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

 

Now you have me wonderin why they spec a 3 day limit. One thought is it's a random time limit to keep some egjit from leaving it plugged in foreva.

 

The 12V gets charged while the battery heater is running, and Canada, plus Alaska, have a system that goes 30 days.

 

No, it does not. The plug monitoring is a continuous operation for safety reasons. The 12-V-battery drain is continuous as long as the HV battery is not charging or the heater is not being used. If they are, then the 12-V battery is on a float voltage (~ 13.0 V), not draining but not charging either.

 

No, it does not. With the car plugged in, the 12-V battery is on a float voltage (~ 13.0 V) when the HV battery is charging or the heater is operating, but it is not charging. So, its charge remains about constant during the traction-battery charging/heating events. If the HV battery is not charging or the heater is not operating, then it is draining with the car plugged in.

Many Canadians on this forum who left their car plugged in had their 12-V batteries drain.

If you want to keep the car plugged in for several days or more, you must also connect a battery maintainer to the 12-V battery regardless of where you live.

 

Corrected below. Sorry Canucks....

So the Prime is sort of a 1 season PHEV, where you can "Plug in and forget"... if it's cold enough to require HV Battery heat.
The other 3 seasons you have to plan around being away from the car, by unplugging it (and not have HV Battery heat if required) or going through the extra expense, time and the safety of walking around with a live 120VAC extension cord out to your 'modern' PHEV while it's also plugged in with a safe L1 - L2 EVSE.

Is any of this mentioned by the car sales force when purchasing this PHEV?

I've owned a PHEV (Volt) and two EV's (Spark EV and now a Bolt). None of them required ALL this extra care and attention and expense to plug it in twice or unplug it after it's topped up.

If this is not called out in the Owner's Manual and if enough owner's make warranty claims for a ruined 12V battery,,
Toyota might get off their azz and offer a SW update to get the Prime into the 2020's of technology.
You'd think it's just SW that is needed to make it work like a normal PHEV or EV.

Good Grief,,, I have a friend that will be getting a Prime, , one of these days,,, and now I have to warn him about this goofy engineering on Toyota's part.

 

It is called out in the manual. Like other notices there, it feels more like a legal CYA from Toyota. As I said elsewhere, not every owner even on Priuschat is having battery issues. It seems mostly to be a case of there being a batch of bad batteries, with the factory installed one already being on the weak side, plus a possible case of the system not going into sleep mode at times. New batteries and software updates appear to have fixed it for users here.

Toyota's choice here works fine for a daily driver without having the owner running out to unplug after charging is finished. It is how things worked with the gen4 without issue. It could lead to a dead battery if the battery is already compromised or there is some other issue, but having the system charge the 12V doesn't fix those problems, just mask them for some time.

 

How do you know they didn't? Maybe there were people on forums insisting on all sorts of weird things you have to do to make them work right, and you just didn't read it.

You are free to just ignore all this nonsense, just as I do.

I'm now thinking this is possibly a North America-only problem. No equivalent TSB has been issued in Europe for that, and charging systems are going to be region-specific. Might help explain how it slipped through testing.

 

this has happened too often for toyota not to have a fix by now

 

They do - the TSB I mentioned above.

Edit: Actually, I edited that message above to the point I didn't explicitly mention it. I meant to say that although North America has its TSB, there's no TSB elsewhere.

 

i know, so why is everyone arguing?

 

Because this is the internet?