12V Battery Issues?

Unfortunately reliability is not what you will get with the Gen 5 Prius Prime 12v Battery. Toyota engineers chose to implement a 12v Battery Management System that puts vehicle mileage efficiency first, and 12v battery reliability second. The stock 12v in the 2024 Prius Prime XSE is not a strong battery. As you search thru the posts here you will find that many owners have replaced the original battery either under warranty or at their own cost. After replacement the new battery soon becomes somewhat unreliable if the car is not driven daily, and with time becomes unreliable even if the battery is not driven daily. The new battery is still charged IAW the Battery Management System toyota engineers chose to provide with the car.
In time you will get a battery maintainer device and maintain the battery periodically (either weekly or monthly) to get the reliability you want. Or you will continue to deal with Toyota dealership and your warranty to replace the battery more often than you would expect.
Sad story indeed !

 

One thing you can do to help the battery perform more reliably is to never use the cars ACC mode. Put your foot on the brake and push the start button every time you are in the car. Don't listen to the radio or use any accessories on the car unless the car is in the Ready mode.

 

Not my experience. I've had my SE since March 24 without any battery issues. I've occasionally not used the cars for a few days, most recently it sat for16 days without losing any charge. I typically charge the "big battery" overnight.

 

How did you determine your battery "sat for16 days without losing any charge" ?

 

As it was already stated, it is impossible for the 12-V battery not to lose charge even in one day, let alone in 16 days. SOC drops by several percent everyday, and it would have dropped by about 50% or more in 16 days, and the battery would certainly completely drain in less than a month.

However, there are reports that the Gen 5 Prius Prime charges the 12-V battery when it is plugged in. What I don’t know is whether this charging stops when the traction-battery charging ends, and then, the 12-V battery would drain even faster than as opposed to not being plugged in, due to the charging computer being on.

 

I have a radar detector connected which gives me a reading of the voltage upon start-up.
To be fair, now that I think about it, I might have plugged it in the evening when I got back before driving the next morning, but I can't say for sure. I had left with the hybrid battery 75% charged.

 

75% charger. What resting voltage was that before you put the car in READY mode (pushed on the brakes & pushed the power button) ?

 

Radar?

When the car starts, the voltage you are seeing is the DC–DC converter voltage, which is always about 14.1 V at start, and it tells you nothing about the 12-V battery SOC. To get an idea of the 12-V battery SOC, you need to measure the rest voltage after at least twelve hours of sleep and with the car still sleeping (no key fob nearby).

 

The easiest way to monitor the 12v battery voltage on a Prius Prime is to get a BM2 device and play with it on bluetooth app for your phone.

 

Cobra-RAD-480i-Radar-Detector-Owners-Manual. ...

Quadratec
https://www.quadratec.com › sites › default › files

PDF
Battery Voltage. To display your vehicle's battery voltage, press the MUTE button while no signal is being detected.

 

OK, that won’t give you the battery (open-circuit) voltage but the DC–DC-converter voltage as the car needs to be on (or in the ACC mode with load on the battery). Therefore, it tells you nothing about the battery SOC. It is like measuring the alternator voltage in nonhybrid cars.

 

You just do not get it!

If the car is in the RAEDY mode, you would be displaying the DC to DC battery charger voltage, not the resting 12v Battery voltage using tht method. It tells you nothng about the battery SOC or health. It just tells you whether the DC to DC device is working.

 

Sorry, above my pay grade. In a non-hybrid car, I'm assuming the voltage display once the "car electricity "is turned on does not speak to resting voltage or battery health either?

 

Correct, for a nonhybrid car, it is the alternator voltage, and for a hybrid car, it is the DC–DC-converter voltage. They are not the open-circuit voltage generated by the battery, but the voltage generated by the 12-V charging system.