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

Wow, talk about misinformation...so all of these reports are the owner's fault? Engineers in general "know what they're doing" but are not all-knowing or perfect. If you think otherwise, explain the flaws that show up in things designed by engineers....all the users' fault? No, I think it is the job of engineers to design products that normal people can use without having to take a night course on how to use it, and baby-sit it like the folks on this thread are doing to prevent their battery from going dead.

I have a '24 non-prime with a bit over 12k miles, bought new and in regular use for varying trip lengths, though no extended trips lately. A couple of months ago I went out to use it and got a "hybrid system must be activated" warning due to a low battery. "Have it towed in", said the dealer, which was ridiculous, as I have other things to do than spend the rest of the day doing something I shouldn't have had to do in the first place. The car had been in use at our other house locally, then the previous day driven the 20 miles home. Plenty of time for any alternator to charge a battery adequately.

So I charged it a few minutes from under the hood to get it going, then later charged it overnight to get it fully charged (after reading in this forum about all the others having the issue). It's was fine since, but, just in case, I had bought a new jump starter battery, not wanting to be stranded later in some awkward situation. We left at the beginning of March for a 3+ week trip, in which the car would be at the airport, and I made sure to have that jump starter in there with the spare tire I had to buy, which was also not included with the car.

Sure enough, parked at the airport 24 days, got to the car and saw the same disappointing "hybrid system..." message. Jumped it at the battery and it ran for a couple of minutes and shut off. Now if that battery was "dead", why on earth did the system shut it off so quickly instead of staying on to charge it? However, it did start and stop OK after that, so go figure.

Now I've been driving and working on cars for 64 years, and have learned over time that 1) A two year old car with 12k miles and nothing left "on" to drain it should not have a dead battery after sitting overnight (and no, I don't leave the key in or around it, the lights weren't on, and also no, I don't pamper it by going out and charging it every night...one should not need to do that if the all-wise engineers were so perfect as some seem to think.

These cars either have a design flaw that does not adequately charge the battery...a simple voltage regulator and alternator are smart enough to know how to charge a battery, they've done it for many years. Or, the batteries they're putting in aren't adequate for the job or are defective. It's that simple. For someone to imply that this is "normal" and it's just improper maintenance or dumb people using their car improperly, is ridiculous.

 

I've said this before, but the $26 bluetooth battery monitor is still providing entertainment to me. Then again, I never felt the need for one before this car, and I'm no spring chicken. Maybe there is no need, but again, it was cheap and I feel informative. For instance:

 

It is not necessarily your fault. As I said, chances are that your car sat at the dealer’s lot for too long before it was sold, which degraded its 12-V battery. The dealer should have charged it periodically and definitely before sold.

As I keep repeating myself, once the SOC becomes too low, the battery is not only degraded, but also the BMS will not properly charge it, leading to a downward spiral.

Leaving these cars undriven for three weeks is too long, even with a healthy battery, let alone a degraded battery as in your case.

Since your battery is already degraded and you don’t seem to be driving much, I would recommend upgrading to the Uplus AGM battery on Amazon and also getting the BM2 battery monitor on Amazon @VelvetFoot mentioned.

In any case, there is nothing special about the Gen 4/Gen 5 Prius. All newer cars occasionally experience similar 12-V battery problems.