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:
- 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).
- 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.)
- Worse, the owner’s behavior can greatly affect the lifespan of the battery.
- Worse still, many of the unchangeable defaults on the Prius Prime are not optimized for best battery life.
- 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.
- 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.
