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

Might be worth testing, if there’s any warranty left

 

Question: Is it OK to have the battery maintainer connected AND the PHEV charging cable operating/charging at the same time?

 

When the car is plugged in but not actively charging the traction battery, it is definitely OK. When actively charging the traction battery, I don’t see why that wouldn’t be OK, as the battery maintainer acts like a battery in parallel. The only time you need to disconnect the 12-V battery from the car is when or if you are using the repair mode, to avoid higher voltages in the repair mode from doing any harm to the car.

 

One way to find out...

FWIW, I've had our car fully on (Ready) for a few seconds, shuffling it forward/backwards a foot or two, in aid of my garage projects. No harm done. If you're doing something involving the car being "On" for a protracted time, it's actually recommended by Toyota to have a charger on, albeit a special Toyota spec'd charger. Specific case is doing a brake fluid replacment, in "Invalid Mode". I've keep my CTEK charger connected for that as well.

 

That battery is toast. Lead Acid batteries chemically die when left discharged. Other chemistries can handle it, but not L/A.

A modern car should not require so much baby-sitting.
A PHEV plugged in along with a charger on the 12V??? Come on. A modern car should require patting your head and rubbing your belly.

It totally seems a SW upgrade from the mothership could fix this goofiness. A free recall.

 

Ah capacity reduced? Likely. Toast? Unlikely.

I had a fully drained Gen 4 Prius Prime battery (left it undriven for over six weeks). I replaced it with the Uplus AGM, but I think it was still OK after I recharged it.

Neither the Gen 4 nor Gen 5 will charge the 12-V battery through the plug when the active traction battery charging is completed. In fact, it induces parasitic drain to keep it plugged in. That is how Toyota designed it, and they probably have their reasons like safety. This has been discussed here many times.

 

March 2024 Prime SE - no battery issues so far

 

Early 24 (Dec 23 manuf) - left outside in cold weather unstarted for two weeks - discharged and failed to start. Re-charged and replaced. It had failed to start summer before when a mechanic left the car "on" on a service rack, requiring a boost.

 

my 2024 non prime just failed 20000 miles 2 years old suddenly no power dealer picked up and replaced
no warning just no power bad part is it had to be dragged up on ramp truck with 4 Wheels locked

 

Are you serious?
Can you name a good 'reason' to run the 12V down to nothing ?
Can toyota?

From here, that is UNEXCEPTABLE.
And yet, it seems a simple SW update could fix it.

 

In any case, neither the Gen 4 nor the Gen 5 Prius is a software-defined vehicle (SDV), so there is no such thing as a simple software update. Perhaps the Gen 6 Prius will be an SDV.

 

Another cold winter, another long vacation leaving the car 3 weeks alone. Still no battery issue

 

I have been leaving the maintainer on for the last two years whilst traction battery is charging with no issues

 

The module (s) controlling the 12V battery can not have updated SW added? Doubtful.

 

Of course it can, just not ota

 

I am not sure what you want a firmware update at the dealer to achieve.

1. It would be crazy to have the traction battery charge the 12-V battery when the car is not running, as that would drain and likely kill the traction battery and brick the car.

2. When the car is plugged in and the traction battery is charging, the Gen 5 charges the 12-V battery through the main DC–DC converter, and the Gen 4 keeps it at float through a sub DC–DC converter in the charger. When the traction battery charging ends, the DC–DC converters shut off. They probably don’t want the DC–DC converters to remain on at all times, as that is a lot of wear and tear on the 12-V system and the traction battery. It also wastes electricity, increasing greenhouse gas emissions. There could be safety issues, too. Moreover, it doesn’t help with the nonplugin Priuses. Toyota definitely thought about it and designed it this way as the optimal solution.

3. Priuses are no different than any modern car, hybrid or not. Any modern vehicle will experience 12-V battery failures if it's not properly maintained. The main issue is when the car sits unused for too long, whether at the dealer’s lot before being sold or at the owner’s home after being purchased. The solution, as I've suggested many times, is to use a Noco Genius and manually charge the 12-V battery if there's suspicion of a low 12-V battery state of charge (SOC).