Parasitic battery drain?

My car doesn’t have a tracker, and isn’t dealer financed. I believe that some issues people have with parasitic drain or battery issues must be caused by trackers or other accessories, but I already ruled those causes out before diagnosing the battery issue. It was one of the first things I had to check.

 

12v just died in mine also. 14 months old and about 8000 miles. 5 days ago it was on a 200 mile road trip with no recharge. Plugged in overnight 2 days ago and was driven the following day about 25 miles. Today with a remaining charge of 45% died while trying to start.

 

Six month update: no issues at 4,810 miles, since March 14 12v warranty replacement.

 

I just had my dealership check my 2024 Prius Prime as we had drained battery one morning. They said the battery checked out OK. But they found a bulletin for 12V battery depletion when connected to the charger. They calibrated the plug-in charge control ECU as per TSB-TCI-6016 CODE EE2411.

 

No, once the AC charging is finished, the battery depletes faster if you keep the car plugged in, with the TSB resolution or not. That is because a 12-V subsystem that checks the plug continuously operates when the car is plugged in. Therefore, you should unplug the car as soon as the charging is finished or at the earliest possibility. Perhaps the TSB resolution helped, but it won't stop the 12-V subsystem from being active when the car is plugged in and the AC charging is finished. Note that during AC charging, the 12-V battery receives a float voltage, but once the AC charging is finished, it will start draining.

Also get a charger like a Noco Genius, as the car will not fully charge a fully drained battery. It has probably lost capacity as well because of the full drain, but if you occasionally manually charge it, it could last for a couple of years.

 

Dang, that's some strange engineering to have a PHEV act like this.
Is this handy tip CALLED OUT in the Owner's Manual?
Or is it folklore here?

And as always, when a 12V lead acid battery takes a trip to 'Very Low Voltage Land' it has taken life time away from the battery.
And sitting around close to zero volts is very very bad. It can be 'revived' but it's an old battery after that.

 

The manual does mention there is extra drain on the 12V when plugged in.
Seeing how some have no issue with leaving the car left plugged in for what most would call a reasonable time, The extra drain alone is not enough to kill the battery.

 

The extra drain is non-zero, but it really shouldn't be enough to worry about in normal conditions. Any more than any of the other drains from stuff like leaving the smart key enabled or having the connected services enabled.

I do make the effort to not leave it plugged in for multiple days, but you won't catch me running out to unplug it as soon as it's finished. Mainly because I don't think the car was designed by idiots.

I'm probably at an advantage because I don't have a voltmeter plugged into it, so I don't see any millivolts to obsess over. I just trust the car to do this job along with all the other thousands of jobs it's doing.

I also wonder if the load drops after 3 days - the battery heater has a 3-day timeout, and I wonder if that would indicate the system dropping into a lower-power mode.

 

I have actual data on this for a Gen 4 Prius Prime, and the plug-in 12-V drain is continuous and significant. It probably shouldn't be a concern for an overnight plug-in session as long as the car is driven everyday, but if you make a habit of leaving the car plugged in for more than a day, you could easily end up with a drained 12-V battery.

AGM battery for Gen 4/Gen 5 Prius/Prius Prime and observations on the 12-V charging system | PriusChat