A short rant, and some things I'd like to see changed to improve battery life

This is a bit of a rant about some simple things I'd like to see Toyota do to encourage longest lithium-ion battery life in the Prius Prime. PriusChat members are probably the only group of people in the world who might care about that.

So here goes.

Background

I'm a long-time PriusChat member, but I probably have not posted on PriusChat in more than a decade.

I still own a 2005 Prius with the Hymotion PHEV aftermarket kit. So I've been into the concept of a PHEV Prius for a long time.

My wife now owns a 2021 Prius Prime with the lowest trim level. We absolutely love, love, love the car. About 90% of around-town driving is EV, and maybe 70% of overall miles are EV.

Despite having owned the car for close to a year-and-a-half, I only just recently realized that

a) Toyota offers no warranty whatsoever on your EV range,
b) owner behavior can have a profound effect on loss of EV range over time (which Toyota says in its warranty documents), and
c) the simple-and-obvious way to use the car -- just plug it in when you get home -- is about the dumbest thing you can do, from the standpoint of preserving your EV range.

Which, of course, is exactly how I've been using it.

OK, caveat emptor and all that. That's all on me. I'm smart enough to do the research, if required. I didn't do that here. In my defense, I read that Toyota offered a 10-year/150,000 mile battery warranty, and I stopped there. I figured they had me covered.

Some things I'd like to see changed.

1) Include a four-page pamphlet on the care and feeding of your lithium-ion battery.

Right now, the sum total of Toyota's advice for the battery on the 2021 is buried on page 143 of the 800-page brick of an owner's manual. And much of it is worded so obtusely that it's hard to know what the underlying issue is. And I admit, I should have RTFM. But I didn't. And I bet that a whole bunch of other owners never have either.

And, as I said, the easiest thing to do is plug it and forget it. And that's not good for battery life.

Just four pages of cartoon do's and don'ts. How much trouble would that have been, given that those who buy this model buy it specifically for that big battery and the EV capability?

2) Allow the user to charge to just 80% or 90%.

I know the Prime has a 15% buffer at the high end. Despite that, you can look at National Renewable Energy Labs research on EVs to see that charging to that nominal 100% (really 85%) greatly shortens battery life. Teslas lets the driver charge to 90%. Why not Toyota?

As it stands, I've resorted to putting a $10 count-down timer in the charge circuit, to stop the charge after an hour or two. I shouldn't have to resort to that kind of Mickey-Mouse stuff to keep the car from charging to a nominal 100%.

3) Add parked-car ventilation.

Heat kills lithium-ion batteries. Toyota itself says: don't charge it and park it in the sun. Which is great, but how about giving us some tools to routinely avoid high cabin heat in the summer? (Yeah, other than "roll down the windows", I get that.)

Lots of cars have some variation on parked-car ventilation. They'll run the fan to vent the car when it's parked in the sun. Tesla, BMW have that. And, in fact, the Toyota Prius model with the solar panels on the roof had that. So Toyota can clearly do this.

Let alone Dog mode on the AC, when parked. Teslas have Dog mode. A Prime battery isn't as big as a Tesla, but puppy mode, maybe? Just something to keep the interior from getting into the 120F and up range.

For my part, I'm adding a piece of radiant barrier (think "space blanket") above the cargo-area floor mat. Better than nothing.

4) Allow EV AUTO by default.

High-current events are bad for the battery. Looking at other PHEV forums, a common piece of advice is to avoid highway driving in EV mode, due to the high current draw. All Toyota says is to avoid extended periods near the EV speed limit. My own calculation is that even moderate acceleration at highway speed may draw as much current asfairly abrupt maneuvers at slower speed, due to the simple physics that power = force x speed.

Anyway, I'd like the car to be able to kick itself out of EV mode if it gets stressed. And the only way I can ensure that is to remember to flip it to EV AUTO when I start the car. I don't see why I couldn't have the option to set EV AUTO as default instead of EV.

5) High-current warning light

In the same vein, it would be nice if there were a little light on the dash telling me that I was doing something that was resulting in a high current drain. Clearly, Toyota set limits on current drain, but those had to be high enough to let the car do emergency maneuvers.

The analogy here is the little "eco" light (really, a vacuum-gauge light) that manufacturers put in cars a few decades back. It's purpose was to tell you not to drive with a lead foot and/or shift the car at the appropriate time.

The "eco-meter" on the MID provides excellent guidance on getting maximum mileage out of the car. It would be nice to have just some tiny little reminder to help get maximum life out of the battery.

Summary

I expect these Prius Primes to last a really long time. Not just because I'm a Toyota fanboy, but because electric motors are robust and they spare wear-and-tear on the ICE. I've calculated that it'll be into the next century before my wife has 150K on the ICE.

And, it's probably not going to be cost-effective for most people to replace a Prime lithium battery merely due to range loss. I've done that calculation separately on my blog, savemaple.org. So,once that range is gone, it's gone until the battery fails completely.

So, if not for yourself, then for the string of people who will own your car after you, making small changes that can have a material impact on battery life should be a good thing. Toyota should help the average user to do that.

Thanks for listening.

 

OK, thank you all for your responses, but I guess I've just been forcefully reminded of why I stopped posting here.

Purely FYI -- just laugh it off --- the first graph below shows what the National Renewable Energy Lab says, regarding the simple practice of charging to 100%. In this case, the modeled lithium battery pack has a 10% reserve, unlike the Prime's 15% reserve. Just relabel the curves by 5% to see what NREL thinks the difference in rate of degradation may be. Annotations in red are mine.




Plausibly, real-world experience may trump what the NREL says. But those folks are pretty smart.

That said, best I can tell, the chemistry of the cells in the Prius is the same NCA/C as those in the current Tesla. Both are made by Panasonic. So that analysis of Tesla loss-of-range over time should be informative of Prime loss-of-range, but only more-so, owing to the higher number of full charge/discharge cycles per EV mile.

Second graph is what Tesla shows, plus-or-minus my annotations in red, which I used in my blog.



So Teslas, real-world, using what I think is roughly the same cell as the Prime, show substantial variation in range loss. Possibly that is random, but all evidence I can find points to the contrary.

I guess I am going to keep my own counsel on this issue.

Again, thank you for your consideration.

 

I thank you for this -- didn't even realize that something like this was possible, and I will come back to this -- but the main drawback of the Hymotion retrofit wasn't the battery, it was the 2005 Prius.

Basically, you've only got something like 35 or 40 horsepower before the car kicks into ICE mode, in the 2005. Worse, the upper speed limit for pure EV mode is something like 32 MPH. It's a challenge to drive it to get the most out of the battery, to say the least. And if you just drive it willy-nilly (i.e., in ICE mode), you don't seem to get a lot out of the bigger battery, in terms of improved mileage.

Really, at this point, my main concern is getting rid of that big LiFePO battery. Nobody wants it. I think I could pay the folks who installed it to take it out.

Otherwise, adding that to the car has made it really hard to find a place that will take it for scrap. Last time I checked, nobody wanted a big EV battery. Sure hope that's changing.

 

We've had the car for less than18 months,and to this point, any range loss is indistinguishable from normal daily variation. Might have lost a few percent. But I could not reliably detect that from what I can see on the dashboard.

This is an issue for the longer run. In the EV graph above, time to failure (defined as 20% range loss, for that analysis),was 8 years versus about 13 years, based on the driver's charging behavior. (To be clear, all full EVs on that graph traveled the same annual distance.)

So, sure, if you trade the car every three years, per the comment above, this is going to be somebody else's problem.

The thing is that a) the average driver, looking at the dash over time, really isn't going to realize that range has fallen until enough damage accumulated, and b) I bet a lot of drivers will just chalk that up to fate, and never realize that certain behaviors systematically and materially shorten battery life Which is why I think Toyota ought to include a "care and feeding" pamphlet with the car.

I mean, this is a forum for enthusiasts, right? How many of you recall reading Toyota's warning, in the owner's manual, not to charge the car and park it in the sun? How many read and recall the instruction to routinely charge the car to 100% only immediately before you use it?

For the second point about warranty, I don't see the logical connection.

All EVs warranty their range. The Federal government requires at least an 8-year, 100K mile warranty against failure of EV batteries. But that only applies to full EVs. Manufacturers decide what "failure" means. For Tesla, it's more than 30% range loss. For VW and many others, it's more than 25% of range loss.

But EVs don't warranty your miles per KWH, the EV analog of mileage.

Anyway, this is why I was floored when I found out that, while Toyota touts its 10-year, 150,000-mile warranty (the California standard, instead of the U.S. standard), you have to read a separate section of the warranty information to see that the offer zero warranty on range.

I figured I would see what other manufacturers do.

Hyundai Plug-In Sante Fe warranty seems to specify no more than 30% range loss in 10 years/100K miles. Like Tesla. Although the document is so oddly worded that I'm not sure if that's the warranty only for an EV battery, or for a PHEV battery. It could be that they don't offer a PHEV warranty either, because they say something that implies that later in the document.






By contrast, Ford Escape plug-in has no warranty on range, like Toyota.



The more I dig into this, the more I think that maybe nobody offers a warranty for PHEV range. After, PHEV is really hard on a battery. Day to day, the typical Prime driver does more or less what the typical Tesla driver does, with a battery about one-tenth the size.

All those various owner's manuals come with tips on how to preserve battery life.

I think maybe the most useful way for me to finish up here is to compile a bunch of those, and see what other PHEV producers are directing their owners to do.

 

Well, you have a point, but you're only partly right.

if this were a hybrid battery, point taken. That's fully under the control of Toyota's algorithms, and absent extreme behavior, there's not much the user can do that will affect that battery.

But that's only partly true for the PHEV battery. And that's really my point. There are a lot of things about use of the PHEV battery that are not under the car's control and that plausibly have a significant impact on battery life.

So, for the Prime, as opposed to the Prius, not only does Toyota's software have to run the charge cycle well, the owner has to RTFM and follow the instructions. And I suspect that a lot of folks don't do that. I know I sure didn't.

Here's an example. Toyota tells you to use the scheduling software so that the battery is charged to 100% only just prior to use. I.e.,don't be in the habit of letting it sit around fully charged. See NREL graph above, for why. But nothing prevents you, the user, from getting in the habit of just plugging the car in every time you get home. Which then leaves it in a high state-of-charge a lot of the time. I don't have a fixed schedule, so the charge scheduling software doesn't help me. So that's what I was doing.

Anyway, the more I look into this, the more consumer confusion I see, right down to the question of whether or not any particular PHEV has a "capacity warranty". From what I'm reading, most of the people who have bought PHEVs do not know whether or not they have a warranty on the PHEV range.

Really, the only thing different about Toyota is that their warranty information is written clearly enough that you can understand it.

Turns out that only one PHEV seller actually warranties the range: Volvo. They'll fix your car if you lose more than 30% of PHEV range in eight years. Says so in black and white in their warranty documents.

Otherwise, nada. Staring from a list of all 2022 model year PHEVs, I looked at least one model from each of these manufacturers, looking for any explicit mention of a capacity. All of them either specifically said that range was not under warranty, or said nothing about any warranty of PHEV range, thus by default offering no explicit warranty of PHEV range:

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

(Upon reflection, I'm pretty sure that I misread the Hyundai warranty above, and that the capacity warranty is only for the EV, not the PHEV.)

Am I the only one who finds that odd? You buy it for the PHEV function and (with one exception) nobody will warranty that function, at all, period, not even one little bit?

On the one hand, maybe lithium-ion cells are so bulletproof that nobody figured they had to offer a range warranty. But, for a whole lot of reasons, I don't think that explanation flies. First, we already know they aren't, as any quick internet search will tell you. And second, if they were, why would manufacturers lose an opportunity to brag about their quality, with a good PHEV capacity warranty?

Instead, I think the explanation is exactly what Toyota says in the warranty.




Toyota seems to think that you can drastically (their word) affect capacity reduction based on how you drive and charge the car. I was not aware of that until I actually read all parts of the warranty and the relevant section of the owner's manual.

Maybe, as you say, it's a non-issue. But I think I see a lot of things pointing in the opposite direction. The near-universal lack of range warranty for PHEVs.. The fact that Tesla (using Panasonic nickel-alumimum-cobalt cells, just like the Prime) prefers that owners charge to 90%. Toyota's use of "drastically" above. NREL simulations that are supposed to be realistic models of what happens during actual in-vehice use.

At this point, I guess I've said what I came here to say.

Possibly, I'm just being a fussy old man about this.

If it's not an issue for you, it's not an issue for you. Think no more of it.

 

On the graph, no, the maximum state of charge the car will allow (SOCmax) for that simulation was 90%. The line labeled 100% of SOCmax is a battery literally charged to 90% of true capacity. Charging the Prius to 100% would correspond to the 95% line. And charging it to 80% -- seems to be a typical rule-of-thumb -- would be one line off the chart. That NREL simulation still suggests that this one practice -- keeping away from (what the car tells you is) 100% charged -- would delay crossing 20% capacity loss threshold by years.

I seriously don't think this is an idle question. Just a two-minute search here, I found one guy who reported 15% loss after 4 years in a PIP. mmmodem, in this thread: Prius Prime Battery Life Cycles | PriusChat

My understanding now is that's probably not just bad luck. Or, at least, on average, drivers in certain conditions (very hot areas) and with certain habits will be far more likely to experience rapid loss of range.

Otherwise, I think the moral of the story is that if you don't need the full range to satisfy your driving, don't charge it full. You'll get a lot more miles out of the battery, before serious range loss sets in, if you do that. That's all. And, secondarily, don't discharge it deeply, which is what shows up at the end of the graph you've labeled. Both of which seem to be standard advice for any reputable site discussing lithium-ion battery life.

As for 20%, you must be misreading something. Those lines are pretty widely spaced for most of that graph, until you get to the deep discharge section. I.e., still pretty widely spaced at 50% average depth of discharge.

After that, I kind of lose it. I don't see where 30% range loss is coming from. The chart tells you years until 20% range loss. If you're saying that falling from 11 to 9 in a hard-driven PIP (charge daily to 100%, discharge fully every day) is plausible, then we're kind of in agreement.

And sure, with the shorter range of the PIP, this may not be very practical advice.

But the Prime has a nominal 25 mile range, and many people (incuding me) get mid-30s with low-speed city or suburban driving.. So for folks like that, if they have enough capacity that they can back off 100% charge/discharge cycles and still get their driving done, they'd be well advised to do so.

And, once again, this is just standard advice that all the lithium-ion gurus already know. I just didn't know it. So I figured I'd try to pass the word.

Not at all clear that I've done that well.

Anyway, if you want to see some really eye-popping charts, flip through the entire NREL report. Plenty of other driver-controllable factors affect range loss.

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

The only punchline here is that if you don't have to use the full capacity of the battery, it's probably better for the life of the battery if you don't.