Regen brake efficiency

We've had some spirited debate in the past about how efficient the regen brakes are in the LEAF. I got a chance to read a LEAF technician's workbook and came across a description of the braking system that showed a general chart of how the regen and friction brakes are coordinated. And it tells the prospective LEAF technicians: "During braking, the system can recover up to 39% of the vehicle's kinetic energy and use it to recharge the battery."



That's what Nissan was saying a couple of years ago when the workbook was written. It seems like a reasonable number to me now that I can watch Gids come and go as I go down and up the hills here. Regen never gives back even half of the energy I use to climb the same hills (I did about sixteen thousand feet of cumulative elevation change yesterday). Regen is better than losing all the kinetic energy to brake heat but one doesn't get all that much of it back into the battery.

And that's why the serious hypermilers here prefer to coast when practical, assuming that one isn't coasting anywhere near terminal velocity, which is a loser compared to regen.

FWIW.

Wow. That's a lot lower that I would have thought - can't imagine where it's all going. I can say from observation that it is not too difficult to avoid friction braking (no pressure in the brake calipers). If only up to 39% is going to the battery then this implies the inverter has to eat 61% of that energy. That's a LOT of heat! I wonder if this was a specific example since the graph in the workbook shows a lot of friction braking which is only representative of emergency braking for most Leaf drivers who are aware.

I think most of the 61% is friction brake loss. I've been surprised that the Leaf's brakes do seem to wear, unlike our Lexus hybrid.

DeaneG said:

I think most of the 61% is friction brake loss. I've been surprised that the Leaf's brakes do seem to wear, unlike our Lexus hybrid.

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I don't think so. I can confirm that caliper brake pressure goes back to zero shortly after applying gentle braking (and regen takes over). Unless you are pressing hard and/or suddenly, there is no more friction that when then foot is off the brake.

The generally accepted figure for two-wheeled EVs is 10% recovery from regen. In electric bicycles with regen it's more like 5%, which is why most of them don't have it. Hopefully that makes 39% seem more impressive!

LeftieBiker said:

The generally accepted figure for two-wheeled EVs is 10% recovery from regen. In electric bicycles with regen it's more like 5%, which is why most of them don't have it. Hopefully that makes 39% seem more impressive!

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That brings up TickTock's question again: where does the energy go? It has to appear as heat somewhere in the system and 90% or 95% lost to heat, as opposed to stored as chemical energy in the battery, seems like a lot.

"Up to 39%" of the vehicle's kinetic energy does seem slightly low to me. However, I seriously doubt that the inverter has to absorb any more than a fraction of the losses. Assuming the use of ECO or B mode and no friction braking, here's where I'd expect the losses to be:
1. Aerodynamic drag. As long as a vehicle is moving at a speed of significance, a portion of its kinetic energy is "lost to the wind". That's independent of whether the vehicle is going downhill. (When going downhill, potential energy is converted into kinetic energy, much of which is lost as described here before it can be converted to battery energy.)
2. Rolling resistance and friction within the drivetrain.
3. Battery charging losses.
4. Losses within the inverter, wiring, etc.

Stated differently, suppose one is traveling downhill at a constant speed and employing continuous regen braking in ECO or B mode. In calculating the amount of power or energy available for regen, one must first subtract the amount of power or energy required to keep the vehicle moving at that speed on flat ground. Examined this way, one might end up with a regen efficiency figure of 70% or more.

For what it's worth, if my state of charge is low enough and I descend slowly, I'm able to put as much as 15% in GIDs back into the pack on a 4800' descent over 14 miles. (The net descent from my home is greater, and adds two miles, but with significant up and down.) 15% times 21 kWh (rough usable energy in a new LEAF pack) is 3.15 kWh, or about half of the 6.24 kWh of the potential energy involved. (See http://www.mynissanleaf.com/viewtopic.php?f=24&t=309&p=6025" onclick="window.open(this.href);return false; for the calculation.) So my experience does seem to be a little better than 39%, at least in optimal conditions.

Now, suppose I subtract the energy needed to move the LEAF that 14 miles on flat ground. Assume a constant speed in the 30s (mph), slow enough to obtain 6.5 miles/kWh. That would equate to 2.15 kWh. 6.24 kWh (the potential energy) minus 2.15 kWh (aero/rolling/drivetrain losses) is 4.09 kWh. 3.15 kWh (15% GIDS) divided by 4.09 kWh yields an approximate regen efficiency figure of 77%. Again, this seems to be close to a best-case scenario.

dgpcolorado said:

Regen never gives back even half of the energy I use to climb the same hills

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Don't forget to take into account drag in your calculations. Even if regen could pack back in 100% of your braking energy (for example, when you're not touching the brake pedal), you still only recover a fraction of the energy used to go up the hill.

All that said, 39% does seem very low. Hasn't Ford bragged about recovering over 90% of the energy used under braking using regen? Nissan needs to get some help from Ford - that would significantly boost city economy.

Here's a few Ford references:
Ford Focus Electric gets a 95% braking regen efficiency
FORD REGEN BRAKING SYSTEM SAVES 100 MILLION GALLONS OF GAS

Look at it this way: electric resistence heating uses huge amounts of power to produce modest amounts of heat, right? That's why the heat pump was developed for the Leaf. What you are doing in regen mode is putting some power back into the battery, and using the rest to produce that same relatively modest amount of heat - from the battery, from the wiring, and from the motor/generator. If it weren't so diffuse they could capture it and use it to heat the car - in Winter, anyway. ;-)

39% number is low, and validates why the Fit EV does better than 2011/2012 LEAF in EPA cycle and in stop and go traffic, but has similar range to LEAF at sustained speeds on the highway. For comparison, Fit EV braking energy graph is mid article here;
Honda Reinvents Regenerative Braking for Fit EV

But I also remember hearing the regen recovery is supposed to be better on the 2013 LEAF. Certainly room to improve.

Mine certainly seems to do well enough. I don't even use the "B" mode, because I prefer to add extra regen when needed with the brake pedal. If it was doing poorly I'd be at least trying "B" mode...

drees said:

Don't forget to take into account drag in your calculations. Even if regen could pack back in 100% of your braking energy (for example, when you're not touching the brake pedal), you still only recover a fraction of the energy used to go up the hill.

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You and abasile are right of course. And I did think of that when mulling over the problem. My guess is that the workbook statement I quoted was referring to actual use of the brake pedal, as shown by the graph, as opposed to the efficiency of the regen system. Those are very different things. I threw the statement out for discussion and it has been interesting.

All that said, 39% does seem very low. Hasn't Ford bragged about recovering over 90% of the energy used under braking using regen? Nissan needs to get some help from Ford - that would significantly boost city economy.

Here's a few Ford references:
Ford Focus Electric gets a 95% braking regen efficiency
FORD REGEN BRAKING SYSTEM SAVES 100 MILLION GALLONS OF GAS

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This begs the question: what sort of efficiency is being discussed? Actual efficiency of the regen system (motor, inverter, and the like)? Or kinetic energy recovery, which includes the drag and friction losses you mentioned? I think that calculation about the Ford regen efficiency on a hill descent (and bear in mind the difficulty in measuring enegy levels in a battery accurately) is looking at a different sort of measure than Nissan was describing. But it does appear that the LEAF braking system has room for improvement, as you suggest. And as a number of MNL folks have complained about over the last couple of years.

I realized that a simple test can be performed using the trip meter I recently added to CANary (trip meter). Before traffic this morning, I found an unused flat stretch of road, came to a complete stop, reset the trip meter, accelerated to 60mph, noted the kWh used, decelerated back to a stop watching the brake pressure to ensure minimal friction used and noted the final kWh. Three runs .18, .19, and .19 kWh each to get to 60 and 0.11kWh after regen back to a stop for all three. Comes to about 42% energy recovered. This is energy into the battery so add some battery loss and that 39% sounds pretty close.

P.S. The .18 was measured going east and the two 0.19 were measured heading west on the same road so maybe a slight eastward gradient or breeze present.

TickTock said:

Before traffic this morning, I found an unused flat stretch of road, came to a complete stop, reset the trip meter, accelerated to 60mph, noted the kWh used, decelerated back to a stop watching the brake pressure to ensure minimal friction used and noted the final kWh. Three runs .18, .19, and .19 kWh each to get to 60 and 0.11kWh after regen back to a stop for all three. Comes to about 42% energy recovered. This is energy into the battery so add some battery loss and that 39% sounds pretty close.

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But doesn't that calculation assume that you would have used 0.00 kWh if you had traveled the same distance at a constant speed?

Ray

planet4ever said:

TickTock said:

Before traffic this morning, I found an unused flat Aaronstretch of road, came to a complete stop, reset the trip meter, accelerated to 60mph, noted the kWh used, decelerated back to a stop watching the brake pressure to ensure minimal friction used and noted the final kWh. Three runs .18, .19, and .19 kWh each to get to 60 and 0.11kWh after regen back to a stop for all three. Comes to about 42% energy recovered. This is energy into the battery so add some battery loss and that 39% sounds pretty close.

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But doesn't that calculation assume that you would have used 0.00 kWh if you had traveled the same distance at a constant speed?

Ray

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That's right. My measurement included frictional losses (wheel, air drag, etc) - I can only assume the 39% number must also include such losses. Perhaps their number starts assuming a similar conditions: 60mph starting and the maximum deceleration rate that doesn't engage friction brakes. Even so, since I include these losses during the acceleration, too, my actual starting kinetic energy is less than the .19kWh I measured so I am getting better than 42% even with the drag components included.

Quick sanity check. E=0.5*mv^2 so 0.5* (3375+165 lbs)/32*(60*5280/3600 fps)^2 = 428340 ft*lbs = 0.16kWh kinetic energy at 60 mph. That means only .03kWh lost to drag, resistive losses, etc. during acceleration. So the efficiency of recovering the kinetic energy for this test including all losses during the regen was closer to (0.19-0.11)/.16=50%.

TickTock said:

I realized that a simple test can be performed using the trip meter I recently added to CANary (trip meter).

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I would be very suspicious of using vehicle telemetry to calculate energy recovery.

How does the system know how much energy has been put back into the battery that will be accessible again in the future?

It might well be able to measure the power [thus integrate for energy] from the motor under regeneration, but this does not equate to energy 'recovered'. The system efficiency is also highly dependent on the recharging efficiency, which is variable dependent on regen-power and state of battery.

I suspect that using a lot of regen produces a highly distorted view of driving efficiency on the vehicle gauges, because those measure power in/out of the motor, not battery capacity used/recovered.

I believe that the Nissan figure is cumulative and includes every thing from aero drag, to motor/electronics, to battery losses, etc. Whereas, the Ford figure appears to be ONLY for the motor/electronics and ignores all other losses...

All that said, 39% does seem very low. Hasn't Ford bragged about recovering over 90% of the energy used under braking using regen? Nissan needs to get some help from Ford - that would significantly boost city economy.

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LeftieBiker said:

Mine certainly seems to do well enough. I don't even use the "B" mode, because I prefer to add extra regen when needed with the brake pedal. If it was doing poorly I'd be at least trying "B" mode...

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To get the best efficiency, I only use B mode after coasting to slow almost to a stop.

dgpcolorado said:

That brings up TickTock's question again: where does the energy go? It has to appear as heat somewhere in the system and 90% or 95% lost to heat, as opposed to stored as chemical energy in the battery, seems like a lot.

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Air friction on the car's body as it gradually slows from ~60 to 0...... just like a normal car gradually loses its Kinetic energy and slows. That's where most of the 60% loss happens
.