How much energy does regnerative breaking save?

Hi Shawn,

A quick reply - the Prius battery SOC is done by integrating VA over time. So comparing SOC to these numbers would be comparing the VA data over time to itself.

 

That's interesting. So the Prius SOC indication is not a state measurement at all? I'm surprised that works as slight errors would quickly get out of hand without some sort of corrective measure.

This makes one characterisitic of the SOC display very confusing: how can it drop two bars overnight? (As I've seen it do at times.) There is no VA activity occurrring so the integrated value the next day shouldn't have changed.

 

The SOC display also uses some direct measurements to bring the SOC calculation back in line. I don't know the details for the Toyota system, but the one on my boat does something similar. It integrates VA over time and computes a SOC. Recalibration is done during recharge. When the battery reaches the proper point during the recharge curve, the system assumes that it is fully charged and zeros out the VA calculation. This eliminates the long term accumulation of errors.

Since the Prius battery runs back and forth through a narrow range, it can't use the same recalibration points, but something similar is being done.

Tom

 

A NiCd and NiMH battery, during charge, suddenly begins to heat up (converts a large proportion of the charge current to heat) when it reaches 80% charge. I suspect Toyota engineers use that fact to "return to cal." the SOC system. The battery has both temp and voltage sensors inside, and the voltage drops when the temp increases (commonly reffered to as "negative delta charge indication").
The voltage across the battery is a poor indication of SOC, as it varies with temperature almost as much as by SOC. This, however, may be one cause of the drop in indicated SOC "the morning after". In other words, part of the calculation of SOC may indeed include voltage. I am dubious about this however, as the voltage is inversly proportional to temperature but proportional to SOC.
I think Hobbit has a more complete answer.

 

My opinion on this is it does not change overnight. The observations by people that it does, typically are by people unfamiliar with S1 battery usage. And 30 seconds into the drive they look down and see the SOC has tanked. Which is because they have been climbing a hill on battery, with the engine doing the warmup idle and providing little to none motive torque. Just 30 seconds of accelleration is enough to change battery SOC significantly.

 

No unfamiliarity here. I've seen that sucker drop two bars overnight essentially at start up.

It's especially obvious since it is in charge mode because the ICE is running right after start up.

 

Mine drops overnight.
I can arrive home with all but the top bar lit, that is a green battery but in the morning the battery is displaying all but 3 bars and that is before I move an inch.
I suspect there is some hysteresis in the battery. I suspect the charge reads higher when the battery has just received a charge as the voltage at the polls is higher but overnight the electrons dissipate across the plates more evenly. I know I don't know the correct terminology but I think you know what I mean.

 

Hi Shawn and Pat,

My engine wont kick on for about 10 seconds after power up. Its never starts up right at power up. A one-bar shift is possible just due to quantization variation. With no real change in the level. The last digit in digital meters is random. So, that is why the specs for such things always have a +1 digit in the tolerance.

Which is really why in the new Prius, if they were going to cheap out and go to a mono-chrome SOC display, they should have just dispensed with the bargraph altogether and just put in a 1 % resolution number display - but I digressed.

Observing at 10 seconds after startup after the battery has been used to start the engine, would not be a correct observation SOC. Two bars change is not possible due to the meter issue, and would indicate a real change.

Pat...,

This discussion started when I pointed out that the SOC meter is just tally'ing the Volts times the Amps going in and coming out of the battery, over time. And the SOC really is not a measurement of volts alone. Or even a measurement of volts corrected by battery temperature. So, Shawn indicated the 2 bar SOC overnight drop as a counter to that point. And I replied that the 2 bar drop is a falacy due to the S1 heavy usage of the battery, and if the observation at startup is not right at startup, the observation is wrong. As the battery is used to start the engine, and the battery is used as the main vehicle torque source for the first minute of operation (S1 mode).

Its possible that the car recalibrates using volts and temperature at startup, I guess. Which would then explain the drop.

But, again, the SOC state during vehicle operation is still just the integrated Volts times Amps. On the MYCANSCAN there is a summary of the integrated power from the battery and sent to the battery over the trip. And using the SOC as a double check of hybrid system efficiency is nothing more than comparing car computers' calculated results to numbers you calculate outside of the car's computer.

 

I'm talking instantly on boot up not after the engine starts. I doubt it takes a full bar from the battery to boot up but my battery will drop 2 bars over night.

 

+1 on what patsparks said.

I was experiencing a 1 bar overnight drop back when the overnight
temps were less than 40 degF.

I figured that it was the result of two things;
* the HV battery capacity and charge/discharge rates being reduced
as temps fall, (I'm not all that e-smart so this might not be correct) and
* the SOC display bars overlap. I thought that there was a chance that
the system was showing a high bar count on shut-down, but for the same
SOC numerical value, showing the lower bar count on start-up.

Had I been on-the-ball, I would have checked numerical SOC values on
the ScanGauge on shut-down and start-up. Hopefully I'll remember to do
so this autumn/winter.

 

Exactly, and I've seen this several times. I look at the battery state before I back out (before the engine is on, then I tend to reconfirm it as I start down the road when the engine is always on.) I don't know that I've ever seen a 3 bar change, but I know I've seen several 2 bar changes and they weren't "narrow" ones judging by how much charging it would do following that.

This is what is most likely occurring. It is developing a large residual error and resetting from what I can tell.

I wonder if there is anything similar occurring in cold weather when the battery SOC is showing green and still stubbornly charging the battery despite my attempts to stop it? It might be thinking it has put in a lot of VA...but at 20 F or so I suspect a substantial amount of that has gone into heating the battery, not charging it.

 

I am wondering how many Green Cars 50watthours = 1 gallon of gas?
I filled up my new 2010 P3 last Thursday for the first time. I had averaged 51.9 mpg on the tank from the dealer, I had done some hot rodding to see what she could do so I dropped it from 53mpg. I reset the trip and took a 36 mile drive up along the Delaware River and back home. I made use of hills, especially a very steep hill going into New Hope PA and had my battery charged to the top, then did EV mode through town. By the time I got home I had it at 56.7 mpg. I guess not that hard to do on the first gallon, it's easy to sway the average when there isn't that much milage.

The next day I drove from Philly to Harrisburg round trip 220 miles. I averaged 65 to 75 mph trying my best to find a sweet spot. My milage really dropped down to 48.5 mpg. I am not sure what I should expect? What is the sweet spot to set the Cruise Control on highways 65mph speed limit? I tried figuring out the best setting, lot of up and down hills.

Should I expect better milage after this gets broken in?

Sorry I now this is the Gen 2 area and I have a Gen 3, but I did a search for the Green Cars energy regenerated posting and found this thread.

 

On level road at about 65 mph, 50 watt*hours will give you about 1000 feet of EV driving bliss

The sweet spot for CC is the lowest speed above 25 mph you and everybody else on the road can tolerate.