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Boomer23
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Re: Regen question

Fri Sep 02, 2011 7:43 pm

CJF wrote:Has anyone seen any posts about why Nissan has not (or could not) include a method to allow for "virtual downshifting" of the vehicle thus allowing a higher efficiency re-gen system to be activated by the driver when needing to slow down, as in approaching unexpected slower traffic conditions or while driving down a lengthy grade? Rather then riding the brake (lightly) to engage the re-gen, I would like to just downshift into a "virtual lower gear" to keep the speed down and increase the re-gen.
I routinely flip the drive mode into ECO to provide regen braking when I'm slowing downhill toward a stop light. I realize that this doesn't provide more regen than the brake pedal can, it is limited, as others have said. However, I enjoy using the shifter so that I don't have to feel whether I'm using mechanical braking or not. And it feels a bit like downshifting a manual trans for engine braking.
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smkettner
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Re: Regen question

Fri Sep 02, 2011 7:56 pm

If it is 30% efficient where does the 70% go? I am not talking combo stopping with the brakes but pure coasting with regen.
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CJF
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Re: Regen question

Fri Sep 02, 2011 7:58 pm

Boomer23 wrote:
CJF wrote:Has anyone seen any posts about why Nissan has not (or could not) include a method to allow for "virtual downshifting" of the vehicle thus allowing a higher efficiency re-gen system to be activated by the driver when needing to slow down, as in approaching unexpected slower traffic conditions or while driving down a lengthy grade? Rather then riding the brake (lightly) to engage the re-gen, I would like to just downshift into a "virtual lower gear" to keep the speed down and increase the re-gen.
I routinely flip the drive mode into ECO to provide regen braking when I'm slowing downhill toward a stop light. I realize that this doesn't provide more regen than the brake pedal can, it is limited, as others have said. However, I enjoy using the shifter so that I don't have to feel whether I'm using mechanical braking or not. And it feels a bit like downshifting a manual trans for engine braking.
I agree....I often use the Eco mode as a way of downshifting but it just does not provide much of a "drag" on the system or result in a significant increase in the re-generation of energy. I would like to see that when we shift into a "lower gear" it would produce a much higher retarding force by recapturing more energy. I'm just not smart enough to know what the "costs" involved would be to engineer something like this.
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Stoaty
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Re: Regen question

Fri Sep 02, 2011 8:02 pm

smkettner wrote:If it is 30% efficient where does the 70% go? I am not talking combo stopping with the brakes but pure coasting with regen.
Probably to heat, I would guess. I don't know that to be the correct figure, just felt sure I had read 30-50% in a couple of places, both here and elsewhere. If anyone has a reference, that would help settle the matter. By the way, Ingineer stated in another thread that the loss in one portion of the regen charging is I2R, such that regen at lower currents would be more efficient. That would mean that 20 KW regen loses 4 times as much as 10 KW, while 30 KW loses 9 times as much as 10 KW. If I have a chance I will dig up the link. Just ran across it earlier today.
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LEAFfan
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Re: Regen question

Fri Sep 02, 2011 8:03 pm

EVDRIVER wrote:You can't even get 80% in the LEAF. Remember the motor is capable of 80kw which means it is capable of 80kw of regen BUT it is software limited to about 30kw or so. If you drive say a Ebox it has as much regen capability as it does output, it is really amazing to feel 150kw of regen if desired and O wish the LEAF had more as I constantly watch energy being wasted to the brakes.
+1 on more regen! Do you believe that someone can 'fix' the regen and make it give us max instead of the meager 30kW? Is it just a software/firmware fix or something else?
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Re: Regen question

Fri Sep 02, 2011 8:52 pm

I found a thread on the GM Volt forum that estimates regen efficiency as a function of deceleration. Levels out at about 73%, and does better for higher levels of deceleration:

http://gm-volt.com/forum/showthread.php ... y-AER-Plot" onclick="window.open(this.href);return false;
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Re: Regen question

Fri Sep 02, 2011 9:29 pm

Stoaty wrote:Google search for the Prius put best guess for regen efficiency at 33-50%. 80% seems awfully high to me, it would be phenomenal if that is the actual answer.
It could be that high, it should be equally efficient accelerating or decelerating, but since the motor/gearing is optimized for a sweet spot around 48mph then once you slow down below that your regen efficiency will drop off, probably get quite low around 15-20mph.

The losses will show up as heat in the motor windings and the motor cooling system. Same thing happens in the Volt, GM even changes the virtual gearing to improve regen at slower speeds.

Best policy is always to drive as if you had no brakes at all.

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Re: Regen question

Fri Sep 02, 2011 9:36 pm

CJF wrote:Has anyone seen any posts about why Nissan has not (or could not) include a method to allow for "virtual downshifting" of the vehicle .
You control that with the brake pedal, just press the brake pedal a little harder until you reach the max regen.. if you press harder then the friction brakes engage. I bet you could fine tune it by feel easily.

Nissan could implement different levels of virtual "low gears", I might be useful to adjust the speed coming down a mountain but it may complicate the stupid shifter even more.

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planet4ever
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Re: Regen question

Fri Sep 02, 2011 11:07 pm

LEAFfan wrote:+1 on more regen! Do you believe that someone can 'fix' the regen and make it give us max instead of the meager 30kW? Is it just a software/firmware fix or something else?
I think you need to talk to some battery expert (which I am not). I will point out that Quick Charge will fill the battery from about 15% to 80% in about half an hour. If we assume a usable battery capacity of 21kWh, that's about 14kWh in 30 minutes, which is a 28kW rate. So our 30kW regen is already hitting the battery about as hard as QC does. Nissan seems to be concerned about doing too much QC, so the regen level might also be about as high as they feel comfortable having it. Of course most people don't regen for 30 minutes straight, but if you're dropping 5000' feet like abasile does routinely ...

Actually, a tougher test might be if you were stuck in Labor Day traffic getting home tonight, and you had 40 miles to travel at an average speed of 10 mph, where you were constantly speeding and slowing between 20 mph and 5 mph. Regen 25% of the time for 4 hours would be ...

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planet4ever
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Re: Regen question

Fri Sep 02, 2011 11:15 pm

Nekota wrote:The 80% efficiency is the product of the motor acting as a generator (about 90%) and the inverter box capturing the regen AC power back into DC to charge the battery (another 90% operation resulting in 0.9 x 0.9 = 0.81).
Ah, but you forgot half the story. You then have to convert the stored energy back into mechanical energy, perhaps at 81% efficiency again. 0.81 * 0.81 = ~0.66. I personally doubt the round-trip efficiency is much higher than 70%, though I could buy 85% in each direction, which would get it up to 72%.

Ray
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