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.
