Volusiano
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Regen question

Fri Sep 02, 2011 3:51 pm

I have a couple of questions about regen.

1. I've heard a few different numbers about regen efficiency. Some poster said you can only get back 30% of the power from regen. Others mention 80% efficiency. Which is it? And if it's only at 30%, why is there so much loss?

2. I understand that regen works by operating the motor in reverse of the forward. So basically in forward mode, you pass current through the coil, generating a magnetic field which causes the motor to turn one way, moving the forward gear. In regen, you use the momentum to "somehow" reverse the motor direction in order to revert the magnetic field in order to induce current back into the battery.

The "somehow" is the fuzzy part I don't understand. Exactly how is it done? How do you transfer the momentum of the car into a gear mechanism that can cause the motor to spin the opposite direction as soon as you let go of the forward pedal?

Do you use the same gear mechanism for forward motion and regen motion? It seems like you can't use the same gear for both because the switching of direction has to be instantaneous. So if you use 2 sets of gears, one for forward and one for regen, do they drive 2 different motors then (one for forward and one for regen)? Surely 1 motor can't instantaneously switch direction back and forth between forward and regen.

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davewill
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Re: Regen question

Fri Sep 02, 2011 3:59 pm

I'm no expert, but I know that motor doesn't have to "spin the opposite direction" in order to do regen. It is all about how the motor is controlled.

BTW, reverse in the LEAF is accomplished by running the motor backwards which propels the car backwards. There is no transmission in the LEAF, just a fixed gear ratio.
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EVDRIVER
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Re: Regen question

Fri Sep 02, 2011 4:06 pm

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.

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Nekota
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Re: Regen question

Fri Sep 02, 2011 4:18 pm

The LEAF regeneration or braking power has a max of 30KW so that is probably the source of your 30 number as compared to the max forward acceleration power of 80KW. 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). The basic physics for regeneration comes from Lentz's law - google 'lentz law wiki' for details. Hope that helps.

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Re: Regen question

Fri Sep 02, 2011 4:24 pm

Yep, the motor continues to spin the same direction. The direction of current flow, however, reverses. Current always flows from higher voltage to lower voltage. When charging a battery, the charger has to put out a higher voltage than the battery's voltage to get current to flow into the battery. The inverter is where this magic happens when the car is moving. You can think of it almost as having a continuously variable voltage conversion ratio between motor and battery. To provide power to the motor, it must adjust the ratio so that to motor will draw the desired amount of current from the battery. To regenerate it adjusts the ratio so the battery will draw current from the motor (acting as a generator). This is not really so much different from down-shifting the transmission in a car to get a bit of engine braking effect when going down a hill, except that an ICE engine just wastes all the braking it's doing as more heat exhausted through the radiator, whereas the inverter puts the energy (minus the conversion and charging loss which is exhausted as heat from the LEAF') back into the battery.

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Re: Regen question

Fri Sep 02, 2011 4:50 pm

I use the three part energy read-out to see how much I am generating in the "regen" mode. By anticipating stop lights and gently using the brake pedal, I am able to see when the 30 kW limit is reached. I adjust my stopping distance so I can keep the regen near, but not above 30 kW. This allows maximum regen without using the brake pads.
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Volusiano
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Re: Regen question

Fri Sep 02, 2011 4:52 pm

Nekota wrote:The LEAF regeneration or braking power has a max of 30KW so that is probably the source of your 30 number as compared to the max forward acceleration power of 80KW. 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). The basic physics for regeneration comes from Lentz's law - google 'lentz law wiki' for details. Hope that helps.

Remember Clarke's 3rd law -- "Any sufficiently advanced technology is indistinguishable from magic."
OK, this makes more sense to me. You don't just say the regen is 30% efficient just because it has a max regen power of 30KW. Those are 2 different things. To me, the efficiency of the regen is how well it can convert the kinetic energy of the car back into the battery energy. I doubt that it can only convert 30% of the kinetic energy back. The 90% efficiency of the motor as the generator and 90% of the inverter box is a much more plausible explanation of 80% regen efficiency to me. Thanks.

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Re: Regen question

Fri Sep 02, 2011 5:10 pm

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.
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Re: Regen question

Fri Sep 02, 2011 5:20 pm

As I mentioned in a couple of other threads, you have to subtract aerodynamic and friction losses (which are unavoidable and do not count as drivetrain losses) before arriving at a figure for the amount of kinetic energy available to be captured via regen. If you do this, then it is not unreasonable to expect 80% efficiency. Coming down 5000' from my home in the San Bernardino Mountains, I think I am getting close to this, but it is hard to get a precise number without fine-grained SOC information.
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CJF
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Re: Regen question

Fri Sep 02, 2011 5:41 pm

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.
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