In a nutshell: Toyota offers no warranty whatsoever on the EV range of a Prius Prime. After doing a bit of calculation, I’ve come to the conclusion that’s probably because they couldn’t. Odds are, for some of these Prius Primes, the EV range will be greatly reduced long before the car is ready for the scrap yard.
Now that I’ve reviewed the basics, I think you could plausibly see two- or three-fold difference in battery life, across users, depending on their habits and climate.
I go over five key habits in the final section.
To summarize:
Want to kill your battery? Routinely charge it to 100% and discharge it all the way down to 0%. Leave it 100% charged for long periods of time, ideally, while letting the car roast in sun. Accelerate with a wide-open throttle and stomp on the brakes to stop. And do a lot of high-speed highway driving in EV mode.
Want your battery to live a long and fulfilling life? Stop your charge well below 100%. Only discharge the battery part-way before you recharge it. Keep the car and battery cool. Drive gently, and use the gas engine when you’re on the highway.
In terms of the core question — how long should I expect my wife’s Prius Prime battery to last — I still don’t know. If I do a crude extrapolation based on a Tesla battery (with same cell chemistry and manufacturer as the Prius Prime battery), I come up with a shockingly short lifespan. Something like an expected 40% loss of range after 30,000 electrical miles. And yet, my wife’s car seems to show no appreciable loss of range after about 8000 electrical miles. So something about the crude comparison isn’t right. I just have no idea what it is.
Edit 9/29/2024: The salad days of 35-mile EV range (under the right conditions), in my wife’s 2021 Prius Prime, are now firmly in the rear-view-mirror. Range took a nosedive last winter, and seems to have stayed down ever since. The last time I drove that car, I estimated a full-to-empty-battery EV range between 20 and 25 miles. (We got high-30s on average when new. The EPA-rated range of the battery when new is 25 miles, but the EPA drive cycle is far more stringent than the around-the-‘burbs driving that accounts for the majority of this car’s use.)
As to why this happened, I have no good answer. My fear is that this is just normal wear-and-tear. Range dropped at one point, and now appears to be stable at, say, 2/3rds of what it was when new. As the EV-usable portion of the battery is only about 60% of battery capacity (accounting for buffers for 100% charge, 0% charge, and hybrid use), a one-third decline in 60% of the battery capacity is algebraically equivalent to about a 20% decline in total battery capacity. The car has 19K miles on it, I’d estimate 75% electrical miles (which is also what I get when I take total miles and net out an estimate of gas-powered miles on an average of three tanks of gas per year), which means that loss occurred in about 15,000 electrically-driven miles.
Which, unfortunately, puts it spot-on with my Tesla-based estimate from two years ago, just above. (For details, see “The crude comparison falls flat on its face”, below. Originally, I dismissed the estimate I got by extrapolating from known expected battery life for a Tesla as being implausibly short. Now, I’m not so sure I was that far off. So, FWIW, and crudely done, an estimated 20% loss total battery capacity, at around 15,000 miles is, in fact, halfway to the projected 40% loss at 30,000 electrical miles, which I arrived at by starting from the stated lifetime (2000 full charge cycles) for Tesla batteries, where those Tesla batteries appear to have the same battery chemistry and manufacturer as the Prius Prime battery.
The good news is that the range dropped, all at once, but has stabilized since. Maybe something catastrophic happened last winter, producing a one-time large decline in range, but no error codes or warning lights. But my bet is that the car was simply programmed to show as little loss as possible early on, as a consumer-satisfaction measure. Best guess, that sudden one-time drop in range doesn’t mean that range will sink like a stone from now on. I’m betting that it just means that the software clicked past some threshold, and all the previously-hidden range decline is now visible to the driver.
But arguing against that, nothing I could see about the state of battery charge, using a ScanGauge 3, suggested anything of the sort. So this mythical “software threshold” may be a figment of my imagination as I try to explain away the sudden, seemingly one-point-in-time, steep range loss.
Bottom line is that we lost a chunk of range, all at once, and I have no good idea why that happened.
Edit 10/19/2023: After more than two years now, my wife’s Prius Prime still shows no noticeable loss of EV capacity. We consistently get 36 to 40 miles of EV range (AC/heat off). (That’s much better than the EPA rating of 25 miles of EV range, but all of our EV driving is suburban-low-speed driving.)
My point is, don’t take this post as a slam on Toyota. Car companies typically offer no range warranty for their PHEVs (Volvo being the only clear exception I’ve come across so far.) See Post #1707 for the long list of car companies that don’t offer a range warranty on their PHEVs.
The well-known reality of lithium-ion batteries is this: You can kill them if you abuse them. And hey, guess what, that applies to all lithium-ion batteries, including the ones in your car. Your car’s battery management system will do its best to stop you from killing your batteries. But it can’t do everything.
It’s up to the driver to avoid doing things that shorten battery life. For real. No kidding. As-reflected in the (lack of) range warranty. That’s the only point of this post.
Why Toyota couldn’t provide a four-page leaflet on the care and feeding of your lithium-ion battery, I have no clue. Because I knew none of this stuff, above, before this latest deep dive. In fact, many of the default settings on the car are not optimized for good battery life and can’t be changed. Likely, the Toyota battery management system guards against the worst of your habits. Still, if you want the battery to last as long as possible, you need to get into the habits that will do that.
Background
Yesterday I set off to do a summary of electric vehicle (EV) battery lifetime. What’s out there now, and what’s in the pipeline.
But I got sidetracked on what I thought was a simple explanation of EV battery warranties.
Because, we all know how car warranties work, right? A part gets covered for the lesser of so-many-years/so-many miles. E.g., a five-year, 50,000 mile warranty. The part is covered until you hit five years, or 50,000 miles, whichever comes first.
The Federal government requires manufacturers to offer at least an 8 year, 100,000 mile warranty on EV batteries. Some manufacturers offer more than that. If you experience a battery failure with in the warranty period, the manufacturer has to replace it.
But there’s a catch: Each manufacturer gets to define what “battery failure” means. For Tesla, that’s a 30% loss of range. For VW and Nissan, it’s 25% loss of range. And so on.
This makes an EV battery warranty materially different from a typical car warranty. It’s not enough to know the standard years/miles limits of the warranty period. In addition, you have to know how your manufacturer defines battery failure.
A normal warranty has two parts: Years and miles. An EV battery has three parts: Years, miles, and definition of “battery failure”.
It’s not as if I didn’t look up the battery warranty before we bought the car. I did. I saw Toyota’s 10 year/150,000 mile warranty on the battery, and I stopped there. I assumed Toyota could offer a great warranty because, somehow, they’d developed a great battery. I took that warranty as a signal of outstanding durability of their product.
Wrong. Probably wrong on all counts.
I engaged in fuzzy-think instead of facts. I kinda sorta assumed that because the defining characteristic if this more-expensive Prius model it functions as an EV, any capacity-loss guarantee would kinda sorta look like what other EVs got. Kinda? Sorta?
Yesterday I got the hard facts, and they were not nice. Toyota offers no warranty whatsoever on the EV range of that car. You have to look past the year/mile warranty information, to the “What is not covered”
And there you find this, emphasis Toyota’s:
Reduction of lithium-ion battery capacity is NOT covered under warranty.
Not a reduced warranty. Not, say, the same warranty as the gas drivetrain. Zero. No warrant whatsoever on the EV range of the Prius Prime.
Worse, after mulling this over a bit, I’m pretty sure I know why Toyota offers no warranty. They probably can’t. In part, that’s for the reason that Toyota states in the warranty document — that battery degradation is affected by many factors. Sure, that’s true, but how hard you drive your car affects engine wear-and-tear, too. But in addition, I suspect they can’t because even with “normal” use, there’s a good chance that battery will be significantly degraded by the time that 10-year battery warranty period is up.
Toyota advertises that this battery is expected to last the life of the car. And, it probably will. It’ll probably be good enough to act as the hybrid traction battery for the gas engine for far more than that 10-year warranty period. But if you just charge it and drive it, as you would naively do, there seems to be a good chance that it will lose much of its EV range, in the meantime.
How long do they last?
This is a simple question: What is the expected life of the battery in the Prius Prime?
Turns out that:
- This is vastly more complicated than I thought.
- Simple answers of “N charge/discharge cycles” are highly incomplete, and assume all kinds of things about how those batteries were treated.
- Even those simple charge/discharge counts, under laboratory conditions, are highly dependent on exactly how the charging and discharging was done.
- As I read it, you can do a lot to prolong lithium ion car battery life.
- And you can do a lot to shorten it, as well.
Let me put up an example, to illustrate the issue.
Discharging a lithium-battery shallowly — to less than its full capacity — greatly increases total amount of electricity it can delivery over its lifetime. Below, I’ve done the math and put the number of full-charge/discharge equivalents in red. In this example, shallow discharges — keeping the discharge to just 40% of total battery capacity — means doubling the total lifetime use of the battery.
Source: Battery university.
This is only an illustration because you cannot literally charge your PHEV battery to 100% state of charge, or discharge it to zero. The car prevents that, as a way of protecting the battery from its owner. Best guess, when the Prius Prime shows 100% fully charged, the battery is actually somewhere around 85% full. And when it shows fully discharged for EV driving, it still has the reserve that it uses as the hybrid battery.
You should take a clue from the car’s battery management system. Toyota keeps the true state of charge far away from the extremes, and bars you from literally using the full range of the battery. That’s to ensure a reasonably good battery life. The more you follow in those footsteps by, say, using shallow charge/discharge cycles, the less wear-and-tear you will put on the battery.
Caveat: It’s not possible to quantify how much you can actually extend the life of a Prius Prime battery, from the graph above. The reason is that the chart above — and some of the ones below — come from lab tests that don’t have the safeguards already built into the Prius Prime battery management system. For example, on the chart above, 100% state-of-charge is literally that. There is no 15% buffer, as there would be in a Prius Prime. For that reason, many of these lab findings will exaggerate the benefits that you can get in a Prius Prime, precisely because the car already protects itself to a considerable degree.
That said, these lab tests show you the factors that will hasten the death of your battery. And, by inference, the practices that will increase its life. And they are enough to show you that these factors can have a profound impact on battery life. Even a battery with built-in protections, such as the Prius Prime battery.
So when the Toyota warranty document said this, emphasis mine:
The extent at which capacity is reduced changes drastically depending on the environment (ambient temperature, etc.) and usage conditions such as how the vehicle is driven and how the lithium-ion battery is charged.
Source: 2021 Toyota Prius Prime warranty documents.
That’s not just some legalese Toyota put into the manual to forestall lawsuits. Near as I can tell, that’s a pretty good statement of the facts, including the use of the word “drastically”.
OK, YMMV. What’s your best guess?
I understand that YMMV.
I get the fact that the future will be a battery paradise. Toyota is planning on limited introduction of its next-generation sulfur/solid state batteries in 2025 (reference). Which is fine and dandy. Those will out-perform the current generation of lithium-ion batteries.
But I’m stuck in the here-and-now. I’d like to have some idea what, exactly, is in the lithium ion battery in my wife’s 2021 Prius Prime. And what I can reasonably expect going forward.
There is remarkably little hard data on that.
Below, a Toyota slide presentation shows that the Prius Prime battery is modesty better than the battery that went into the original Plug-in Prius, and that neither battery will retain 90% of range after ten years. (Note that they have specifically prevented any interpretation of the vertical scale of the graph.)
Source: Toyota’s Battery Development and Supply, Masahiko Maeda, Chief Technology Officer, Toyota Motor Corporation, September 7, 2021
But that’s it. In terms of hard data, from Toyota, I think there is literally nothing available to the public. I could be wrong, but I surely found none. So I’m going to have to take a guess.
First, what type of lithium-ion cells do I own, exactly?
Luckily, it appears that Toyota lithium-ion cells are the same as those used by Tesla. More-or-less. It appears that the current generation of Prius Prime uses batteries made by Panasonic. Which, apparently, also makes (or at least, made) the batteries for Tesla (reference). That is confirmed in this detailed discussion of battery chemistry, where both Tesla and Toyota (and others) are listed as using the same NCA/C chemistry (i.e., one electrode is nickel-cobalt-aluminum, the other is carbon)(Google reference).
(I want to be able to find that last one again, so the formal reference is: Houache, M.S.E.; Yim, C.-H.; Karkar, Z.; Abu-Lebdeh, Y. On the Current and Future Outlook of Battery Chemistries for Electric Vehicles—Mini Review. Batteries 2022, 8, 70. https://doi.org/10.3390/batteries8070070 )
In all likelihood, the cells in the Prius Prime are functionally identical to the cells in a Tesla, at least in terms of battery chemistry, and in terms of manufacturer.
That’s handy, because Tesla at least bragged about expected number of charge/discharge cycles. And that’s probably the best information I’m going to get, as a guideline to how I would expect the Prius Prime battery to behave under normal (naive, no-special-precautions-taken) use.
How long do Tesla batteries last?
Most of the scholarly (lab research) data on those Panasonic NCA/C batteries is unhelpful for estimating the actual in-car battery life of those cells. Based on a smattering of papers, it appears to be standard practice to cycle the cells from literal 100% state-of-charge to literal 0%. I know that’s bad for the battery, and, correspondingly, these studies typically show horrendously quick degradation. (Which, makes sense, as that means a shorter overall experiment time, and they are only looking for relative effects, e.g., how much does overcharging shorten battery life. In effect, in lab tests, researchers are trying to destroy the battery. It’s just a question of what conditions kill them quicker.)
The upshot is that if you mistakenly look at lab studies of those Panasonic batteries, you’d expect to get fewer than 800 charge/discharge cycles before losing 25% of capacity. Gotta tell you, I had a bit of a heart-stopping moment when I did the math and realized this chart was telling me that a mere 800 cycles would cut 25% off the battery capacity. (See below, that would leverage in the Prime to about a 40% cut in EV range.)
Source: Kemeny, M.; Ondrejka, P.; Mikolasek, M. Comprehensive Degradation Analysis of NCA Li-Ion Batteries via Methods of Electrochemical Characterisation for Various Stress-Inducing Scenarios. Batteries 2023, 9, 33. https://doi.org/10.3390/batteries9010033
But that’s not what car battery management systems do. They don’t discharge to literal zero state-of-charge, nor do they charge to literal 100% state-of-charge. When your car tells you zero, there’s actually some significant battery capacity left. When it tells you the battery is fully charged, ditto. In particular, the Prius maintains quite a large buffer at both high- and low-charge states. It does this specifically to allow the battery to last longer (reference PriusChat).
And, as it turns out, that’s just the tip of the iceberg. That is, keeping the charge range between (say) 15% and 85% of the full capacity of the battery is just one of many factors that can radically affect battery life. That is, affect the total amount of electricity that you can withdraw from that battery over its lifetime.
Arguably, once you assume that the car will do its best not to under-charge or over-charge the batteries, and will keep discharge rates within some reasonable range, then the two most important factors affecting lithium-ion battery life, are battery temperature and battery state-of-charge. Moderation is the key. You want to avoid high temperature, and avoid extreme states of charge. As the Toyota manual says, one of the worst things you can do is charge that battery to 100%, then let it bake in the hot sun.
The upshot is that anything you read about battery life — either in terms of total charge/discharge cycles, or years — assumes some sort of pattern of use, and charge, and discharge. It’s typical or average or normal battery life. It has to be based on some notion of what the typical driver does. Or maybe it’s based on some idea set of conditions. It’s hard to say.
The upshot is that the worst possible habit to be in, for a lithium-ion car battery, is to charge it to 100%, then run it down to zero.
And guess what I’ve been typically doing, with my wife’s Prius Prime? Why don’t car manufacturers tell you this when you buy a car with a lithium battery in it? Give you a little pamphlet of tips, or something.
That, apparently, is just the tip of the iceberg, in terms of factors affecting battery life.
But you have to start with something. Some estimate of typical battery life.
The whole topic is a really squishy. The closer I look, the squishier it gets. All I know for sure is that, in 2019, CEO of Tesla tweeted about battery longevity. And, sad but true, that’s about the sum total of hard data I can get my hands on.
That same guy, by the way, also advises you NOT to charge a Tesla to 100%, if you want to get maximum battery life. (reference). Secondarily, being charged to 100% prevents use of regenerative braking, until you’ve discharged the battery a bit.
The crude calculation falls flat on its face.
Edit 9/29/2024: Or does it? See edit at start of post.
Gotta start somewhere. Let me take the Tweet above as Gospel, with the understanding that battery failure for a Tesla means 30% loss of battery capacity.
So, as a first approximation, I’m guessing I’ll lose 30% of battery capacity once I’ve done 1500 full charge/discharge cycles. Same as a Tesla, because it’s more-or-less the exact same Panasonic cell.
At this point, the arithmetic gets pretty simple.
If a Tesla, with a roughly 300-mile pack, under normal driving, can go 300,000 electrical miles before it loses 30% of capacity, then a Prius Prime, with a 30-mile pack, can go roughly … 30,000 electrical miles before it loses 30% of range?
Worse, in a Prius, that’s leveraged. The entire battery pack is 8.8 KWH, but the car reserves the bottom 2 KWH for use as the hybrid battery. If I lose 30% of total battery capacity, then that should reduce the total PHEV range by something closer to 40%.
At this point, I’m going to stop and scratch my head for a bit. Because we’ve already put about 8000 electrical miles on the car (best guess). By this calculation, we should have already lost 10% of our EV range. And I don’t think that’s true.
Based on car’s own range estimate, we appear to have lost little or no battery capacity in the first 8000 electrical miles. Just today, with the heat off, the vehicle estimated that we should get 35 miles on a full charge, given the way we drive. (It’s all suburban low-speed driving, and I pay pretty strict attention to the guidance the car offers regarding acceleration and braking.) That’s about as high a number as we’ve ever gotten.
So this is a puzzler. The crude calculation tells me this ought to be a pretty serious issue. The math seems to suggest much faster battery degradation than we are currently experiencing. But the actual gauge on the car says all remains well.
More research is needed.
A quick summary of EV battery do’s and dont’s
General caveat: Most of the findings below are out of laboratories. They don’t really account for how well your car’s battery management system (BMS) protects your battery. In particular, how well it protect it from your bad habits. So I suspect that what you see below gives you the general idea, but in some cases may exaggerate the impact on real-world, in-the-car battery life.
1: Don’t let the battery sit around fully-charged.
This is one that the BMS can’t save you from. This one’s on you. The Toyota’s owners manual indirectly tells you not to do this. They suggest that you use the charge scheduling software, so that the battery is fully charged just before you need it. But what they really mean is, don’t charge it up and leave it sitting around for long periods of time.
Here’s what the National Renewable Energy Labs found, regarding lithium-ion battery life, and the fraction of time that the battery spends sitting idle, but fully charged. Note: I believe this is supposed to be a real-world, in-the-car estimate of impact on battery life. So this one is no lab-bench exaggeration.
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.
Just ponder what that chart tells you, about the impact that your charging and use behavior may have on battery life. The peak on the top line represents somebody who never leaves that battery sitting around at 100% state of charge, and who only uses shallow discharges. That person gets 22 years of life out of that battery. By contrast, the bottom line represents a user who always leaves the battery fully charged. If they are in the habit discharging the battery deeply, that person gets just 6 years of life out of the battery. The same battery.
Upshot: Don’t plug the car in when you get home, unless you’ve got something in place either to prevent it from going to full charge, or to delay the charge until the next time you’re likely to need the car.
2: Don’t routinely discharge the battery completely.
This one applies to EVs, but not, I suspect, to the Prius Prime. And probably not to other PHEVs as well..
It doesn’t apply because the Toyota uses the last 20% (?or so) of the battery as the hybrid battery. So when your EV range reads zero, you actually have at least 20% of the battery left. You literally can’t drain the battery to any state near true zero.
Nevertheless, the chart below makes the point that deep discharges are bad for battery life. And that’s another habit where the car is completely at the mercy of the user. The BMS can’t nag you into doing shallow (less than full EV range) discharges. But you should do that if you want to prolong battery life. (Note that the chart below is lab-bench data, and so likely exaggerates the impact on actual in-car use.)
Source: Battery university. Annotations in red are mine.
Upshot: Use the full capacity of the battery only when you need it. You don’t drain your gas tank dry before filling up. The same should apply to your battery.
3: Don’t routinely charge it to 100%
Again this one’s on you. The BMS won’t stop you, it only prevents you from committing total battery destruction. It is already programmed to stop well short of a true 100% state-of-charge. (When it shows 100%, you’re probably at 85% of the battery’s true total capacity). Still, e.g., Tesla not only recommend stopping at 90%, that’s the default.
I believe this one represents estimated actual in-car use of the battery. If I’m reading that right, habitually charging to 85% instead of 100% increases battery life by about one-third.
Source: NREL, op. cit.
Upshot: If your car’s software has no provision for stopping short of 100% charge, your only option is to put a timer in the charge circuit to interrupt charging before you reach 100%. In my case, we don’t have a fixed schedule, so we don’t use the Toyota charge-scheduling software. I bought this “countdown” timer for that purpose. We’ll see how it stands up to the 12-amp charging current.
Source: Amazon
4: Avoid high current flows.
Avoid jackrabbit starts. (In a Prius? OK, bunny-rabbit starts. Full-throttle starts). And avoid quick stops, because those dump current into the battery. Basically, obey the advice offered by the eco-meter on the dashboard.
I’m going to add, you should probably avoid driving in EV mode on the highway. I see this stated plainly in on-line groups for other PHEVs. But it’s not clear exactly why that would necessarily be an issue. Toyota’s only guidance in this area is not to drive extended periods of time near the upper speed limit for EV mode.
The reason goes back to Post #1618, There ain’t no disputin’ Sir Isaac Newton. It’s this simple equation, ultimately based on Newton’s laws of motion:
Power = Force x speed.
Since amperage (current) is proportional to power (via the West Virginia law, Watts = Volts x Amps), this means that that a modest rate of acceleration at, say, 75 MPH, is going to draw three times as much current as the same rate of acceleration at, say 25 MPH.
My point is that your senses will fool you. What may feel like modest acceleration and deceleration at may, in fact, call for the same level of current flow as rates of acceleration that you would deem unacceptable at slow speed.
Put this another way. If you follow the guidance of the eco meter, you know what an acceptable rate of acceleration is for speeds around 25 MPH. If you want to limit your power draws to the same level, at 75 MPH, you’re going to have to accelerate and decelerate one-third as quickly.
My point is, cruising down the highway at a reasonable steady speed, on flat ground, is not the issue. The issue is that may feel like the modest acceleration you need on the on-ramp, or when dealing with traffic, may actually be a fairly high-current event.
The only way I’m going to know that for sure is buy another ScanGauge or similar OBDII reader suitable for hybrids, and use that to read instantaneous battery current. Until that time, I’ll be using the gas engine when I’m on the highway.
Anyway, the blue line below is the battery subjected to high currents. The black line is the battery that wasn’t. Even though this is lab-bench data, and may not accurately reflect in-car use, I’d say that’s enough said.
Source: Kemeny, M.; Ondrejka, P.; Mikolasek, M. Comprehensive Degradation Analysis of NCA Li-Ion Batteries via Methods of Electrochemical Characterisation for Various Stress-Inducing Scenarios. Batteries 2023, 9, 33. https://doi.org/10.3390/batteries9010033
Upshot: Drive gently. And be mindful that for any given rate of acceleration or deceleration, power draw increases linearly with speed. Maneuvers on the highway are inherently high-current-draw events.
5: Avoid temperature extremes
To a large degree, your battery management system does this for you. (Unless you drive a Leaf, which does not). Teslas and many other EVs have a liquid-based temperature management system for the battery. The Prius has an air-based system. The Leaf, unique among EVs, has none.
The temperatures below would be the temperature of the battery, not necessarily the temperature of the outside air. For me, living in Virginia, there might be some reason to avoid using the battery at the peak of summer heat. That said, one of the aspects of the Prius Prime that peeves a lot of people is that if the car wants to use the AC when charging, it has to ask your permission, every time. I believe this one is intended to show true impact in actual in-car use.
Source: Via ResearchGate: Rezvanizaniani, Seyed Mohammad & Liu, Zongchang & Chen, Yan & Lee, Jay. (2014). Review and recent advances in battery health monitoring and prognostics technologies for electric vehicle (EV) safety and mobility. Journal of Power Sources. 256. 110–124. 10.1016/j.jpowsour.2014.01.085.
Upshot: The main thing you can do is not park your car in the hot sun. Particularly not with a full charge. The Toyota manual specifically warns against that exact practice. Otherwise, once the car is running, the battery management system does what it does to keep battery temperatures within range. Plausibly, you might get a bit of an edge by avoiding battery use during peak hot and cold seasons. It’s not clear how much that matters.
Radiant barrier for summer? In the distant past, I added a Hymotion plug-in hybrid kit to my wife’s 2005 Prius. The battery for that was located under the cargo area floor, same as the Prius Prime. In the summer, I got in the habit of covering the cargo area with a piece of radiant barrier. It wasn’t a space blanket, but it was functionally equivalent. That prevented much of the radiant energy that entered through the hatch glass from directly warming the floor of the cargo area. But it was a nuisance — it was a great big shiny thing sitting under the back window.
This time, I believe I’m going to laminate a literal space blanket to the bottom side of the fabric of the tonneau cover. In fact, if Toyota is so worried about you cooking that battery in the hot sun, I wonder why they didn’t do that, as standard equipment, on the Prius Prime.
The weird thing about radiant barriers is that putting that on the bottom of the cover — where I can’t see it — will work almost exactly as well as putting it on the top. (Don’t believe me? Note that the radiant barrier in roofing plywood is installed on the inside of the roof, facing the air in the attic. Doesn’t do any good at all if you bury that shiny side under shingles.)
