Calculating ICE vs Electric Miles

You're aren't accounting for the use of the traction battery while in hybrid mode to reach the 50mpg. The actual efficiency of the ICE is lower than that. It can get complicated because it will use the electric motor to assist even when the ICE is running.

There are a couple of drive record screens on the MFD with a EV % but I'm not exactly sure how accurate they are or if they roll over after a certain number of miles

 

Sounds like the EV % display is a carry over from the Prius, where it is more of an ICE off %. In the Prime, it is operation from the grid charge plus the times the battery was used while in hybrid mode.

 

The individual trip odometers show EV percent, so if you just set Trip A and never reset it, it'll cumulate your EV percent. I don't think there's any way for you to retrieve a vehicle-lifetime record of that,short of some fancy scan tool that can read the car's memory.

Getting the information is a two-step, two-hand process.

This is for the base trim level. Might be different for higher trim levels?

You have to pull up the Drive Monitor screen on the MID, using the controls on the right side of the steering wheel.. It'll show %EV miles on your last trip (car on to car off), which is pretty close to useless.

BUT, if you then hit "trip" on the left side of the steering wheel, you can see that same information for the Trip A or Trip B odometers.

You have to bring up that trip odometer, on the other display, to read the total mileage this EV% figure pertains to.

At one point, I considered putting a simple induction hour meter on the ICE, just so I'd have some clue as to total run time. I figure this is good enough. Once I figured this out, I just never reset Trip A.

As long as your driving habits are consistent, once you have enough miles on Trip A, you'll have a pretty good estimate of your overall EV percent.

 

pip recorded ev only when it was pure wall charge.

prime changed that to anytime engine is off.

 

Ah, I stand corrected. I should have tested that myself, to see that I cumulate EV% in HV mode. It just never occurred to me that Toyota would do it that way.

So (1 - EV%) per Toyota's display is more-or-less what I'd have gotten off an induction hour meter on the ICE. It's just total ICE miles, instead of total ICE hours.

What I actually want is total HV miles, which by subtraction would give me total EV miles. Which Toyota does not display now.

Looks like there's no way to recover actual wall-powered EV percent from what Toyota reports.

Thanks for setting me straight on that.

 

Just make sure that when the trip flips, that the ratio screen doesn't. Or keep detailed records.

 

Wait. Algebra to the rescue.

What I want to know is percent of miles spent in HV. Call that HV%. (So, 1 - HV% is the true EV mode percent of miles.)

What I know from the display is percent of total miles spent on battery. Call that Batt%.

All I need is a good, solid estimate of the fraction of HV-mode miles that show up in the overall battery % figure.

Let me take my Trip B odometer, reset it, and go on a long, "typical" HV trip. From that, I'll get an estimate of the fraction of HV miles that show up as part of Batt%. Call that my HV_Batt%

Yeah, with that, I've got one equation and one unknown:

Total Batt% (per display) = (1 - HV%)*100% + HV%*HV_Batt% (estimated from long typical HV trip).

The things in bold are known or estimated quantities. The only unknown is HV%.

Numerical example. Suppose my Trip A tells me that 85% of my miles are on battery. Suppose my Trip B tells me that, typically, 50% of my HV miles are on battery. Then:

0.85 = (1 - HV%) + HV%*.5

Solving:
-.15 = -.5HV%
0.30 = HV%

Under those conditions, I can infer that 30% of my miles were spent in EV mode.

As long as you can get a solid estimate of the fraction of your HV driving that is actually on battery, you can back-solve for your true wall-powered EV%.

Not perfect, but better than nothing.

I think that, from now on, I'm going to reset Trip B whenever I'm in HV mode, and record the Toyota-reported EV% (what I called battery % above). Record the trip mileage and EV%. And with enough such trips, I ought to end up with a solid estimate of typical reported EV% of my HV driving.

Not clear to me that, in practice, this is going to be any more accurate that back-solving from the gas mileage, as was done at the start of this thread. But it at least gives you an alternative way to calculate it.

 

I'm on a tier rate electric bill. I've set the car to charge after midnight and get discount credit for any significant energy usage between midnight to 6 AM. I take the KwHr credit and assume it's mostly EV recharge and split the bill; that portion of the bill is charged to auto:fuel - gasoline charges also gets charged to the same account. My other cars auto:fuel is tallied in another sub-account. This way; I get a fairly accurate accounting of actual fuel usage (both gas & electricity). For my driving style and commute, I've calculated a gas vs electricity break-even at about $0.43/KwHr @ $3/gal. gasoline.
The only other way is to place a KwHr meter on your charging outlet.

 

Yeah, I can do math. I'm crap at explaining math. So no surprise there.

My question back to you is, where does the 0.44 come from?

 

Fwiw, that is exactly what I calc for break even KWH rate here in Virginia.

https://savemaple.org/2023/02/07/1705/

 

Got it. I think some of the comments were led astray by calling those ICE miles. If you substitute the term "hybrid mode", or "HV" miles, then I think that should be clear and make sense to everybody. People were interpreting your statement to mean something like "when the gas engine is literally running", instead of, when the car is in HV mode.

(Maybe, to be fully accurate, HV mode with EV battery portion depleted? I'm a little hazy on the extent to which the car will continue to draw from the EV portion of the battery, in HV mode. Seems like it doesn't, to me, but I've never much paid attention.)

Anyway, that uncertainty aside, this makes perfect sense to me. If you get 50 MPH in HV mode, and your overall MPG is ~89, then 44% of your miles must have been in EV mode.

FWIW, when I do your calculation, with data for my car, I get exactly what I expect. Back-of-the-envelope, I think about 70% of my overall miles are in EV mode. My odometer shows 213.3 MPG, and I think I get about 60 MPG, on average, in HV mode. When I crank that through your calculation, I get just over a 70/30 split, EV/HV miles.

Consider that a second data point to confirm that your calculation is telling you what you think it's telling you. As long as your 50 MPG in HV is accurate.

Separately -- and I think it doesn't much matter -- there's an odd minor issue, in that Prius Prime wall-socket-driven miles might actually a bit larger than EV-mode miles, because when you charge, you charge the HV portion of the battery to full, which you could then consume (in part) as you drive in HV mode. Not clear to me the extent that the Prime will do that.

 

Those comments are directed at the use of EV % reported by the car. That isn't a percent between EV and HV mode. It was ported over from the Prius, where it reporting EV use. For a hybrid without a plug, that works out to whenever the engine is off, though it could be when the motor is applying power to the wheels. Either way, it appears what that reported ratio is was not changed for the Prime, so it is basically useless for figuring out many of your miles were powered by grid charge, and which by gasoline.

It's one big battery. The system is monitoring things like battery temperature, state of charge, and rate of draw from it to determine if it needs to turn of the engine. All those variables means there isn't a hard set point in the battery's charge level at which the switch from EV to HV happens. There's a soft point, but some days it will happen sooner, and others later.

Then things like weather and route will influence the level of charge left in the battery when the car is turned off at the end of the day. With the engine running more in the winter, my Prius had a higher SOC when I got home than in the summer. So the HV 'portion' could be fully charged by the time the car is plugged in.

As you said, tracking the exact miles from the grid charge is a minor issue. The effort required to figure it out isn't worth it for most. It's like filling the tank. Without extra effort, the tank isn't filled to the exact same point every time you get gas. Sometimes the fill stops short, or more gets pumped in. Over time, the fills will average out for the majorities' purposes.

 

I calculate mine a little differently. I bought mine new March 30, 2017. Since then I have a small book kept in the door panel where I write down the amount of gas and date for every fill up. I have added gas 28 times in the last 6 years for a total of 220 gallons. At an average cost of $3.50 per gallon that is $770. for gasoline. I roughly average 50 mpg, so 220 gallons of gas equals 11,000 miles. My total mileage is 25,500, so I assume the remaining 14,500 miles is on the traction battery. Between Summer and Winter I can go an average of 24 miles on a full charge. At 13 cents per KWH it costs roughly 80 cents to charge the car at home, which where It has always charged. 14,500 miles divided by 24 miles means I have charged it 604 times. 604 times eighty cents equals $483. for electricity. Looking at those numbers, it costs me $770. to go 11,000 miles on gas or a cost of 7 cents per mile. It cost me $483. to go 14,500 miles as an EV, or 3.7 cents per mile.
Of course there are a lot of fixed costs which have nothing to do with mileage, such as depreciation, insurance and license fees plus tires, a new 12V battery and oil and filter changes.