Relationship of Regenerative Braking Intensity VS. Efficiency - What is best?

e) let off gas pedal just enough to glide, or use neutral.

 

Whenever possible I would use "a", ie: coast down to the lower speed without touching the brake. For that reason I tend to leave generous following distance to the cars ahead. I'm by no means sure this is the best option, but it seems to me any time you touch the brakes you're throwing away inertial impetus.

 

High current regen. can be cancelled by the car (due usually to battery heat, but also battery charge getting high). Low current regen. will not overheat the battery, but can also get the charge high. Overall, low current regen. is most likely to succeed in getting the most charge in the battery as it is less likely to cause the issues mentioned. The efficiency vs charge current is mostly the same, but the other factors can make a big difference.

DO keep in mind that, while better than NOT recharging by regen., the charge/discharge cycle of the battery is -at best- about 50% efficient overall. So avoiding it entirely is -more- efficient, as already mentioned. But, if it is required in driving, it is energy that other cars throw away, and it contributes to the overall efficiency of the Prius.

 

Why is that? I have heard that statement multiple times, but don't really understand the underlying causes of this inefficiency. I'd imagine the inverter is highly efficient (90%+) and modern motors are highly efficient (85%+). Why does over half the energy get wasted and not recovered?

 

Because there are so many stages of energy conversion getting energy from wheels to motor to inverter into the battery, then back out of battery to inverter to motor to wheels. There are losses at each conversion. Even if (to oversimplify) each stage is 90% efficient, that's .9×.9×.9×.9×.9×.9×.9=43%

 

the premise of this hypothetical question seems a little confused, in that the different options would use very different amounts of space, which is supposedly limited to some fixed distance. Aside from that, option "d" would put the most energy into the battery, IF the relevant components of the car were all equally efficient at every level of regeneration braking. The reason is that option "d" would theoretically minimize loss of energy to aerodynamic drag. The difference would be small, though. I'd favor option "a," in the real world, though, because it puts less stress on the components.

 

Batteries are not efficient when charging. At best, 80%, typically less than that. I hear the LiIon ones are better, but I have little experience with them. Now, it's difficult to measure, but when a NiMH battery is discharged it is also not efficient. It's better than when under charge, but only a little better.
All numbers I use are -approximate-. They are close, but only that.

So, you apply the brake lightly to charge the NiMH battery (the HSD does so to about 80% capacity, then stops - I'll explain why at the bottom) - you loose about 5% in the MG, another 5% in the inverter and 20% in the battery. Remember, these numbers are cumulative. That is, you charge the battery with only 90.25% of the energy and only retain, in the battery, 80% of that, or 72.2% of the energy you started with.

At the next light or uphill or whatever, the HSD draws energy from the battery to power the car. It only gets out about 85%, the rest is lost to heat. This then goes into the inverter (to convert the approx. 200V to approx. 350V three phase) and you end up with a further loss of 5%, then this goes to the MG which also looses about 5%. Result, you get 76.7% of the energy from the battery to the wheels.

Overall, you get 55.4% of the energy you started with (multiply the 72.2% in by the 76.7% out). I'm overusing the decimal point here, (too may significant figures ), but whatever. There are other losses in the cables (battery in rear, MG up front) and connections.
So we "round off" to 50%. Temperature will also have an effect (or is it an affect).

So, why does the HSD operate the NiMH battery over such a limited capacity range? (it's used from about 30% to about 80%).

First, when charging, a NiMH battery is moderately efficient, -UNTIL- you reach 80% charge. It then converts a lot of the charging current to heat. This has been used for years to detect the 80% charge point and switch to "trickle charge" in uses such as hand-held two way radios. This effect is probably NOT the main reason however.

Remember, the "traction battery" is a series connection of 168 NiMH cells, each one slightly different, even though they are "matched". So, when you discharge the battery, -one- cell will have the lowest capacity of the bunch, and if you continued to discharge to get close to full discharge that one cell would run out before the others. Because they are series connected, that one cell would be "reverse charged" by the current from the other cells. This is -death- for NiMH cells! So we avoid it by stopping at 30%!

When you charge the traction battery and try to get to 100%, that one cell will end up being overcharged. It will convert that charge current totally to heat, overheat, and vent (don't let the battery manufacturers fool you, NO cell is -ever- fully "sealed". If they didn't have a vent of some kind they would burst)! This venting will cause that cell to loose electrolyte (or water if you are lucky), but either way loss of electrolyte/water will drop it's capacity some more. You end up with a battery with a short life! So we avoid that by limiting the charge to 80% capacity.

Sorry, it's a long explanation, rather technical, and not totally accurate, but close enough for our use here.

 

The battery is probably happier and more efficient when the charge current is lowest, as in your option "a." The wiring definitely is. I'm not sure about the other relevant components.

What's more important is that option "a" provides the most distance, if you have space to use it safely, therefore is most efficient overall, even if it puts slightly less energy into the battery. It's better to spend your valuable kinetic energy on keeping the wheels turning than on charging the battery.

 

Many years ago here, it was cited that either C or D would be your best option; someone apparently tested it out on a GenII Prius, and found that regen had some amount of fixed energy cost, plus efficiency peaked at higher amounts of regen.

 

If what you say is true, then what is to be gained by the "pulse and coast" scenario that is so frequently promoted in these forums? Wouldn't the most efficient strategy be to maintain a steady throttle--at least, while driving in steadily moving traffic?

 

Not necessarily, because the "pulse and coast scenario" done optimally does not involve regenerative braking and the associated energy conversion losses. During the coasting phase, spending stored kinetic (or potential) energy directly on keeping the wheels turning is a lot more efficient than spending it on charging the battery, then withdrawing from the battery to turn the wheels---conditions permitting, of course.

That said, the "pulse and coast scenario" works best at fairly low speeds when there's no traffic to deal with, or when you can take advantage of hills. On flats in the "steadily moving traffic" you mentioned, holding the accelerator pedal steady with engine running continuously is likely to be more practical, safer---and more efficient too if the speed is highish.

 

CR94--Thanks for your reply. I can see where I was confused about just how to benefit from "pulse and coast". I now understand that the goal is to keep the battery out of the equation as much as possible.

 

David Beale "charging efficiency of nickel metal hydride batteries is typically 66%".
NiMH battery technology, how to charge Nickel Metal Hydride
Batteries tutorial for design engineers, as well as NiMH chargers.
Nickel–metal hydride battery - Wikipedia, the free encyclopedia

I got some reading time for you KyleSTL. If you're interested of course.
Simplified efficiency problems:
http://windstreampower.com/wordpress/wp-content/uploads/2013/01/EFFICIENCY.pdf

This might be interesting for everyone.
Extensive 2010 Prius motor test:
https://info.ornl.gov/sites/publications/files/Pub26762.pdf
Page 68 (Voltage generation), 71 (Summary).
In short "Motor efficiency measurements reached 96% are above 90% for a great portion of the operation range".

 

"...will also have an effect (or is it an affect)."
The temperature will only have an effect on the results unless it gets angry with you for some completely incomprehensible reason, or maybe the temperature gets hot and bothered because it's too sunny that day, or maybe it gives you the cold shoulder because it's winter and you don't have the car's heat on. Oh, wait, we're not talking about my DW, are we?

 

Isn't the Hybrid System supposed to stop charging when the battery reaches 80% or even 60%? And I don't mean 80% of the 8 bar meter, I mean the overall capacity.
I think when the 8 bar meter shows full on the dash in reality the battery is at 80% or 60%.

 

Yes, but he is not claiming the hybrid system will charge to 100%. He is explaining what would happen if it would be charged to 100%.