving at a fixed speed. Always.

> I know almost nothing about the chemistry of the batteries, and even less about their control software.

Natsu13y ago

You lack knowledge of physics, not chemistry. The implications of entropy are what you do not seem to realize. These things are independent of the car's inner workings.

j / k navigate · click thread line to collapse