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

Much more expensive BMWs, Audis, Mercedeses, etc. are a lot worse in 12-V-battery longevity than Gen 4 and Gen 5 Priuses and Prius Primes.

Again, there is no problem with the Prius battery 99% of the time—you only need to observe certain precautions. The main two things are (1) to use a battery maintainer at least once if the car has any history of not being driven for more than a week in order to bring the battery SOC back to a level maintainable by the car’s native charging system and reduce the sulfation and (2) not to have a persistent habit of leaving the car plugged in for longer than overnight.

 

Exactly, you must use a battery maintainer after everytime the car has been sitting for longer than one week, as the car’s native charging system is not capable of fully charging or healing batteries with a low SOC.

 

My guess is that the 12-V-system operation during solar charging is very similar to that during AC charging.

This means that the DC–DC converter is on (possibly at a low voltage such as 12.92 V, albeit reportedly higher in Gen 5) during traction-battery charging and turns off when the traction-battery reaches 100% SOC. So, once the traction-battery is at 100% SOC, the parasitic drain on the 12-V battery will increase due to the charging computer still being on but the DC–DC converter being off.

This brings an interesting question: Do the vehicles with a solar roof have a higher parasitic drain on the 12-V battery, as they must constantly monitor the status of the solar roof?

 

So to answer the inital question - I recently came back from a 3 week vacation. For the whole time the car was parked outside with temperatures slightly below freezing at night. Fortunately no battery issues.

The car has solar panel and there was sun but probably not enough (~25W when I checked today) to go over the threshold (50W?) so no solar charging during my absence (and generally not in winter months).
The traction battery was left at ~40% SOC and did not change - the tires lost a little bit pressure though

 

Just wondering how you determined that the battery was left at ~40% SOC ?

 

No battery issues? You don't know that.

Three weeks is a long time. There is a 2.3% parasitic drain everyday in my Gen 4 Prime. Chances are that your starting SOC was around 80%. So, after three weeks, your SOC would be about 30%, which means your battery has had serious sulfation and capacity degradation. If you couldn't connect a battery maintainer because you didn't have a garage, you should have disconnected your battery before you went on vacation.

Your car's charging system will never be able to bring the SOC to a healthy level now. You should hook up a battery maintainer like Noco Genius for a couple of days to bring the SOC to 100% and remove some of the sulfation. It is unlikely that your battery will survive the same thing with the capacity degradation caused, but again, use a battery maintainer to eliminate some of the sulfation and bring the SOC back to 100%. Your SOC is probably stuck around 60% at the moment.

 

That is the traction battery.

 

You might have a point, I will not doubt your knowledge... but then you kinda miss the point. People have long vacations here and none really cares about battery maintainers or even disconnecting the battery. Especially not the typical Toyota customers. You dont buy one of the most reliable (and boring brands) brands (which is kinda expensive these days) with a 15 year warranty to deal with this kind of stuff.

If the battery stops working after my next big break I will just call Toyota so they can come over to fix it.In worst case nothing too bad happens besides the battery having to be replaced earlier - which is an insignificant price to pay compared to everything else this car costs. No point in overthinking it now though.

 

Not just knowledge—but also experience with such long-vacation situations with my Gen 4 PHEV. The first few times the battery was still alive, but it eventually completely drained. I then investigated what was going on.

I don’t know if the solar roof charges the 12-V battery when parked. If it does, perhaps it helped.

You say that, but you will find out that it is not fun to come back to a dead battery from your vacation to say the least—especially if you need the car right away. It is not just about money.

My recommendations stay: Use a battery maintainer now to recondition and fully recharge the battery. In the future, you probably want to disconnect it for long vacations if you can’t garage and hook a battery maintainer to it. You can also leave the key with a friend and let them turn the car on to the ready mode for half hour a couple of times a week.

By the way, even if you do not have a garage, you can simply remove the battery and take it to your apartment and hook up a battery maintainer in there.

Noco Genius 2-ampere on Amazon DE

 

At what 12v battery voltage does sulfation begin or at least get to a level of concern?
I have a Bluetooth monitor on my 12v. I have a 2023 Prius (not Prime) and this monitor says the battery is low when it reaches 12.39v. The car is brand new and only takes about 2 days to go from 100% to 64% which, as I mentioned above, the BT monitor calssifies as "low".

 

Sulfation can start at any SOC below 100%, but it gets worse and worse with decreasing SOC. I would say you should start worrying if the SOC falls to the light-green zone.



If 12.39 V for your flooded cell is at least after 16 hours of rest, you are looking at about 78%, which is typical with the Gen 4/Gen 5 BMS. My AGM cell is falling down to as little as 12.57 V after 16 hours, which is 74%.

I am thinking of hooking up my Noco Genius to bring the SOC to 100+% to reduce/prevent sulfation issues, as 74% is a little too uncomfortable for me, but then, again, that's what the Gen 4/Gen 5 BMS does.

 

I'll put my battery tender on the 12v and when it completes it's charge cycle I'll set a 16 hr alarm and see what things look like.
Thanks for your expertise,
J

 

I haven’t had a single problem with my battery or with anything really, except when I drove over a nail and needed to replace a tire.

 

That is exactly the point—you don't have a problem until you have one.

 

How often and how long do you drive the car?
My issue is I don't drive the car enough for it to keep the 12V charged sufficiently.
If I drove it daily for 10->20 minutes I too would have no issues but I don't. I'm retired so, homestly, an Uber would probably be more cost effective but I don't want to totally give up my independence.

 

In that case, you should hook up a battery maintainer once a week, as you lose about 2.3% SOC everyday. Noco Genius can be left hooked up indefinitely. The 1-ampere version is cheap.

 

But I don’t do anything special though. This is a multi-page thread about batteries with all sorts of advice and instruction to limit potential problems. I just don’t do anything special with my car and I haven’t had any issues with my battery, or with anything else.

 

Same here, but I imagine the folks who do have problems appreciate both the arcane and the practical info in this thread.

 

Shortly after I got my 2024 Prime, about a year ago, I had problems with my 12V battery going dead. My dealer said the battery passed the test, and a second dealer replaced it, but the problem persisted. One incident I remember as the worst; I parked for 2 hours to go on a hike, and I returned to a dead battery. By "dead" I mean the voltage fell below the threshold where I got a message saying the battery level was too low to start. I measured parasitic drain to be highly variable, probably due to over the air updates.

I installed a battery maintainer, and an auxiliary battery, which quadrupled my battery capacity. I haven't needed a boost since.

 

Nothing like installing lots of parts to fix your new car.
Why not have toyota fix this problem?