Regen recharging

[DJWilson2]

I have a question about regen recharging, how much of a battery charge can you get going down a 30 degree grade for 5 miles and how much charge do you use going up the same 30 degree grade for 5 miles?

Is there a way to recharge the car going down so that you can go up the grade?

[ClarksonCote]

I recovered 60% of my battery charge going down the Mount Washington Auto Road, which was a pleasant surprise. The efficiency of regenerative braking is a function of how quickly you're decelerating. At and above 0.3g (where g is 9.8 m/s) the graph I saw said the regeneration is about 75% efficient, and it starts to drop off below that.

Conservation of mass and energy won't let you get enough recharging from braking regeneration to then go back up the same grade using only that energy. Efficiency losses will always prevent that.

[SharkVolt]

One of the GM engineers said you can reclaim roughly 50% of the power used to go up the hill when you go back down.

[bonaire]

30* hill is probably overstated. That is like a ski-slope angle. Are you thinking 30 percent slope? That is also a bit dangerous. They put up signs saying trucks should be careful on 10 percent slopes. No matter what the angle, regen braking is a good thing.

Regen was stated by Chevy to be about 50% of the energy retained. Knowing how much energy it takes to go up is a formula based on elevation change and the weight being elevated. A certain number of watts (units of work) are needed to raise the car but you cannot reclaim it all by going back down. If you did, you would break or come close to breaking the law of conservation of energy So, imagine if you recharge at the bottom and go up to the top, if used 6 kWh to go up and came back down, you probably will get back in the neighborhood of 3kWh or slightly more.

[ClarksonCote]

For reference, below is the graph I was referring to (taken from http://www.driveforinnovation.com/regenerative-braking). Actually, I think I misinterpreted the 75% number. This site claims the efficiency of the mechanical to electrical conversion is over 90%, but there are other factors that decrease the practical amount of energy that gets put back into the battery. The explanation on the above site is probably worth reading.



Also, I would keep in mind that there is additional regeneration available above and beyond what you experience in "L" -- to a point -- when you apply the brake. It's been suggested before that the disc brakes only begin to get applied when the spinning leaves on the efficiency ball disappear during deceleration.

[Rusty]

It's been suggested before that the disc brakes only begin to get applied when the spinning leaves on the efficiency ball disappear during deceleration.

I believe it was VA Trevor who said that. It's probably usually true on flat ground with typical braking. It's not anywhere near absolutely true. I've seen nearly 100% regenerative braking with the spinny ball nearly into the yellow (though it takes some brake pedal pressure control to do it). And I've seen 100% friction braking with a happily spinning ball right in the middle (going down a 16% slope in low coming to a stop at the base of the slope will do it repeatably).

If you want to maximize your AER it's best to keep the spinny ball happy. If you really want to know when the friction brakes are applying (and how much - the Volt really does a remarkable job of blending the two) the only ways I know of telling are to monitor the OBD bus or externally instrument the brake hardware.

[ClarksonCote]

I believe it was VA Trevor who said that. It's probably usually true on flat ground with typical braking. It's not anywhere near absolutely true. I've seen nearly 100% regenerative braking with the spinny ball nearly into the yellow (though it takes some brake pedal pressure control to do it). And I've seen 100% friction braking with a happily spinning ball right in the middle (going down a 16% slope in low coming to a stop at the base of the slope will do it repeatably).

If you want to maximize your AER it's best to keep the spinny ball happy. If you really want to know when the friction brakes are applying (and how much - the Volt really does a remarkable job of blending the two) the only ways I know of telling are to monitor the OBD bus or externally instrument the brake hardware.

That's interesting Rusty, thanks for the insight. I'm surprised that at times the ball can be near yellow and still be nearly 100% regenerative braking. That's comforting to me.

But to be clear on your second example with the 16% slope... The friction brakes were never engaged without applying the brake, right? That is, using "L" without any brake pressure ensures that the friction brakes are never used.

[Rusty]

But to be clear on your second example with the 16% slope... The friction brakes were never engaged without applying the brake, right? That is, using "L" without any brake pressure ensures that the friction brakes are never used.

Oh right. Absolutely. No friction brakes in L going down a 16% slope without touching the brakes. The car just speeds up constantly... For the 16% hill I test going down (25 MPH) "L" is not enough to hold speed. This is an un-upgraded MY '11. The MY '12s and software upgraded MY '11s may have a different amount of regen for that case. And the hints I've heard is "different" means "less", but I haven't tested that.

I've not seen any instance where the friction brakes deployed in "L", without pressure on the brake pedal.

[Roadburner440]

I can speak for MY12, and there is A LOT of regen braking in L... So much so that the seat belt has to hold me in place if I completely lift my foot off at 45 or 50mph.. So I do not think they tuned it down at all. Granted I have never driven a MY11. With a Prius as our other car I expected L to be like the B mode and not do much. I couldn't have been more wrong that is for sure, and scared the crap out of me the first time I did it.

[solar_dave]

I don't know for sure but a subjective test between a 2011 non upgraded and a 2012 seems the 2012 is less. I have in my neighborhood driven the 2011 for a few months and have established some point estimation on where I need to lift the throttle to get near zero mph at certain traffic lights. I have only had the 2012 for a day or so but those estimation points don't seem to get the same decrease in speed hence less regen.

Not definitive but it certainly feels that way.