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

I'm pretty sure other manufacturers figured this out.
The 12V is monitored, Load Shedding begins at X SOC. When necessary turn on the DC-DC to transfer X amount of Whs to the 12V.
Monitor the HV pack. Give up the ghost, throw your hands in the air, at 40% SOC, (months later?)
It's not Rocket Surgery

Again, I changed out my +8 year old Bolt 12V battery because I got tired of waiting for symptoms. Did GM figure it out?

 

The Gen 4 and Gen 5 Priuses have no 12-V battery issues when properly maintained, and their 12-V batteries will last 10 years or longer, too. You don’t fix something that is not broken. Besides, Toyota engineers know what they are doing. Every Toyota technical manual, whether it is for Prius or not, instructs that the battery must be fully manually charged if it is drained (due to not having been driven, etc.), which is the only reason for the failure.

 

1* What does 'properly maintained' mean to the typical owner? Some word of mouth extra procedures?
2* A 'Technical Manual'...? Does the typical owner know any of this by reading the Owner's Manual?

I understand that you have brand loyalty and appear to be an apologist for this Problem.

Is this the problem?
You can't leave your PHEV plugged in after it has charged to 100%.
You can't leave your PHEV while gone on a +2 week vaca? Or any combination of those?

I really don't know. No skin in the game. But there sure are a lot of reports and questions on this and other related threads.
From here, it looks like if the hardware could support it, a simple SW update would make this 12V system play nice, just like any other EV and PHEV.
You seem to have studied this system for a long time and might know why a SW update can't be done.
What about a hardware and SW update? And toyota foots the bill?

 

After finding my 12 volt consistently sitting at less than 50% state of charge, I’ve unplugged sensor on the negative terminal of the 12 volt battery. This fixes the charging voltage at 14.3 volts whenever the car is in “Ready” mode. It doesn’t light up any warnings and now my Prius keeps the 12 volt topped up just like most other vehicles.

I did the same thing with my 2014 F150 after the first battery lasted less than 3 years, with no adverse effects. The second battery lasted 7 years.

I don’t think that Toyota can do anything because additional 12 volt charging would lower fuel economy slightly and require new testing, reporting and affect CAFE ratings, etc.

 

Is this a Prime only or Warm Weather phenomenon for the HEVs?
I've only had my HEV since November, so not that warm yet.
I've been keeping my eye on voltage, and it's always 14.3-14.4 v in Ready.
Time will tell I guess.

 

Or, alternatively, hook up a Noco Genius every three months or so.

The 12-V BMS keeps my AGM battery between 75–90% SOC, which is OK. I would definitely hook up the Noco Genius if the car is not driven for over a week. The temperatures are never too cold here for the car to automatically turn off the BMS. I see no reason to manually cancel the BMS (by disconnecting the sensor) in my case.

Canceling the 12-V BMS may actually reduce the 12-V battery’s lifespan by increasing the number of charge and discharge cycles. I believe the 75–90% SOC target chosen by Toyota is optimal for battery longevity, while also improving fuel economy and decreasing greenhouse gas emissions.

 

July 2023 LE awd with zero battery problems. Never trickle-charged and has always been good to go. This Prius is kept in an unheated W. PA garage.

 

I highly doubt that would be the reason. Do the math.... Trickling a 12V up to a decent SOC would be X kW's, probably not even a kWh.
This would be hard to quantify in 'reduced mpg'.
I don't get the same mpg numbers ever when I do paper and pencil calc's and the display is no better.

 

Has your car ever sat unused for long (7+ days) periods of time?

 

Yes, it’s probably minuscule but every little bit contributes to the overall fuel economy.

The engineers at Ford thought that it was worthwhile to install the system to back off the 12 volt charging on my F150!

 

I beg to differ

Our 2024 Prime is a daily driver and is plugged in nightly using the on board timer scheduler. Our daily commute is about 30km round trip. In warm weather the vehicle rarely runs the gas engine and we have up to 5,000+ kms on a tank of fuel. When the outside temps dropped below freezing and the car parked outside during the day the 12V battery has failed twice now within the first year of ownership requiring a boost to get the car started.

I am now doing a PM routine to charge the 12V battery once a month to top it up. I suspect that the 12V battery is degraded right from new.

This is an OEM vehicle design defect IMHO. My experience with 3 other Toyota HEVs is that the 12V batteries have lasted 10+ years without issue. The PHEV 12V battery should be getting charged or at least maintained during the traction battery charge cycle. The Gen5 Prime 12V system is designed to fail.

When I talked to the dealership about it they said yes it's a known issue. Toyota chooses to ignore the problem and stick it to their customers. That's why the Gen5 Prime's have a low reliability ratings at the current time.

 

I don't know about these 'ratings', but I know owners here have spoken.
It could be fixed. It's not Rocket Surgery.
An owner shouldn't have to rub their tummy and pat their heads to keep their PHEV alive and well.

 

My infrequently driven ‘21 Venza is prone to the 12v Battery issue as well. My NOCO GENIUS 5 has become my friend to both of our Toyotas. I suspect the Telematics system both cars have contribute to the 12v drain.

 

I never had an issue with gen 2&3.
Sometimes I think these modern connected cars are in constant communication with HQ

 

Neither did 99% of Gen 4 and Gen 5 owners.

 

Again, I explained why these new Gen 5s had battery issues. They sat unused in the dealer lot for too long, probably because of the months-long stop sale due to the door-switch recall. During that time, the state of charge (SOC) drops too low, and the battery management system (BMS) can no longer fully charge the 12-V battery. Manual charging with a battery maintainer is necessary. The dealer was required to do this but did not, which caused your 12-V battery problems.

In a Prius, the engine does not charge the 12-V battery. Instead, the traction battery charges the 12-V battery via the DC–DC converter. Whether the engine is running or not, it has no effect on the 12-V battery.

You should buy the BM2 battery monitor from Amazon to assess your battery’s condition. You should also use a Noco Genius to fully charge it and restore its health. If the battery is too degraded, I recommend the Uplus AGM battery sold on Amazon.