Author: chogan@directresearch.com

Post #1711: State-of-charge hypermiling and a generalized theory of pulse-and-glide

Why pulse-and-glide saves gas.

Gasoline engines run most efficiently when under a fairly heavy load.  Load them too lightly, or too heavily, and their efficiency drops.

Below is the “efficiency contour” of a hypothetical 2 liter Atkinson cycle engine. Engine RPM is on the X-axis.  Engine load (output) is on the Y-axis.  The labels on contour lines are percents, and show the fraction of the energy in the gasoline that is converted into motion by the engine.  Those contour lines define a sort of hill, with the peak of the hill — maximum efficiency — occurring when this engine is running around 2500 RPM, putting out about 100 horsepower. And converts just shy of 39% of the energy in the gas into usable power.

Source:  Kargul, John & Stuhldreher, Mark & Barba, Daniel & Schenk, Charles & Bohac, Stanislav & McDonald, Joseph & Dekraker, Paul & Alden, Josh. (2019). Benchmarking a 2018 Toyota Camry 2.5-Liter Atkinson Cycle Engine with Cooled-EGR. SAE International journal of advances and current practices in mobility. 1. 10.4271/2019-01-0249. Accessible though this link.

The engine modeled above is a 2.0 liter Atkinson-cycle engine.  That’s just a bit bigger than the 1.8 liter engine that’s actually in the Prius Prime.  But it’s close enough.

Below, there’s the crux of the problem.  Much of the time, the engine is inefficiently lightly loaded.  I’ve marked the power required to cruise on level ground at a steady 55 MPH in a Prius.  The car only needs about 12 HP.  (I infer this from the ~12 KW of power drawn to keep the car at that speed in electric (EV) mode.   That power, less about a 20% loss in the electric motors, is the energy required at the wheels to keep the car moving forward at that speed.

And so, if you cruise along at a steady 55 MPH on the gas engine, even though the car won’t be burning a huge amount of gas, what little it burns will be burned inefficiently.

Instead of running that engine steadily at 12 HP output, you could alternatively run it hard — run it briefly at 100 HP — then shut it off.  And repeat as necessary.  That’s pulse-and-glide.

And that’s why pulse-and-glide saves gas.  You extract energy from gasoline as efficiently as possible, by running the engine under heavy load.  And then you match the engine output, to the average power required by the car, by cycling the engine on and off as needed.

Traditional pulse-and-glide makes you a rolling hazard.

With traditional (or kinetic-energy) pulse and glide, you first run the gas engine and speed up.  Then switch it off, coast, and slow down.  And repeat.

Practically speaking, this is of almost no value on the public highways, because this makes you a nuisance to other drivers.  It makes you into a rolling traffic hazard.

Potential energy pulse-and-glide requires the right terrain.

Speeding up, however, is not the only way to store the output of the car’s engine.  Going up a hill works just as well.  You store that excess output in the form of potential energy (height) instead of kinetic energy (speed).  Apply gas on the uphills, coast with engine off on the downhills.

I can attest that this most definitely works.  This is how I achieved my last two 80-MPG all-gasoline (no energy from the grid) road trips.

Needless to say, this only works where you have significant hills.  Ideally, hills large enough that the car will maintain the posted speed limit on the downhill with no or minimal energy input from the drive train.

A new/old concept:  State-of-charge pulse-and-glide.

Both methods described above can be done by a standard gas car.  No electric motors are required.  In fact, in a Prius, you achieve maximum efficiency under either method if you never use your electric motors.  (Using the gas engine to charge the battery, then run the motors, wastes about 30% of the power produced.)  Champion Prius hypermilers actually shift the car into neutral on the “coast” phase, specifically to avoid moving electric current into or out of the battery via the motor/generators.

But a plug-in hybrid electric vehicle (PHEV) like the Prius Prime has yet a third option, which I’m going to call state-of-charge pulse and glide.

To be clear, what I’m about to describe is something that the car does, on its own, anyway.  The only question is whether you can modify your driving behavior to take exceptional advantage of it.

If you use the gas engine to charge the battery, then run the electric motors, that wastes about 30% of the energy produced by the gas engine.  So, at first blush, it seems like you’d want to avoid using those electric motors.

But, if you charge that battery at the peak of the gas-engine efficiency curve, that means the electric motors are using up your gas with somewhere around (0.7*38% = ) 27% efficiency.

This leads to the section that I’ve labeled “EV carve out” above.  Roughly speaking, if the driving situation requires less than about 25 KW of power, it’s more efficient to run in EV (electric-only) mode, as long as you can later recharge the battery at relatively high engine load.  (So that the recharge happens near peak gas engine efficiency.)

In the Prius Prime, assuming this engine chart is a reasonable proxy for the actual Prime 1.8 L engine, that has the following practical implication for running the car in hybrid-vehicle (no-grid-power-used) mode.  If you can, you should run on electricity-only up to a current draw of about 90 amps.  That’s the point at which the electric motors, less their inherent 20% loss, are producing about 25 KW of power. That’s the point where switching to gas propulsion is more efficient.

But the closer you get to that 90 amp limit, the less advantage electricity has over gas, and the less gas you are saving.  So, from a battery wear-and-tear perspective, it’s probably best not to push it that far.  You will likely get the bulk of your savings with a more conservative limit of (say) 50 amps, or roughly a “2 C” discharge rate.  (The rate at which the entire EV battery would be dead in half an hour.)  Assuming the car will let you do that, in hybrid mode.

So, a conservative rule-of-thumb is that a power output of somewhere around 17.5 KW (25 HP) is where you should try to flip the car from gas to electric and back.

To be clear, the car does something like this on its own.  At low power demands, it shuts off the gas engine and used the electric motors.

What I have noticed, however, is that there’s considerable hysteresis in the car’s decision.  In particular, once the gas engine is on, it tends to stay on until power demand gets quite low.

So I believe that driver intervention can improve mileage, using (e.g.) terrain anticipation.  If you’re coming to a stretch of road with likely low power demands — cresting a hill, starting a slow deceleration, or just coming up to a level stretch — you may be able to beat the Prius’ internal algorithms.  Conversely, when you see a high-power-demand situation coming up — a hill, say — you can flip the car into gas mode before it begins to bog down in electric mode.

My simple initial rule-of-thumb will be a 50-amp cutoff.  When in hybrid mode, drive the car on the electric motors up to 50 amps current or low state-of-charge cutoff, whichever comes first.  Anything over 50 amps, nudge the accelerator to kick the car into gas mode.

Edit:  I decided to do a little acid test of the concept.  As every driver knows, the worst trips for a gas engine are short, around-town jaunts.  I decided to do a little run to a couple of stores, in hybrid vehicle mode, total trip of about 8 miles, divide into three legs with stops in-between.  After the mandatory gas-engine warm-up period, whenever the gas engine came on/power was needed, I loaded the gas engine heavily.  I gave it enough throttle to bring it immediately to the “power” zone on the dashboard.  But, once up to speed, I let off the accelerator to shut the gas engine off, and drove for as long as feasible on electricity only, respecting a maximum draw of 50 amps.

Results:  71 MPG.  And it was clear that if I’d had a longer distance between stops, that would have increased. 

One short trip does not prove the concept.  And the Prius chastised me soundly for those hard accelerations, basically giving me a flunking score on the eco-meter.  Nevertheless, I consider this first test to be encouraging.

By the book, and by the dashboard readouts, I was doing everything wrong. And yet, it’s hard to argue with the MPG.

Edit 2, 2/19/2023.  Not so fast.  Building on the above, I went to a local disused office building and circled the parking lot.  Roughly a 1.3 mile circuit, 25 hour speed limit, three full stops per loop.  On one set of loops, I tried this hypermiling approach.  On another set, I drove gently, then used the “charge” function to bring the battery state-of-charge back to its original level. 

Results:  In both cases, I got about 75 MPG.  Which, in hindsight, may simply be what the Prius Prime gets, driven in hybrid (gas-using) mode, around 25 MPH.

I think the moral of the story is that I’ve done so little around-town driving in hybrid (gas-using) mode that I’m not sure sure what sort of gas mileage I should expect as a baseline.

Conclusion

Anyone who has ever used the Prius cruise control in hilly country knows that it’s quite “reactive”.  It doesn’t anticipate the hills, but instead holds speed steady, then pushes the car far out onto the power curve in an attempt to maintain speed on the uphill.  For that reason alone, I don’t use the cruise control on hilly roads, as I feel that I can drive the car more efficiently in manual mode, making some modest adjustments in speed on the downhills and uphills.

Similarly, I’m betting I can squeeze a little extra mileage out of the car, in hybrid mode, by manually selecting the point of switch-over between gas and electric propulsion, and pushing the gas engine at high load to maximize efficient use of gasoline.  Then, once at speed, or over the crest of a hill, lifting my foot off the gas briefly to shut the gas engine down, and continuing in electric-only mode as feasible.

You need an extra bit of instrumentation to be able to do that well.  I’m using a Scangauge 3, which will show me quantities such as battery current, engine RPM, and engine output.

What makes this work, as a form of pulse-and-glide, is, of course, the traction battery.  That’s where the excess power production of the gas engine is stored if not needed.  So the right way to view this is state-of-charge pulse-and-glide.  Instead of letting the speed vary (kinetic energy), or the height vary (potential energy), you let the battery state-of-charge vary (electrical energy).

Same concept either way, you just choose a different place to store the excess power output of well-loaded gas engine.  With different implications for how usable pulse-and-glide is, in actual highway traffic, for a given terrain.

Finally, I note that there have been recent patents issued for systems that would automatically pulse-and-glide large trucks, based on a system that anticipates changes in terrain.  They seem to be characterized as a more fuel-efficient form of cruise control.  With everything in modern cars being controlled by a computer, it doesn’t seem too far-fetched to think that some form of automated pulse-and-glide — a fuel-saving cruise-control mode — might eventually become a standard option on vehicles capable of doing it.

With that point of view, driving a gas engine at a constant, low engine load is something of a relic of the past.  It dates to the era when there was literally a metal cable connecting your gas pedal to the throttle body on the carburetor.  With everything computer-controlled these days — and carburetors a thing of the far distant past, for cars — it doesn’t seem like a stretch to ask your computer to do your energy-saving pulse-and-glide for you. As long as you have some safe way to store that excess gas-engine output.

Post G23-003: Garden plan, 2023, step 1.

As my store-bought organic potatoes chit (sprout), in anticipation of planting on St. Patrick’s day, I am in the process of figuring out what else I’m going to grow this year.

In a surprise move, I’m going to take my wife’s advice and … wait for it — only plant stuff that we actually like to eat.

Then it occurred to me that I should only plant stuff that the deer don’t like to eat.

Now that I was surfing that intellectual tsunami, I went out on a limb by saying that I probably don’t want to plant stuff that my local bugs like to eat.

Finally, drafting in the wake of that runaway 18-wheeler of brilliance, maybe I should avoid plants that are susceptible to diseases frequently found in my garden.

Fantastic.  Easy-peasy.  Planning accomplished.

I just need to fill in the details.

Unfortunately, when I do the Venn diagram of those four insights, I’m left with:

And I’m not all that sure about the red one.

(Plus, as I understand it, that “guy” on the left is now part of a lefty-liberal plot for the emasculation of America.  Pink ears, blue shoes — that’s certainly a mixed message, but that’s the way the libs go after it.  Slow rot.  Next thing you know, they’ll be taking away his angry eyes.  And then they’re coming for your potato cannon and spud gun!!  You’ve been warned!!!)

Time for a bit of a rethink.  More soon.

 

 

Post #1710: The best thing that ever happened to all my friends’ gas mileage.

My wife bought her first Prius in 2005.  We tend to forget, but there was a lot of hatred expressed toward that car, at that time.  Which sounds hilarious now, but is true.  There was also disinformation spread about that car, similar to the disinformation you’ll hear these days regarding electric vehicles.  E.g., that the Prius had single-handedly ruined Sudbury, Ontario due to the need for nickel for the battery.

There was also a lot of just-plain-ordinary denial.  That car got an EPA-rated 46 MPG, which, for the time, and the size of the car, was absolutely outstanding.  This was a time when you could not find a traditional gas car with similar interior volume that broke 30 MPG.

It was, as I have noted before, alone in its level of efficiency.  That’s expressed below by an index combining gas mileage and interior volume.  (This is my calculation, from EPA mileage data.)

At that time, if you were willing to drive a small car, and required that it get at least a whopping 35 MPG overall, your choices were:

  • Honda Insight (basically, a tiny 2-seater).
  • Honda Civic Hybrid (as shown on the chart above).
  • Three small VW models with 35 MPG turbo-diesels.

This per the federal website fueleconomy.gov.

And yet, I used to joke that my wife’s Prius single-handedly improved the gas mileage of the U.S. automobile fleet.  Because, every time we mentioned 46 MPG, the universal response was, “Big deal, I get almost that good of a mileage in my fill-in-the-blank.”  That Prius was the best thing that ever happened to the gas mileage of all of our friends’ cars.

The first year we owned that car, we heard about all kinds of mythical non-hybrid vehicles that easily got over 40 MPG.  Easily.  All the time.  Without all that fancy hybrid nonsense.

In reality, none of these folks had a clue what they were talking about.  None had actually carefully tracked mileage.  Most had some impression of some road trip they once took where they think they got great mileage.  Nobody was talking about city mileage.  And so on.

But they all knew that hybrids were just so much hype.

As I continue to learn how to drive my wife’s 2021 Prius Prime for greatest fuel economy, I keep setting new personal bests.  Most recently, we drove out to a local scenic byway (the Snickersville Turnpike) and back.  Door-to-door, using “hybrid mode” (no energy from the grid), we managed to get 82.4 MPG over the course of the 80-mile round trip.

That was a mix of 55+ MPH urban arterial highways, country roads, and then small-town streets.  So, no high-speed interstate driving.

Back in the day, people could fool themselves into thinking that their non-hybrid vehicle was just about as efficient as a Prius.  Even though the U.S. EPA clearly said otherwise.

But this most recent generation of Prius, when driven with an eye toward best mileage (Post #1624), gets such eye-popping numbers that I don’t think you can kid yourself any more.  This is now my second trip where I’ve ended around 80 MPG, driving the car in hybrid mode (i.e., not using energy from the grid.)  Even our interstate trips now routinely yield high-60’s MPGs (admittedly, without the extreme speed limits present on Western interstates.)

And, separately, more than 70% of our miles are run purely on electricity from the grid.  Which means the 65-to-80 MPG observed in hybrid mode is our version of gas-guzzling.  In “EV mode”, using the battery and not the gas engine, we manage somewhere around what the EPA would term 150 MPGe.

This isn’t by way of bragging.  It’s by way of setting the record straight about what’s routinely and reliably available these days.  For not much money, as new car prices go.

I continue to read articles about how hard it is to move to electric transport, what a huge expense it entails, and so on.  And, yeah, you can make it hard, and you can make it expensive, and inconvenient.

But none of that has to be true.  Buy a quality plug-in hybrid electric vehicle (PHEV).  If you’re like us, you’ll get most of the benefits of electrical transport and none of the drawbacks.  Sure, you have to have some faith in the technology.  You need to learn the do’s and don’t of taking care of that big battery.  In a few areas,  electricity is currently a more expensive fuel than gas, by a modest amount.  But as far as I can tell, hybrids started out pretty good, and they just keep getting better.

I’m no longer satisfied when I only get 80 MPG, driving my wife’s hybrid.  And I find that absolutely mind-blowing.

Post #1709: Penultimate COVID update, I hope

 

Yesterday the Washington Post reported that Johns Hopkins is winding down its three-year effort to track the COVID numbers.  That’s a pretty good signal that it’s time to wind this down as well.  For now, both the New York Times and the CDC continue to track the data.  If you want something reasonably up-to-date, you can look there.

As it stands, the number of new cases continues to recede slowly.  We’re now down to an average of 10 new cases / 100K population / day, down from 12 last week.  And it’s getting on toward Spring, when incidence of viral respiratory illness normally declines.  So I expect that the numbers will continue on that slow downward trend, moving forward.

I might check this one last time, a month from now, just to see where things stand.  But right now, this is what we live with.   No point in saying anything more.

Continue reading Post #1709: Penultimate COVID update, I hope

Post #1707: Nobody offers a warranty on the electric range of their plug-in hybrid vehicles?

 

Edit 2/11/2023:  I grossly underestimate the replacement cost for a Prius Prime lithium-ion battery.  Per this thread on Priuschat, the cost of new Prius Prime battery, from the dealer, is $12,595.  Others suggested the dealer took some markup, as the list price from Toyota is just under $10,000.  I say, potato, potahto.

In round numbers, the cost of a new replacement battery is 43% of the cost of a brand-new Prius Prime, base model, current MSRP $28,770.  As a footnote, literally none were available in North America, and the battery has to be shipped directly from Japan.

I should put in the usual EV-weasel-wording:  By the time the battery dies, there will be plenty of good-used batteries in junkyards, from wrecks.  That did, in fact, happen with the original Prius NiMH hybrid battery.  Plus, there may be much cheaper aftermarket replacements at some point.  And so on.  But right here, right now, what I cited are the hard numbers for battery replacement cost.

Original post follows.

Only Volvo offers any warranty on your plug-in hybrid electric range, near as I can tell.

And I think I have finally nailed down why that is.

A typical battery guarantee for a fully electric vehicle (EV) is that a car will lose no more than 25% of range in 8 years/100,000 miles.

Based on research shown below, using actual driving behavior, for a Prius-Prime-like plug-in hybrid electric vehicle (PHEV), you would expect about 5% of batteries to fail, under that 8-year, no-more-than-25% loss definition.  Just from normal wear-and-tear, as-typically-driven.

So, my guess is that PHEVs don’t get those guarantees because manufacturers would end up replacing too many batteries.

All the more reason to treat your battery gently.

Background

Last week, I found out that my wife’s Prius Prime had no warranty on its electrical range.  Currently, as we drive it, we get mid-30-miles on a charge.  But if that drops to zero, tough.  As long as the car will still run as a hybrid, they battery has not “failed” under Toyota’s 10-year/150,000 mile warranty.

So I got curious.  I already have a list of all 2022 plug-in hybrid electric vehicles (PHEVs), from a just-prior post.   I decided to look up the warranty information for as many manufacturers as I could find.

Here’s the results.

Volvo offers a 30% loss-of-range warranty.  If you lose more than that, during the eight-year warranty period, they’ll fix it.

Near as I can tell, none of these other manufacturers offer any warranty whatsoever, on the electrical range of their PHEVs.

Toyota
Kia
Porsche
MINI
Ford
Chrysler
Mitsubishi
Jeep
Hyundai (I think)
BMW (but maybe they decide case-by-case?)

The Hyundai warranty covers EVs, PHEVs, and hybrids, and in separate places says that it definitely covers loss of range, and that it definitely does not cover loss of range.  Your guess is as good as mine, but I’m guessing they cover range for EVs (as required by law) but not for PHEVs.

Originally, I could not understand why Toyota offered no PHEV range warranty.  That situation has now improved.  I’m now baffled why almost nobody offers a PHEV range warranty.

Unfortunately, I think that “no PHEV range warranty” is the industry norm  for precisely the reason stated in the Toyota warranty documents:

 

I’m an economist by training, and I find it this interesting, I guess. When there’s a de-facto industry standard, there’s usually a reason for that.

And I think I understand why no-PHEV-range-warranty is the industry standard.

How many would “fail” under normal driving conditions?

I’ve been searching for an answer for this for the better part of a week.  I posted my thoughts on preserving battery capacity on a chat side dedicted to the Prius (PriusChat), and with a few exceptions, got met with derision.  For sure, nobody there had ever heard of a Prime or the prior version (Plug-in Prius) showing any signs of premature loss of range.  Almost nobody thought that any sort of battery-protecting behavior was necessary.

I finally came across what I believe is a realistic projection of the fraction of Prius-Prime-like vehicles that would fail under the typical EV warranty of no more than 25% range loss in eight years/100,000 miles.

That’s:  Comparison of Plug-In Hybrid Electric Vehicle Battery Life Across Geographies and Drive Cycles, 2012-01-0666, Published 04/16/2012, Kandler Smith, Matthew Earleywine, Eric Wood, Jeremy Neubauer and Ahmad Pesaran
National Renewable Energy Laboratory, doi:10.4271/2012-01-0666

They used actual driving data from about 800 trips taken by Texans in PHEVs.  The then extrapolated that to eight years of driving behavior.  Their model is not quite perfect, as the modeled vehicle only provides a 10% “buffer” at maximum allowable charge, while the Prius Prime provides 15%.  On the other hand, for the key chart, they did not include (e.g.) the effects of high temperatures on battery life.  (So, no parking your car in the sun in this model, so to speak).

Here’s the key graph, where the most Prius-Prime-like vehicles is the PHEV40.

Source:  Cited above.

In a nutshell, only counting the wear-and-tear from normal driving and charging, they expect the average user to lose 20% of range by the end of the eighth year.  And about 5% of users would experience in-the-neighborhood-of 25% range loss.

If they were to throw in the effects of variation in climate (hot climates kill batteries quicker), and variation in practices regarding storing the car fully charged (which also kills batteries quicker), I might guess that around 10% of drivers might exceed that 25% loss threshold within an eight-year warranty window.

To put that in perspective, car manufacturers as a whole spend about 2.5% of their total revenues on warranty repairs (reference).  A ten percent failure rate of this part, replaced at new-battery cost shown above, would by itself account for (roughly) 4 percent of Toyota revenues from Prius Prime sales.

As far as I’m concerned, this solves the mystery of the missing warranty.  (Almost) nobody offers anything like the standard EV warranty, because if they did, they’d have to replace an unacceptably large fraction of batteries under warranty.  And that would lead to an unacceptably high warranty cost.

All the more reason to avoid unnecessary wear-and-tear on a PHEV battery.

Post #1706: When is electricity the cheaper home heating fuel?

 

Today the Washington Post had an article on electric heat pumps displacing oil and propane in Maine.  Not only do modern heat pumps work reasonably well in that cold climate, but they were reported to save a lot of money, compared to oil or propane furnaces.

I had a hard time believing that they were big money-savers, as electricity rates in New England are pretty high.  So I decided to check the math, using reasonably current prices and some reasonable guesses for technical performance of each type of heating.

The answer is yes.  If you replace on old, inefficient oil stove with a heat pump, you should expect to cut your heating bill in half.  Probably more interestingly, not even a modern high-efficiency oil furnace can compete with a heat pump, at Maine’s prices.

But I note one fact that makes Maine’s situation different from that of Virginia, where I live.  I don’t think you can get natural gas most place in Maine.  It would be slightly cheaper to heat with natural gas, at national average prices, if you used a 95% efficient natural gas furnace.  In Maine.  Given Maine’s high electricity prices.

As a final footnote, near as I can tell, the interesting thing about these new generation heat pumps is that they will work in extreme cold.  Near as I can tell, they are not hugely more efficient that the prior generation, as long as temperatures are moderate.

But what about electricity versus natural gas, in Virginia?

Source:  Clipart Library.com

My home heating system was designed by the internally-renowned HVAC engineer Rube Goldberg.  The original 1950s gas-fired hot water baseboard heat is now the secondary heating system.  That’s run by a 95%-efficient gas furnace, which also provides domestic hot water.  Layered over that is the new primary heat source, consisting of two elderly ground-source heat pumps, fed by just over a mile of plastic pipe buried in the back yard.

I have pipes, wires, and ducts running every which-away.  In the house, throughout the yard.  In the attic.  Under the slab.  Up the outside of the house and over the roof (no joke).

It works.  Except when it doesn’t.  As was the case this week, when the super-efficient gas hot water heater failed.  That finally got fixed today.  Which is what got me thinking about this.

From the standpoint of carbon footprint, after I put that high-efficiency gas furnace in ten years ago, I was more-or-less indifferent between electricity and natural gas as the fuel source. Pretty much the same C02 per unit of heat, either way.

But as the Virginia grid has more than halved its carbon footprint over the past two decades, electricity has become by far the lower-carbon option.  (Underlying source of data for both graphs is the U.S. EIA).

Every once in a while I revisit the issue of cost, mostly because natural gas prices fluctuate all over the place.  Using the same framework as above, here’s the current match-up between my ground source heat pumps (with assumed coefficient of performance of 3.3), and my 95% efficient gas furnace.

Turns out, at current prices, and with my setup, electricity now beats the pants off natural gas, cost-wise.  Not hugely different from the situation for oil in Maine.  I didn’t expect that, and I’m pretty sure that’s a consequence of currently high natural gas prices.

In any case, it’s nice when you can do well by by your bank balance by doing right by the environment.

My only real takeaway is that I should minimize my use of gas-fired secondary heating, within reason.  I figure if the citizens of Maine can get by with nothing but heat pumps, I should be able to do that as well, in the much milder climate of Virginia.

Post #1705: When is electricity the cheaper motor fuel?

In prior posts, I noted that my “break-even” price of electricity for my wife’s Prius Prime is currently around 24 cents per kilowatt-hour.   That’s the point where running the car on electricity costs as much as running it on gas, with gas at $3.24 a gallon

I can say that with precision because the Prius Prime can use either fuel.  As long as I know the EPA ratings for miles-per-gallon and miles-per-kilowatt-hour, it’s trivial to figure out the break-even rate, for that one car.

Breakeven electricity price = Gas price x (miles-per-KWH/miles-per-gallon)

In other words, if one KWH takes you 7% as far as one gallon of gas, then the break-even price for that KWH is 7% of the price of a gallon of gas.

Call that term at the end —  (miles-per-KWH/miles-per-gallon) — the “break-even ratio”.

Here’s something that I find interesting.  All PHEVs have roughly the same break-even ratio.  To show that, I downloaded the EPA 2022 model year vehicle mileage database.  Using the MPG (gas) and MPGe (electric) figures, and the constant that one mile per KWH is 33.705 MPGe, I was able to calculate this break-even ratio for every PHEV offered in the U.S. in 2022.

Roughly 70% of all the PHEVs offered in 2022 currently have a break-even electricity price between 23 cents and 25 cents per KWH.  That’s using today’s current U.S. average gas price of $3.36 per gallon of gas (per the Federal Reserve Bank of St. Louis).

Note that each one of those ratios is a straight-up apples-to-apples comparison, because it’s literally the same vehicle being driven either as a gas hybrid or as an electric vehicle.

By contrast, for a lot of pure electric vehicles, there is no obvious way to do that apples-to-apples comparison.  Most famously, there is no such thing as a gas-powered Tesla.  Less obviously, even if a vehicle manufacturer offers the same vehicle in gas and electric versions, the versions won’t be identical because factors such as interior volume will change between the models.

An important caveat for the table above is that all of these PHEVs are hybrids, when they are burning gasoline.  That’s going to translate to above-average gasoline fuel economy.  And, because the efficiency of the electrical side of the vehicle does not change much across manufacturers, that’s going to lead to a low break-even price.

But what about those cars where the gas “twin” uses a conventional gas engine?  What would the theoretical “break even” price be, for those nearly apples-to-apples comparisons?   You’d expect that without the hybrid efficiency in the gas “twin”, the break-even price of electricity ought to be higher.

It’s much hard to do this electric/gas “twins” analysis from the EPA data, for a couple of reasons.  First, there aren’t many examples of conventional gas/EV twins.  Second, you have to find them by searching the EPA database for instances of the same model, but different propulsion system.  I also have to rely on manufacturers using the same base model name for both gas and electric models.

I only found four plausible “twins”, and two of them have so many power trains listed for both electric and gas that I’m not sure I’ve made an apples-to-apples comparison.  That said, the two at the bottom appear to be fairly unambiguous twins.  They both suggest a break-even gas price (say) 34 cents per gallon.   Which makes sense, as standard (non-hybrid) gas engines are inefficient relative to the hybrid engines of the prior table.

The outlier is the Mustang, and there’s good reason to believe that’s not a coincidence.  Performance cars have notoriously fuel-inefficient engines.  Likely, the more you move toward the performance end of the gas-car spectrum, the higher your break-even electricity rate is.  And the higher your fuel cost savings would be in switching to a performance electric vehicle instead of gas vehicle.

So, if your only two options were a pair of seemingly-similar cars — one using a standard gas engine, the other using electricity — and you would not consider a car with a hybrid gas engine — and you’re not looking for a performance vehicle — then, plausibly, you could start counting you fuel cost savings at 34 cents per KWH.  Because your gas-vehicle comparison is so inefficient.

Obviously, YMMV.  For any two cars that you consider to be close substitutes — one gas, one electric — you can simply look them up on fueleconomy.gov and do the math.

For example, the Tesla and BMW above have nearly identical interior volume, and are similar in price.  Doing the math, the 132 MPGe equates to (132/33.705 =) 3.92 miles per KWH.  The break-even price of electricity for this pair of cars, at $3.36 per gallon of gas, is $3.36*(3.92/30) = $0.44 per KWH.  Presumably that’s due to the fuel-inefficient engine in the gas BMW, for performance driving.

In summary:  If you’re the sort of person who is considering buying either a hybrid or an EV, at today’s gas prices, your break-even electrical rate is going to be somewhere around 24 cents per KWH.  If you insist that your only realistic choice is either a standard gas vehicle or an EV, your insistence on using the less-efficient gas technology means that your break-even electrical rate is going to be plausibly somewhere around 34 cents per KWH.  But if you insist that your comparison is between gas and electric performance cars, you can plausibly boost that break-even electrical rate to around 45 cents per KWH, or so.  YMMV.

What’s the policy point?

Almost all discussion of electric vehicles either explicitly or explicitly assumes that electricity is a much cheaper fuel than gasoline.  The standard reasoning is that sure, EVs may be more expensive up front, but they’ll pay you back in fuel savings.

By inference, then, there’s an assumption that sooner-or-later, EVs will be the economically preferred choice, owing to their lower fuel costs.

My point is, that’s only true sometimes.

The only true apples-to-apples comparison of gas versus electric fuel costs comes from PHEVs. In that case, the exact same car can use either fuel.  There, the break-even price of electricity is centered around 24 cents per KWH currently, with gasoline at $3.36 per gallon.  Anything cheaper than that, and electricity is the cheaper fuel.

There’s a caveat.  The gas engines in those cars are all hybrids, so that benchmark really only applies to individuals who are considering a purchase of either a hybrid or an EV.  My guess is, that’s most of the EV market.  For those folks, that current 24-cent break-even price is appropriate.  But as you move up the scale of inefficiency, from hybrid to standard gas engine to gasoline performance car, your savings from electricity grow, and your benchmark break-even electrical price rises.

That said, for anyone driving a PHEV now, or anyone considering buying either an EV or a hybrid, that’s the correct current benchmark rate at which gasoline and electricity are equally costly fuels (with gas at $3.36/gallon).

As long as we are talking about PHEVs, or electric versus hybrids, large portions of the U.S. population face electrical costs for vehicle charging as high or higher than that break-even rate.  At current electrical and gas prices, there are no fuel savings from going electric.

First, in New England, recent spikes natural gas prices have resulted in unprecedented electrical rates.  Prices seem to be easing a bit in most states, but only a bit.

Source:  US EIA.

A PHEV user in New England will get little-to-no cost savings from driving on electricity rather than gasoline, assuming they pay somewhere near the U.S. average price for a gallon of gasoline.  Which appears to be true, per the American Automobile Association.  By inference, a New Englander choosing between similar hybrid and EV models probably could not count on significant fuel cost savings from going EV.  At today’s gas and electric prices in that area, a hybrid and an EV would have roughly equal fuel cost per mile.

Source:  AAA, accessed 2/7/2023

 But a far more important population is individuals who cannot charge at home, and must use public charging stations.  This probably includes most of the roughly 30% of the U.S. population that does not live in single-family dwellings.  For these individuals, charging is expensive enough to eliminate any material fuel savings from electricity, compared to driving a gas hybrid.

My experience is that for most public charging stations, it’s just about impossible to figure out the cost.  But I think the following ad is representative of the best rates you are likely to find.

Source:  EVgo.

Ignoring the weasel-wording (“as low as”, “TOU pricing applies”), and paying attention to the monthly fees, none of these options offers any material fuel savings for the PHEV owner or for the individual considering electrical versus gas-hybrid transport.

Bottom line is that at current prices, EVs are going to be a hard, hard sell for people who have to rely on expensive public charging stations.  At least at current gas prices.

Rather than turn a blind eye to that, public policy needs to acknowledge it.

I’m a big believer in electrical transport.  Right now, it doesn’t make good economic sense to a large portion of the population, looking narrowly at purchase price.  And for a pretty big chunk of the population, there will be no material fuel cost savings to offset that higher purchase price.

Maybe that will change, somewhere down the road. Some combination of higher gas prices and lower electrical prices might result in universal fuel savings from EVs compared to hybrids.  But right now, you really shouldn’t based policy on the assumption that everyone will see fuel cost savings from EVs.

Post #1704: My $10 battery-saving device

 

Source:  Amazon

They say there’s no saint like a reformed sinner.

And, I swear this is going to be my last post on electric vehicle batteries.

I just need some closure.  Because I’m still fairly ticked about this entire episode.

For a year and a half, I adopted the obvious but destructive habit of plugging in my wife’s car as soon as I returned from a trip.  That way, it would always be fully charged when we wanted to use it next.  Easy-peasy.

As it turns out, discussed in the just-prior post:

  • charging it to 100% shortens battery life
  • charging it to 100% and letting it sit around shortens battery life a lot

(And when I say 100%, I mean to the highest charge level the car will allow.  I realize that Toyota built in a roughly 15% buffer, so that the literal state of charge is around 85% when it says the battery is full.  All car makers do that.  And some people say that provides all the protection you need.  But I don’t.  More importantly, the National Renewable Energy Lab (NREL) doesn’t.  The battery life simulation below assumes a 10% buffer, so SOCmax is 90% true state-of-charge.  You can take their chart, relabel the lines by adding 5% to each label, and that ought to be a pretty good estimate of what you should expect with a Prius Prime.  And, based on that chart, you would expect to shorten the life of the battery substantially if you always charge to (what the car tells you is) 100%.)

Source:  Optimizing Battery Usage and Management for Long Life, Kandler Smith, Ying Shi, Eric Wood, Ahmad Pesaran, Transportation and Hydrogen Systems Center, National Renewable Energy Laboratory, Golden, Colorado,
Advanced Automotive Battery Conference Detroit, Michigan June 16, 2016  Annotations in red are mine.

 

If I had only:

  • Read the fine print in Toyota’s highly-touted 10 year/150,000 mile battery warranty to realize that there is zero warranty for loss of range.
  • Scrutinized page 143 of my 800-page owner’s manual, and realized the significance of this sentence:
  • “Use the charging schedule function as much as possible in order to fully charge the hybrid battery (traction battery) immediately before starting off.”
  • And had the wit to realize that while Toyota said “Use the charging schedule function” they actually meant “don’t let the battery sit around fully charged.”

If I had put all that together — for this new car that was functioning and driving perfectly, getting better-than-EPA gas and electrical mileage — I would never have made that mistake.

Instead, I probably would have figured out that the $10 countdown timer, pictured above, would have prevented almost all the abuse I was heaping onto that (plausibly) $5000 battery.

The only new thing to report is that the cheap timer picture above seems able to handle the 12-amp charging current just fine.  And I’ve changed my bad habits.  My new policy is to give the car an hour of charging, if the charge is low when I get back from a trip.   It’s a simple as plugging it in and pushing a button.  But otherwise, I’ll put the car on to charge, for a few hours, when I make the coffee in the morning, so it spends the greatest amount of time a some moderate state of charge.

That cheap, simple change is all it took to eliminate a potentially battery-killing bad habit.

My sole remaining concern is that some EV charging systems only “balance the battery pack” or equalize the voltage across all cells at the very end of the charge cycle.  If that’s true for the Prius Prime, I’m going to want to do an occasional 100% charge in order to get that done from time to time.

An unexpected bonus is that I can take advantage of the “charging curve”.  The closer you get to 100% charged, the slower the charging gets.  A rough rule-of-thumb is that the last 25% of range takes half the total charging time.  And so, while the car takes more than five hours for a full charge, it only takes an hour to go from ~40% to ~80% charged.

Anyway, no saint like a reformed sinner.  I hope I can be the person that I want to be.  As pictured below.

Source:  Electrek.co.  Annotations in red are mine.

 

Post #1703: Four simple rules for protecting the lithium-ion battery in a Prius Prime

 

Background

Source:  Geotab.

So far, on average, the lithium-ion batteries in the Prius Prime appear to be holding up well.  The small sample of 2017 Prius Primes used for the graph above lost range at a rate of just four percent over the first three years of operation.  That’s just a touch better than the average EV.

Within that overall good average, some individuals are going to get outstanding battery life, and other’s won’t. 

That’s not a matter of luck.  For example, the Geotab site (source of the graph above) summarizes the predictable loss of battery life due to high heat, fast charging, and so on.

As Toyota itself says (emphasis mine):

 

Source: 2021 Prius Prime warranty booklet.

As far as I can tell, the use of “drastically” above is correct.  Based on the National Renewable Energy Laboratories analyses presented in the prior post, treating the battery gently could result in two-to-three fold increase in battery life, compared to abusing it.

Here’s a bit of data from Tesla to illustrate.  The X-axis is how much the battery has been used, in total KWH.  The Y-axis is the remaining range.  (Note:  Full range of the vertical axis as shown is about a 20% capacity loss of the battery.)  There is, in fact, quite a spread around the average capacity loss.   Of the two data points highlighted, for roughly the same battery use, one has lost about 5% of capacity, the other has lost nearly 20%.

Source:  Electrek.co.  Annotations in red are mine.

 

The rules.

The rules for long battery life given below are based on the evidence and analysis in the just-prior post.  But, in fact, these are all well-known rules for extending lithium-ion battery life.  If you look around, you’ll see that more-or-less everyone says more-or-less the same thing.

Rule 1:  Avoid charging to 100%.

  • Don’t charge to 100% unless you absolutely need that full range.
  • More importantly, don’t charge to 100% and let the car sit unused.
    • If you’re going to charge to 100%, use the charge scheduling software so that the car reaches 100% just before you drive it.
  • Even more importantly, don’t charge it to 100% and let it cook in the sun.
  • The most common suggestion is to charge to 80%.  Not clear if that specific number is anything more than a rule-of-thumb.

Rule 2:  Avoid temperature extremes, particularly high heat.

Rule 3:  Avoid high-current events in EV mode.

  • Avoid rapid acceleration.
  • Avoid fast stops.
  • Arguably, avoid driving at highway speeds in EV mode.
    • The faster you go, the gentler your driving should be.
  • Minimize high-current events by driving in EV AUTO mode — punch the right-most button on your driving mode selector.

Rule 4:  Use shallow charge-discharge cycles whenever possible.

  • Get out of the habit of charging to 100% and discharging to 0%.
  • Get into the habit of charging/discharging over a narrower range, e.g., charge to 75%, recharge when it hits 25%.

There are a handful of rules that aren’t cited here because they don’t apply to the Prius Prime.  Frequent use of a fast charger reduces battery life.  But you can’t do that in a Prime anyway.  Discharging the battery down to zero is bad, but, again, you can’t do that in a Prime.  The Prime reserves the last portion of capacity for use as the hybrid battery.

In a sense, this is just a natural extension of what prudent drivers have done all along to avoid unnecessary repair costs.  In a conventional car, if you want your brake pads to last, you aim for nice, gentle stops.  And now, if you want your battery electrodes to last, you do the same thing.  Plus some.

Discussion, Part 1:  An unusual automotive situation.

For the last two posts (#1702, #1701), I’ve been getting my mind around the fact that there’s no warranty on the EV range of a Prius Prime.  The more deeply I dug into this, the more appalled I got.  Briefly:

  1. Most people buy this car, instead of a standard Prius, specifically because the car can be driven as an EV for a considerable distance (25 miles, per EPA).
  2. But Toyota provides no warranty on that key EV capability.  If your EV range drops to zero, but the car still runs as a gas hybrid, tough luck.  (You have to read the “exclusions” section of the warranty document (above) to know that.)
  3. Worse, the owner’s behavior can greatly affect the lifespan of the battery.
  4. Worse still, many of the unchangeable defaults on the Prius Prime are not optimized for best battery life.
  5. The simplest way to use the car — plug it in when  you get home, drive it the next day — is really bad for battery life.
  6. Toyota’s directions on best practices consists of a brief section buried in the middle of the 800-page owner’s manual.

Source:  2021 Prius Prime owner’s manual

In short, the lithium-ion battery in a Prius Prime is an expensive, effectively un-warrantied car part that you, the owner, can easily screw up over time.   The obvious default consumer behavior — plug it in when you get home, and let it charge — is absolutely the wrong thing to do.  Many of Toyota’s default settings do not optimize the life of the battery, and you have to work around those manually if you want to get best battery life.

This is so out-of-touch with modern automotive engineering that I’ve had a hard time getting my mind around it.  If you want to get the most out of that battery, then you, the owner, have to go out of your way to do that.

Think about it.  When the car needs an oil change, it tells you.  If you run low on oil, it’ll shut itself off to avoid damage.  But if your behavior is quietly cutting years off the life of your lithium-ion battery?  Nada.  It’s entirely on you to figure that out and adjust accordingly.

Discussion Part 2:  YOLO, or once you’ve lost EV range, there’s probably no going back.

Premature battery wear just gets worse when you put it in the context of what should be an extremely-long-lived vehicle.  I’m guessing that as long as the car runs as a gas hybrid, few people will be willing to pay to replace that battery merely to restore full EV function.  Best guess, once that EV capacity is destroyed, it’s gone for good.

First, all other things equal, I would expect these cars to have an extremely long service life.  That’s a consequence of the robustness of electric motors, and the fact that you have both EV and internal-combustion-engine (ICE) power on board.  For example, my wife’s car has about 11K miles on it in a year-and-a-half of use.  But I’m guessing the gas engine has no more than 3K miles on it.  At that rate, that car will hit 150K on the gas engine literally next century. 

I don’t expect it to last that long.  But if our 18-year-old Prius is still running well with 230K on it, I see no reason this car — and many others like it — couldn’t make it to 500K miles.

Extreme car lifetimes are the trend, not the exception.  When I was a kid, odometers only had five digits, because it was almost-unheard-of for a car to make it to 100,000 miles.  You more-or-less expected to need an engine rebuild (“valves and rings”) over that period.  Today, a car that failed with only 100K on the odometer would be considered a lemon.  (Well, surely a Toyota that failed at that point would be.)  So why shouldn’t the next generation of cars kick that up a notch?  Tesla, for example, predicts 300K to 500K service life before the batteries need to be replaced.  I don’t see why Toyota can’t match Tesla in that regard.

My point is, Toyota might consider 150K miles to be “the life of the car”, but I sure don’t.  And I expect that for this particular model, a whole lot of them are going to last much longer than that.  So the question isn’t “will this battery last 10 years”, the minimal question that needs to be asked is, “how’s this going to drive 20 years from now”.

Here’s the final reason you want to take really good care of that battery:  Replacing the battery to restore EV range will not be cost-effective.  If you lose most of your EV range, but the car still runs fine as a gas hybrid, replacing that battery, solely to restore EV range, will almost certainly not pay for itself in fuel savings, for most users.  So, if not for your own use, then for the string of people who will own the car after you, you really want to make the battery last as as possible.

Above, you see how the calculation looks for me, under the assumption that the battery lasts 3000 full charge/discharge cycles.  (Tesla, which uses more-or-less the same cells, originally claimed that their batteries could do 1500 cycles before losing 30% of range.  Real-world data from Tesla suggest slightly better performance: just 10% capacity loss at 200,000 miles (reference), which projects out to about 22% average loss of range over 1500 full charge/discharge cycles.)  This calculation uses my current gas and electricity costs, and grid footprint, and assumes a new battery could be installed for $5K, which is the best rumor I’ve read so far about that cost.

The gas savings from restoring full EV range wouldn’t come close to the (assumed) $5K cost of battery replacement.  Based on that, I’m guessing that as long as the car still runs well as a gas hybrid, lost of most or all EV range will not motivate most owners to re-battery the car.

Edit 2/11/2023:  I grossly underestimate the replacement cost for a Prius Prime lithium-ion battery.  Per this thread on Priuschat, the cost of new Prius Prime battery, from the dealer, is $12,595.  Others suggested the dealer took some markup, as the list price from Toyota is just under $10,000.  I say, potato, potahto.

In round numbers, the cost of a new replacement battery is 43% of the cost of a brand-new Prius Prime, base model, current MSRP $28,770.  As a footnote, literally none were available in North America, and the battery has to be shipped directly from Japan.

I should put in the usual EV-weasel-wording:  By the time the battery dies, there will be plenty of good-used batteries in junkyards, from wrecks.  That did, in fact, happen with the original Prius NiMH hybrid battery.  Plus, there may be much cheaper aftermarket replacements at some point.  And so on.  But right here, right now, what I cited are the hard numbers for battery replacement cost.

Original post follows.

My conclusion is that as far as the Prius Prime battery is concerned, it’s a straight-up case of YOLO.  I expect these cars to last a long time.  And I expect that almost all of them are only ever going to have that original factory battery, no matter how long they last.

So, if you bought this car for the EV capability, the moral of the story is, do what you can to take care of the battery.

Discussion Part 3:  Manual timers, radiant barrier cargo area mat, and other workarounds for unhelpful Toyota defaults.

This last is just a list of things I’ve come across that I wish I could change.  Perhaps some future software update/production change will address some of these issues.

No way to charge to less than 100%.  This is probably the most critical problem.  The default is to charge until the battery is full (100%).  Near as I can tell, there’s no way to change that. 

Other vehicles, such as Tesla, allow the user to charge to less than 100%.  That’s good for battery life.

As it stands, the only way to keep the charge below 100% in a Prime is to interrupt the charge circuit yourself.  I’ve bought a “countdown” timer for this purpose (see prior post).  Based on the car’s state of charge, and with a target of no more than 80% charge, I’ll set the timer manually to stop the charge at roughly the right point.

It doesn’t get more Mickey-Mouse than that.  But Toyota does not provide any way to stop the charge before 100%.

No way to set EV AUTO as default on startup.  The default is hard-coded as EV.  That is, you lock the car into using the battery no matter what.  If you want EV AUTO — so that the car will automatically switch on the gas engine if it’s stressed, rather than withdraw high current from the battery — you have to remember to punch that button every time you start the car.

So I now have a sticky note, on the steering wheel, that says “EV AUTO”, to remind me.  More Mickey-Mousery, but Toyota does not allow you to change the default mode at startup.  Or if they do, I sure haven’t seen it.

No warning for excessive current draw/no native monitor for battery current.  I understand that Toyota set the car up with limits on peak battery current.  Those have to be set to allow adequate emergency acceleration.  The almost certainly are NOT set up to provide peak battery life.

I’d like to have something that lets me know when the car is drawing a high current out of the battery.  Not prevent it, just let me know when that’s happening.

In the past, I’ve had cars that had an “eco” light on the dash.  Push down on the gas too hard, and the light would come on to remind you to back off for better gas mileage.  Or to shift, back in the days of manual transmissions.  That’s all I’d want, really.  Just a little reminder not to drive in such a way as to shorten battery life unnecessarily.

As with the first two, I’m going to have to roll my own if I want that capability.  I assume the current generation of ScanGauge or similar will let me see instantaneous battery current.  So, in effect, I’m going to have to add an aftermarket gauge to the car, because Toyota does not provide that as a native capability.

Edit 1/27/2024:  I bought and plugged in a ScanGauge III, and it works perfectly for this purpose.  (It also lets you check battery temperature, battery fan operation, and other more routine stuff, such as tire pressure.) 

My main observations are that a) for high-current events, the brake pedal is more dangerous than the accelerator (even a moderately hard stop can generate 125 amps of regen current), and b) the “eco” bar on the Prius display is set to encourage you to draw no more than about 50 amps of current on acceleration.  That 50-amp draw works out to a “2C” rate of discharge (the amount which, if you kept it up, would drain the battery in half an hour), a reasonable rule-of-thumb for limiting current draw of a lithium-ion battery.  That also works out to about (350V x 50A = ) 17.5 KW of power, or about 23 horsepower.  Which, in turns, works out to a rate of acceleration that pisses off Northern Virginia drivers, so I routinely push the car over that limit when I’m in traffic.  If left to my own devices, I do what the car tells me to do.

Bottom line is that the eco-meter on the dashboard tells you all you need to know about acceleration.  Obey it if you can.  For braking, though, it’s not helpful.  (Which, when you think about it, is no surprise.)  Absent a ScanGauge or similar, you just have to realize that a heavy foot, at high speed, generates a lot of power.  Per Newton’s laws (Post #1618), to stop the car in a given distance, with minimum peak current, you start with a light foot and press harder as the car slows.   

No radiant barrier, parked-car ventilation system or other summer heat protection.  Toyota specifically warns you not to charge the battery up, then let the car sit in the hot sun.  Which is great, but it would be even better if there were some entirely-passive or partially-passive methods built into the car to limit interior summertime temperatures when parked.

BMW, for example, offers a “parked car ventilation system”, which is exactly what it sounds like (reference).  You can ask the car to run the fan and blow fresh air through the car while it is parked, to keep the temperature down.  Tesla offers a similar function as “cabin overheat protection (reference).  (In addition to a “Dog” setting, which will run the AC when parked.)

You know what’s even more irritating?  Toyota has one too, but it was only offered on the Prius when you got solar panels on the roof.  Toyota literally knows how to do this, already, they just didn’t bother to offer it for the Prius Prime.  Given that summer heat is quite destructive to a lithium-ion battery, you’d think that Toyota could have modified a bit of computer programming to add this already-existing feature to the Prius Prime.

And even more irritating than that?  The Prime is perfectly capable of running the AC for a few minutes before you get into the car.  You can trigger that with the fob, or with the Toyota phone app.  But there’s no way to automate that to (say) keep the interior temperature below 100F.

For my part, I’m at least going to add a sheet of radiant barrier in the cargo area.  Basically, a space blanket, but tougher.  Without getting into the physics of it, as long as there is an air gap on one side or the other, radiant barrier prevents passage of infrared equally well whether the shiny side faces up (into the sunlight) or down (into the cargo compartment).  (Weird but true, which is why I’m not going to get into the physics of it.)  So if the cargo compartment is empty, it would work just about equally well if laminated to the underside of the tonneau cover cloth, or just sitting on the floor of the cargo compartment.

Given the critical role that heat plays in damaging lithium-ion batteries, you’d think that this cheap-and-simple aid would be standard on Prius Primes.  Something as simple as reflective mat for the cargo area.

Edit 1/27/2024:  In the end, a “reflective floor mat” is exactly what I ended up with.  I took a piece of construction radiant barrier and covered the floor mat in the cargo area.  That will work as as radiant barrier as long as that’s open to the air above it, whether or not the tonneau cover is open or closed.  The surface is dull enough that I don’t have problems with reflections showing up on the back glass then the cover is open.

I might go so far as to add one of those stick-in-the-window power vents.  Those always struck me as gimmicks.  But given that heat is bad for the battery, I guess $20 invested in testing one of those may be money well-spent.

Summary

I don’t want to hype the issue of preserving battery life.  The car does a pretty good job of protecting that battery from abuse.  And, at this point, there’s little doubt that most Prius Prime owners are likely to get a satisfactory amount range, over a satisfactory lifetime, for that battery.

But some of what it takes to preserve battery life is up to you.  If you simply plug your car in when you get home, then drive it away fully charged in the morning, you are definitely not doing right by your battery.  Modifying your habits, based on a few simple rules, will go a long way toward preserving the range of your battery as the car ages.