Okay. Who HASN'T had a 12V battery issue with their 2023-24 Prime?

*Knock on wood* but I've never had 12V battery issues. I leave my Prime plugged in overnight on a level 1 charger even after it's done charging.

 

No issues here. This thread needs a poll.

 

I am guessing 99% haven't.

As long as you do the following, you will be OK:

• Do not keep the AC charger plugged in for longer than overnight.
• Do not sit in the car unless it is in the READY mode or it is entirely off with all the lights off.
• Do regular long trips.

 

Addressing only the issue of “why not leave it plugged in after it’s done charging?”, wouldn’t it be better to ask “why leave it at 100% for long periods of time?”

The problem with leaving it always plugged in isn’t just the health of the 12v auxiliary battery – it’s also that it isn’t the healthiest thing for the traction battery. The ideal method is to charge it before you need to use it and to just take it easy in between uses.

The manual says:

It’s probably better to charge the car when convenient or when prices are good. Optimizing for a full battery at all times, or keeping it plugged in constantly, isn’t necessarily the healthiest for any part of the car. If you’re driving it more than 2/3 EV, it’s going to be cost-effective and a good balance for the car, but it doesn’t need to be charged daily unless you are using it daily. Your ideal situation is only having it at 100% right when you are about to drive it, not leaving it at 100% in case you need to drive it.

 

Design flaw or not, it is true, as it is stated in the owner's manual, because it uses power from the 12-V battery to run system checks when plugged in.

 

Correct. The PHEV battery should never be left at higher than 60–70% SOC for longer than several hours. As you said, if you are going to fully charge the battery, it should ideally be done immediately before you drive. Charge schedule can be used for that purpose if you want to charge overnight as you said.

While the owner's manual says leave it at 0% (or a very low) SOC, I believe the ideal SOC for long-term storage is about 25% SOC. Other than that SOC being optimal for battery longevity, it would also avoid the possibility of the battery going entirely dead and possibly damaged if the car is left undriven for a year or longer.

 

I lost power on my 12 volt as I passed the 1,000 mile mark on my 2024 Prius Prime and won’t bore you with speculation as to why… just the immediate results. The car gives you quite a light show in the MID but never takes you to READY so it cannot be driven. I shut down and went in to read the manual. Knowledge in hand I grabbed my portable battery charger and headed for the battery. The car was now completely dead. Suspect when I thought I shut it down, I didn’t. As a result, I could not open the rear hatch and the rear doors were locked. I went in and opened a rear door from the front. Crawled over two bags of mulch to try to access the emergency hatch latch from the inside. It is behind a small panel that cannot be opened by hand (a problem all by itself). So, with no easy access to the 12 volt battery, I went to plan B. Access battery charge points in the engine compartment as specified in the manual. The two contact points had the + connection on the right side of the engine compartment and the point for the ground (-) on the opposite side of the engine compartment. My black ground cable would not even get close. I was reluctant to select another point closer to the + pin as that is the area where all of the electric motor components are located and didn’t want to fry something. I called Toyota Road Service and they responded quickly. The technician on the truck also had a charger with short cables - had to go retrieve another charger. Thanks to one of our members here, a closer access point that had worked for him was pointed out and I tested it out and it was fine. The saga at the dealership is too long to go in to but the battery test they did showed a bad battery that would power up but would not hold a charge… battery replacement went poorly. None in stock, two wrong ones installed, and at the end of day 2 they got it right. Thanks for the loaner car Toyota.

Bottom line - my battery charger is now permanently stowed in the cargo area and soon I will have a spare tire taking up what little room is back there. The spare will be used on long journeys to Canada and other rural New England areas with no cell service or Toyota Road Service.

What would be helpful here is to assemble a list of items that may cause this phantom battery discharge. The dealership speculated the battery’s weak ability to hold a charge may have been exasperated by my magnetic charger that I had plugged into the front C port; the in-car Toyota phone charger does not work with a Mag Safe cover on my phone. The car was off and no phone was connected so that didn’t fly as a reason. But I have heard that aftermarket dash cams, key fobs stowed too close to the car, and remote starters will impact the 12 volt battery. It would be great to find out what other culprits have been validated so we can all avoid them.

 

Use the charge schedule as we explained.

 

None of the owners of Gen 5 with 12 volt battery issues have figured out why, yet, at least that I've read about here. There is plenty of speculation about what it is and how to deal with it, though !
And each of us has our suspicions about the issue and what advice to give to others who ask about it. So my advice is to take the advise that makes the most sense to you and run with it. It may be right and it may not be. We wouldn't know for sure until we get some hard data about why.

 

It’s up to you! You might find it is less work than you think, but everyone’s situation is different. A charge schedule could help, or maybe just plugging it in half the time.

One thing that I find surprising (because it’s hard to check until I actually drove a PHEV daily) is how few days I need all 100% of the battery. Charging is something I do less frequently than I thought I would, even with only Level 1 charging. I’m still driving the car on EV most of the time, but it’s sitting on a charger less than I would have expected. As a result, it’s rarely sitting at 100% charge.

 

Wow, that's some story! By the way, the Walmart EverStart Maxx 140R might be a better choice than the OEM GS Yuasa, as it has a higher Ah capacity and is probably of higher quality—if you can convince your dealer to install that instead. It has the proper vent hole and red plug as well.

Walmart now carries the Prius Prime battery—EverStart MAXX 140R | PriusChat

 

I’ve had similar issues (documented extensively elsewhere on this forum). I had a battery replacement under warranty a few weeks ago, since my battery did go bad.

Things that I don’t do:

• no long charging sessions
• not driving exclusively or excessively short drives, and do drive longer distances at least weekly (735 miles at first failure, now up to 1300+ miles after a couple of months of ownership)
• no accessories plugged in constantly (a couple of USB plugs are plugged into nothing at all except while driving)
  
• no remote starting – only done once to diagnose the 12v issue
• no dash cam
• no dealer accessories

Things that I have probably ruled out (only for me – anyone else will have to rule these out individually):

• key fob is not kept nearby the car
• key fob is usually turned off to prevent interacting with the car
• dealership says no constant parasitic drain – although what about large spikes of activity when garaged?

Things that it still could be:

• the Toyota app, particularly since my issue started up immediately after the v2.5 app update, and my bluetooth “remote key” kept removing itself
• slow/inadequate 12v charging while driving – let’s say that hypothetically it normally charges up, but on some drives it was under-charging for some hard-to-diagnose reason
• cold weather – not as a sole cause, but as a contributing factor
• bad 12v battery (I guess – I don’t find this convincing)

Things that I am now doing to mitigate any possible issues:

• monitoring any failures or abnormal behavior (smooth sailing for 500+ miles since a new battery)
• documenting my drives so I know how long/short drives are so I can rule that out as a cause
• got a battery maintainer/charger to prevent having to jumpstart my car in case of failure
• got rid of the bluetooth key – it was de-registering in the Toyota app almost daily so I just stopped configuring it

 

How many checked their battery when they died? Like a full cycle testing at some parts store?

I have two dash cams, a USB light always connected, added back footwell and ambient light, I sometimes plug in my car on it's normal Friday schedule and don't use it until Monday morning, other times I use it on weekends and fully use my EV battery weekdays or 90% and recharge the next day. I'm closing in on 20k miles. I normally put my key in sleep mode at night but not always.

I have a battery monitor from the 12v outlet plug where I also power my dash cams and usually battery is around 12.6-12.8 on acc and 14-14.2 on startup.

I don't have to deal with cold currently. I don't know how people wire their dash cams but the issue for the 12V battery seems random.

It would be interesting to see a poll on how many people have had the issue (I've seen maybe 12 so far?) and which trim to see if one trim is more affected than the others.

 

After only 3 months, I plug in the OEM (Level 1) charger every time I'm in the garage.
Charging schedule is 8:30 - 4:30 to take advantage of the solar panels on the roof.
Went over 6 weeks without using any petrol, but longest trips were 17 miles each way out of town.
I cannot imagine problems with either battery -- after 70 years of working with both cars and electronics. The chargers are designed to back off after the battery is fully charged (or, in some cases 80%).

 

Yea, poll would be good. I am sure the folk with issues are real, but are they isolated or systematic. No problems with my Prime that I've had since August.

I have had issues where I've accidentally opened up the car hitting keyfob buttons by accident and not realizing it for hours.

Will

 

Unfortunately I had my battery replaced at 3467 miles. I run on ev everyday. Don’t do many long trips about once every two months. Commute to work is all ev. 40 miles for 3 days a week. Once every two weeks I’ll run hybrid for 5 miles on my way to work. However I do use the scheduled charge at night prior to work. May sit up to 3 hours after full charge still plugged in. On weekends I fully charge on Friday evening and run on EV the entire weekend. Lucky the dealership had a battery in stock. At first they did not find anything wrong until I left it overnight and in morning it was dead again. They said battery could not hold a charge.

 

I periodically leave mine plugged into the AC for a couple days. Haven't had issues. Usually though I drive it every day, and have a charging schedule that usually brings it up to 100% overnight. I would not worry about it so much. Periodically leaving it at 100% charge is also not a disaster. Reduction in battery capacity over time will be greater if you regularly do that. I suspect if your habits are extreme in some way (leaving the key in the car with it plugged, on a regular basis, for long periods of time, and run in accessory mode), you might have some issues. But it's not particularly delicate in my experience and I sometimes leave the keys in, sometimes have it plugged in for a long time, have not had problems, and at 11K miles. The accessory mode is a big one, really should be removed, there's no disadvantage I see in just turning the car on.