Hi all! I bought a 2024 Prius Prime about a month and a half ago. I read probably a hundred alarming threads here and on reddit. Problems present in multiple ways, and end up being diagnosed as a flat 12v battery. This thread briefly summarizes that reading and adds my observations. I put a battery monitor on my car a couple days ago and let it sit (no driving, no app use, no keyfob use). I can already see what the issue is (I have it too). It loses about 0.1 v/day if nobody uses the 12v (including opening the Toyota app, which produces a little spike every time you do it). The battery was at 11.7v when I hooked up the monitor. (New car, 800 miles, had 8 when I got it, driven 20 min at a clip 10-12x/week, battery state not checked upon delivery of car, presumably it was pretty dead.) I charged it to full. It floated at 14.35v, and rested at 12.95v with the charger disconnected. It lost 0.1v in 8 hours, then settled in to losing 0.1v every 24 hours. (The experiment ends tomorrow, because I have to go to work.) My takeaways from a lot of reading and a little observation are: 1) The battery in any new Prius Prime is likely damaged by deep discharge already, as they discharge fast, and the cars tend to sit on the lot. Dealers could charge the batteries once a week or leave a $20 solar charger on them... maybe some do. 2) The people posting about dead cars seem to fit into a few groups: infrequent drivers people who use ACC mode the way the manual says it works people who have third party apps polling the car frequently Frequent drivers who have a battery that was severely discharged before they got it, and will be fine once the 12v is replaced. From reading, there's also a group who killed their 12v by using HVAC while charging- but my car does not seem to discharge the 12v for this. Maybe there is a model year difference there? 3) A car starter battery is not well suited to this application in the first place. This car needs 1 cold cranking amp, not 1000, and it needs tolerance to deep discharge. That's called a deep cycle battery (if you want to stay under $200). It can get into the hundreds or low thousands for a better battery than that (for example lifepo4 needs built-in overcharge and discharge protection, plus a temp sensor and an onboard heater). 4) Toyota can check the tire pressure on a parked car and report back on the app, but it can't check the 12v and charge it off the traction battery when it's low? Come on. My wife's car does this exact thing. I've seen a few apologists frame this as an insurmountable engineering challenge, but it's not. Personally I don't expect to have my starter 12v replaced for years, because I am going to keep it topped up, but this isn't the kind of problem I should need to solve, since it has such a simple answer. What do I suggest as a practical work-around? - Monitor your voltage. Bluetooth battery monitors are cheap. Maybe you already have a multimeter. - Charge as needed. This could be an external charger, or just leaving the car in READY. - Carry a jump pack in the car. I also, at the risk of angering the service department, mounted charging leads to the battery so I can plug in a solar panel (spring for one with a charge controller), or a wired battery maintainer (again, make sure it has a charge controller). I put the same plug on both so it's quick to connect both to my charging leads. Mine tucks away nicely under the battery cover.
also, there was a software update that some were able to get done. are there really a lot of primes 'sitting on the lots for a long time'? i thought they were hard to come by.
I measured 330 mA parasitic drain at one point, but I doubt it was always that. The stock battery is 33 Ah, but the car will fail to start when the voltage falls somewhere around 11.5V, I'm not sure exactly, but I have a volt meter in my car, and mine wouldn't start when it was somewhere in the 11V range. This was a surprise to me because with my Gen 3, all I needed was a partial charge to activate the startup sequence to start the car. I used to wonder why the Prius doesn't take some power off the hybrid battery to activate the DC-DC converter to top up the 12V battery, but I can understand their rationale for not doing so. They don't want to risk running the hybrid battery too low to take a charge, and needing an expensive replacement. But this problem could have been easily solved with a simple logic circuit that would decide if the hybrid battery had enough charge to spare to activate the DC-DC converter, providing enough voltage to start the car. But I don't think Toyota wants to make things that easy for us; they'd rather have us go to the dealer to pay for whatever BS they tell us we need for the problem. Like a new battery, or at least a service call. If it wasn't a risk for voiding my warranty, I'd like to see a mod to do what I described above. It would certainly help solve what seems to be a very common problem with the Gen 5 Prius Primes. I'd like to see them take that as a suggestion, but I don't see that happening.
Agree with the basic point that it doesn't need a lot, but "1" is a bit optimistic. I don't know about this generation, but measurements in earlier gens tend to see a second or so of around 30 amps with some spikes to maybe 80 or so, before the DC/DC converter comes online and takes over. Maybe your service department is more easily angered; the guys at my dealership see this any time they dig down to my battery, and they've never given me any grief about it. (ok, they don't actually find a multimeter stuck in there; it's just the pic I had.)
At the dealer I went to, I kind of got the impression that I was getting one from a mothball fleet of 2024s. It had 7 miles on it after they drove it over from the "other lot", so I don't think it had even been on a test drive. They had 4 primes in the trim I wanted, according to the website... not sure how many total. I think it's even crazier now that I'm watching it every day. I drove to lunch in EV mode today, and it 12v lost power all the way. In HV mode, it seems to charge the battery at 14.4v. I'm not sure if it always does this because I don't use HV mode much. In EV mode, it often declines to charge the battery. When it does decide to, it charges at 13.58v. (I have also once observed it using this voltage to charge the 12v while the traction battery was on charge and at 100%.) No idea why it uses 2 different voltages unless it actually has an alternator (which I doubt). My brand new battery was at 11.7v in a car that I was driving 10x a week; there's no excuse for that when it's 24/7 connected to a charger powered by a (relatively) giant traction battery, and a computer that already monitors its voltage and can charge it as needed.
I seem to remember that as a safety feature the HV battery stays disconnected until the computer can perform all its safety checks. Which requires a functioning 12V to boot up. Hmm, my brainstorming side suggests a circuit that can operate with a near-dead 12V and if necessary switch in a long shelf life lithium primary battery to handle the boot sequence. FWIW my '18 puts 14.3 on the DC bus in EV mode, which raises more questions. Why is it more than the recommended float charge voltage, and how does it manage to leave the 12V undercharged anyway?
Even the deep-cycle batteries really don't like to be pulled down beyond a certain point. Especially not repeatedly. I used to work with solar-powered industrial controllers. We used the deep-cycle batteries you talked about. The solar controllers that disconnect the battery once the voltage gets too low would save the battery. Probably not the best situation for a Prius though as you'd lose all the settings and have a hard time getting into the car. It sounds like that Gen 5 has too much of a drain for the small size of the 12V battery. 330 mA is definitely too much of a draw, especially for a 33 Ah Battery. Even a full sized battery won't hold up to that long. With as many people as I've seen on here with the same problem, Toyota has a real issue that they're trying to not have to deal with. Hooking it up to an auxiliary charger seems to be a lot of trouble if you're driving the car at least once a week that should be enough to keep the battery going. Just my opinion. I've only got a 2019 Prime, and had to replace the 12V battery shortly after I got the car (used). I figure the car sat around on a lot, maybe it didn't get disconnected or put on a battery maintainer when they detailed the car. We also sat in the garage on ACC mode for a while figuring out all the features of the car, which also caused the battery to be pulled down one more time. I was able to charge up the battery and it seemed like normal, but my wife had troubles starting the car after work a couple days later, so that was it for that battery. We haven't had any troubles in the year since, but I've got the previous generation.
I talked to an installer about installing a switch to make the 12V socket stay on when you shut off the car. I have a few accessories I'd like to leave on all the time, like my cellphone. He said he wouldn't like to risk it with a 2024 Prius Prime, because he didn't know if it would mess something up. I can't see a problem with installing a separate socket that's live all the time, controlled by a switch. I have a 20A switch with a light in it. I trust myself to turn it off when necessary.
Bulk charging voltage and float voltage are different animals (and vary a lot with temperature). There's a nice writeup that I am not allowed to link (google powerstream sla basics). I can answer the question for my 2024, because I have a bunch of drives and charges logged on my battery monitor now. It manages to leave the battery undercharged because the battery is constantly draining, and it charges randomly and sporadically, and low battery voltage does not seem to be one of the things that triggers it to charge the battery. If you keep the 12v charged manually, the car actually pops up a large error message on the instrument cluster saying that 12v charging is not available due to the 12v already being charged. So when the charging algorithm (randomly) runs when you start the car, the battery being at 12.5v+ is a surprising error condition. The car even has special software to alert the driver about this disturbing turn of events! It's a baffling amount of complexity given how simple the problem is, and the length of time that it has been solved for. I would expect a $40,000 car to be able to match the performance of a $20 two stage battery charger (or, if that's too much to ask, maybe it could at least emulate a car with a dumb alternator and put some voltage on the 12v bus when the car is turned on). The good news is that all the needed hardware is in the car; they just need to sort out the software and tell it to charge the 12v when the 12v needs a charge. The bad news is that there's no indication that anyone in a position to help knows or cares. As for me, I added a dashboard solar panel with a built in charge controller, and I put a lithium jump pack in the space where the spare tire should be as a backup. So while I am baffled by this nonsense from Toyota, it's not going to actually stop me from driving.
It does occur to me that the lawyers could have something to do with this. They put the 12v in the cabin of the car, so maybe the algorithm's first priority is to avoid gassing at all costs. If it starts with the assumption that the cabin is always at 120°F, and the top priority is to avoid offgassing, the once-in-a-while 13.5v charge makes a lot more sense.
Mine's under a little cover in the hatchback area, near the rear passenger side wheel. (That's a 2024 Prime.) If anyone's curious to see some data on this... Here's a workday with battery events annotated. 00:00: The car is parked and on charge. Voltage starts at 12.67v and creeps down to 12.65v. 00:27: The car charges the 12v at 13.59v for about 1h 50m. Then it draws on the battery to do something (??) for 20 minutes. Voltage bonks around in the neighborhood of 12.5v for 20 minutes. (Couldn't you have done this while the traction battery and/or L2 charger was supplying power to the 12v system??) 02:47: The post-charge load stops and voltage goes back up to 12.71v. It slowly drops to 12.69 by.... 04:27! I remote start the car, which is barely even visible on the 12v voltmeter. 04:41: I unplug and start the car in EV mode. Car starts charging the battery at 14.4v. (It usually does this, but not always.) 04:55: Stop for coffee. Charging stops. 04:57: Back to driving. Charging resumes at 14.4v. Battery voltage sits at 12.3v, which is funny given that it was at 12.69 before we charged it. 05:00: Arrive at work. Charging stops. 12v is doing something for ~15 minutes and holds 12.3v, gets back up to a resting voltage of 12.6v. 05:51: ??? Unknown low amperage activity, 10 minutes. Spike down to 12.5, recovers back up to 12.62. 08:15: ??? Unknown activity, 10 minutes. Spike down to 12.39v. Sun comes up around this time and the solar starts charging, so we can't see where the battery levels out. From now until dark, it can be hard to tell if the car is applying load to the battery, or if the sun just went behind a cloud. 10:35: Solar charging has pushed the voltage up to 13.24v. I start the car to go to lunch; it charges at 14.4v. 10:41: Arrive at lunch. Car is doing that whirring servo thing it sometimes does after a drive; battery hits 12.16v with the solar panel providing charge. 11:29: Battery has slowly crept up from 12.16v to 12.94v with a few watts of charge being applied; clearly being driven and parked again is hard on the 12v. I start the car; it charges at 14.4v. 11:33: Arrive back at work. Car is doing that whirring servo thing again; battery hits 12.03v while charging. Being driven a short distance and parked again is clearly pretty hard on the 12v. This vehicle just spent 18 hours on L2 charge, was driven 3 times, and had a 20w solar charger providing a few watts of power directly to the 12v all morning, and it's at 12.03v with the car turned off. 12:05: Sitting quietly and under solar charge, the system voltage recovers to 13v. (Bear in mind, this isn't the voltage that the battery would be resting at if it wasn't being charged; the voltage being supplied by the solar charger is driving the voltage up.) 12:23: Unknown small power draw. 14:49: Unknown larger power draw. 16:37: Unknown small power draw. 17:23: 12.83v. Solar charging is little to none at this point. I start the car and drive home. Car charges the battery at 14.4v for the trip. 18:13: I park the car and plug it in. 12v charging is turned off. Battery starts sliding downhill from 13.08v. 20:00: Voltage has slid from 13.08 to 12.90. Unknown power draw spikes it down to 12.63; it recovers back up to 12.89. 21:15: Unknown power draw drags us down to 12.59; recovers to 12.79. 22:45: Unknown power draw drags us down to 12.62; recovers to 12.76. 23:59: Day ends at 12.75v. ...so clearly the car *does* charge the battery sometimes, including sometimes when we'd expect it to (for example, when the car is turned on and driving, it usually applies 14.4v to the 12v system). Observations: - The battery drains a little all the time, at a rate faster than you'd see from regular lead acid car battery discharge. It has a small load on it all the time. - The car puts some decently large, long loads on the battery without bothering to charge it during the loading on the 12v system, and even without checking it afterwards to see if it's okay. - The car usually charges the 12v at 14.4v when it's in READY, but sometimes not. - The car infrequently charges the 12v at 13.59v when it's not in READY. It does this when it's in the mood; the 12v system being down in the 11.x volt range doesn't trigger a charge. - Every problem observed here can be corrected in software. You don't need a team of engineers and a custom AI to analyze a thousand battery parameters. If the 12v system drops below whatever voltage threshold you want to set, charge it from the traction battery. If the traction battery's too low to charge the 12v, tell the driver that they need to charge one or both of their batteries before the car dies. - Every problem observed here could ALSO be corrected by getting rid of all these loads that we apply to the 12v system when the car is off. Really, Toyota, do we need all that crap to be going on while the car's parked? (And even if we do, could you please put some juice on the 12v bus before you do it? Thanks.) - As a driver, you should carry a jump pack until they fix the 12v charging logic. If you want your 12v battery to live a long life, you should also have a charging routine for it. (Although if you want the problem fixed faster, skip the charging routine and put in lots of warranty claims for new batteries. This will help bring attention to the problem. The rest of us appreciate your sacrifice. )
Interesting observations. I was at the dealer last week while they found and fixed my tyre problem. They admitted the 12V issue and they opened the Prius hood in the showroom and they had it on a trickle charger. Apparently they do this with more Toyotas as the 12V are "too small". This is the picture of their top of the line solar model on a trickle charge in the showroom.
When I got my Prius Prime about a year ago, a dead 12V battery was a very common occurrence. It seemed to happen whenever I left the car parked for more than a day, and once while I left it parked for about 2 hours. I had to use a booster pack. I complained to the dealer, but they said the battery tested "fine" registering 29 Ah, against the factory spec of 33 Ah. I eventually settled on highly variable battery drain, depending on software/firmware updates and such. I may never know the exact cause, but installing a NOCO battery maintainer solved the dead battery problem. My Gen 3 used to engage the DC-DC converter as long as there was enough power to enter READY mode. No such function with my Gen 5. Somewhere around 11.5V, the system refuses to start, and displays a message saying something like "battery level too low." A boost or a service call to the dealer are the only options. I'm sure plenty of batteries were replaced to no avail. I'd like to see a mod that would charge the 12 V battery from energy taken from the hybrid battery, but only when the hybrid battery could safely spare it. As designed, there is no way to access power from the hybrid battery, once the 12V battery runs too low. This is likely to prevent the hybrid battery being drained too low, but it could easily be protected with simple circuitry. If the hybrid battery is capable of providing a small boost to the 12V system, it should be able to do so, but only when the hybrid battery has a sufficient charge to do so. It need not drain the hybrid battery continuously; only when the car fails to start, due to a flat 12V battery. I'd love to make that suggestion to Toyota, but they don't seem open to such ideas.
The car already does this exact thing. Sometimes. When it feels like it, not when the 12v battery is low. If you look at the graph I posted, the car is doing this exact thing, by itself, from 00:27 to 02:17. And then, as soon as it's done charging, it does some battery-draining thing for 20 minutes and beats the battery back down to 12.5v. This is a software problem which is causing a hardware problem. Toyota could fix it in everybody's car tomorrow without a recall or service appointment if they cared to do so.
If you'll look closely at the picture, you'll note that there is no battery visible. The battery itself is in the back of the car, and there's room to put a bigger one in that spot (which I wouldn't bother doing, because the problem isn't the size of the battery). As far as it being "too small": A bigger battery will just have the same problems a little slower. The problem is that the car is putting a lot of different loads on the battery without charging it.
If you'll look closely at the picture, you'll note that there is no battery visible. Well yes, they hooked up to the connector in the fuse box which is the first place they look to jump the car. It can be that the trunk cannot be opened if the 12V is flat. Toyota should face the issue instead of convoluted design arounds. I am beginning to think whoever runs the 12V team is related to the Chairman of the Board. It must be costing Toyota a fortune with all the call outs and replacements.
