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