Parasitic battery drain?

There are plenty of threads/posts about this subject, you may want to check them out. The obvious things to check are: don't leave the AC charger plugged for an extended period once the traction battery is fully charged as that will cause more drain on the 12V battery. I've left mine plugged in for up to 24 hours but usually no more than 12 hours. Also, avoid using ACC mode. I never use ACC mode. I just put the car in Ready mode. See if there is a lo-jack type device installed on the car. I would suspect autos that are leased may possibly have this installed. Also, do you have any aftermarket devices or equipment installed?

 

Get a free load test at auto zone.
If it passes, give it full charge and take a static voltage measurement every morning to see how much it drops overnight

 

I have to say... I am getting ready to purchase a Prius Prime but this parasitic battery discharge issue is really sounding foolish. I have a friend with a Chevy Volt that leaves his charging cable in whenever it's parked in the garage and nothing bad happens. As an engineer designing electonic stuff, I can't really warm-up to the idea that leaving the traction battery charge cable plugged in causes discharge of the 12V battery. If that's the case, sounds like Toyota should fire their engineering "B" team and hire in people who know what they're designing. If anything, the AC charge cord should be charging both batteries. Or at least trickle-charging the 12V while charging the traction battery. The 12V would be charging effectively zero compared to the big battery so it really wouldn't increase the load on the AC mains. My 2010 Prius never discharged it's 12V battery in 11.5 years (until it finally died). The new battery is trouble free since.

This could still be a software bug improperly enabling things that shouldn't be during charge also. Still... Fire the engineering "B" team.

IMHO. ;-)

 

I have a 1-month-old Prius Prime that is having significant levels of 12v discharge (killing the battery twice so far). The struggling 12v battery doesn’t seem to be related to the charging cable at all, since I only occasionally charge the traction battery to 100%, and never leave it attached to a charger longer than it needs to be. The traction battery gets charged and cycled frequently, but I haven’t yet let it go to 0% SOC, and it isn’t spending more than an overnight period charging ever.

The AC charging cord does appear to charge the 12v indirectly while the traction battery is charging, which seems to be working as designed, but I am not checking voltage before and after on my car to confirm this.

Whatever is going on with the charging, my car is on the verge of dying with 700-800 miles on it, and it hadn’t been charged once within a few days of the 12v battery failures. The dealership doesn’t care and says everything is fine.

 

I’ve had my 2024 Prius Prime SE for three weeks now. In the weeks prior to pick up, I was concerned about the same 12V battery issues alluded to in this and many other threads. But after reading a bit (including the owner’s manual) and interacting with some folks in this forum about the 12V battery, I’m not as concerned anymore.

Specifically, I think there are several best practices to follow:

• Don’t leave the charging cable plugged in for an extended period of time after charging is complete:
• If you’re going to be in the vehicle for more than a minute and the vehicle is parked, put the Prius Prime into Ready mode.
  
• Be wary of any third party electronic accessories, including external meters and dealer installed anti-theft systems, as these can be sources of 12V battery drain.
• Keep the key fob a distance from the parked Prius at home, to reduce communication between car and fob.

Following these four tips has worked so far. My Prius Prime is not a daily driver (I think I’ve driven it 4 times in three weeks) and I have a tendency to sit in it for hours while playing with system settings and looking up stuff in the manual (with the radio on or car play active). Considering my habits with the car so far, I think I would have drained the 12V without these tips.

 

I wish it were that easy. I followed all of this advice from my first drive, and the battery or electrical system failed anyway at 735 miles.

I have:

• never left the charging cable attached for more than 2 hours after finishing charging, and I haven’t left the car charging for more than 8 hours at a time.
  
• not left the car in ACC mode while parked. If I’m in the car, I try to keep it in READY mode.
• not installed any third party accessories.
  
• not kept the key close to the car. I park in a single car garage completely separate from my house, and I put the key into sleep mode (long press lock, double press unlock) so it can’t communicate with the car.

 

I’ll admit that the key fob thing is a bit suspect, but I don’t think the others are wild guesses. For example, regarding the charging cable being left connected when charging is no longer needed, the manual on p81 states “When the vehicle is left with the AC charging cable connected, the electricity consumption amount of the 12-volt battery increases due to control, such as the system checking, operating. When the AC charging cable is not needed, remove it from the vehicle.” I’m not sure where 12V leakage and back flowing into the cable came from, but this is what I was referring to.

Additionally, I didn’t mention anything about ACC mode. I did mention Ready mode, and I think we’re on the same page there, where, in general, the 12V battery is being charged by the traction battery in ready mode.

But yea, I get it. Your 12V battery died and when you spoke to your dealer about it, they blew it off. That sucks and I am sorry that happened to you.

 

That is unfortunate. I hope your 12V battery issue gets resolved soon.

 

ACC mode shouldn’t kill a 12v battery on its own, but it isn’t healthy for it, so I err on the side of not sitting in the car with lights on or ACC running, as the traction battery is better at dealing with that power draw.

One service advisor accused me of leaving it overnight in ACC mode. Another service advisor at a dealership admitted that ACC being on when you leave the car shouldn’t kill the battery, since ACC eventually turns itself off. Yet another service advisor said “you left a map light on” which was absolutely not true. So there is a lot of nonsense in diagnosing this at the technical level, and many Prius owners seem to understand their cars better than the people who are being paid to make warranty claims on them.

Thanks for the well wishes and I hope you don’t run into the same issues.

 

When in doubt measure parasitic draw. Rather simple and any mechanic can do it with multimeter in series using the millamp scale.

 

The reasons for the advice about the keyfob and the charger cord are the same: they can keep the car from going into, or staying in, the deepest level of sleep.

The keyfob problem has mostly been solved, so it's probably not good advice anymore for new cars. But there is a good reason to still do it in older (Toyota) hybrids. It mostly has to do with brakes and a few other systems. On a non-hybrid, the brake master cylinder is powered by the engine(either through the power steering pump, the vacuum system, or a dedicated hydraulic pump depending on the car). That means as long as the engine is running, you've got brakes. But in a hybrid, you might not have the engine running right away to build up pressure in the brake system. So hybrids use an electric motor to create the brake system pressure.

The only problem with this method is that it takes a few seconds to build up pressure if the car has been sitting for awhile. Toyota(and other carmakers) were worried about people jumping in their cars and hurriedly starting them and putting the car into D/R before the system had a chance to fully charge. They also knew customers would be upset if they were forced to wait until the brakes were ready before being allowed to shift into gear. The solution was a proximity sensor for the keyfob.

Once the keyfob comes within range of the car, the car detects it and starts to come out of hibernation. If the car has been parked alone for at least a few hours, you can hear a series of noises as you approach it. Stuff like the brakes prepping are the source of that noise. And this leads to the problem of storing your keyfob too close to the car. If you leave the fob in range, the car stays awake. Eventually it will try to go to sleep, but the presence of the fob will just start the cycle all over again. Wakes up -- timer expirers -- shuts down -- wakes up -- etc.

This was(and still is) a problem with older hybrids. Eventually Toyota fixed the problem by making the keyfob smarter. The newer fobs can now detect the cycles happening and the fob itself can go to sleep if it realizes it's stuck in one. That's also why a fob might not open a door on the first try. If a fob goes to sleep because of being unused for a long time, being stored too close to the car, or a couple other reasons, it can take a bit for it to wake up. I don't know when Toyota added the sleep function to the fobs, but I'm really sure the gen3 didn't have it and the gen5 does. As for gen4? I don't know.


The charger cord is a similar situation. As long as it's plugged in, it forces parts of the car's electronics to stay in an active monitoring mode. The power for that comes from the 12V battery. Why Toyota doesn't let the charger or the HV battery keep the 12V topped up when not charging, I have no idea. But it doesn't, so owners are stuck with the situation. In cold temps, it doesn't matter so much because of the battery heater. When the car is plugged in, it uses the HV battery to run the heater. Eventually the HV battery drops low enough that the car needs to charge it up. And when the HV battery is charging, so is the 12V. It's just when the car is left plugged in and the temps stay high enough to not need heating does the problem show itself.




All that being said, I'm suspecting either a firmware coding issue keeping the Primes from staying in deepest sleep no matter whether the cord is plugged in or not(most likely) or a manufacturing issue at the battery supplier(less likely). And a coding issue is likely to be a cast-iron b!tch to find. I'd bet at least another six months, more probably a year until they figure out what obscure fragment of code among the other millions of lines of code is the problem. But I hope I'm wrong and they release a firmware update tomorrow that solves the issue.

 

I agree. My assumption right now is that it’s a software/firmware issue, or even the car’s integration with the Toyota app that has some sort of awful and hard to reproduce edge case that is keeping the car from sleeping properly. This would explain why some owners are fine and some end up having chronic issues.

A dealership technician (actual tech on the shop floor, not a service advisor relaying the issue) said that they checked for parasitic drain on my car and found normal levels of power draw. Hopefully they actually did this and didn’t just scan the battery.

The battery may test “good” according to the Toyota diagnostic, but I doubt it will for long.

 

I read your notes in the other thread. Your experiences with this sound pretty terrible. No one wants to go through stuff like this with a new car.

 

I wouldn't rule out a bad batch of either betteries *or some other component* that goes in and out of sleep - that isn't necessarily a software thing.

If it was a software thing I think many many more of us would be reporting this issue. It's not *quite* so common as to likely be a straight software issue right?

Maybe I just drive too often? But even if I leave the car for 2 or 3 days I've had zero problems. Plugged in, not plugged in, etc.

It really is a curious issue.

 

Would think a software/firmware issue would affect the majority of owner rather than the less than handful we heard from so far. I've owned my Prime XSE since August 2023 with 8000 miles and have not experienced any battery issues, and I think the majority of owners here would say the same.

I think something else may be causing a parasitic drain that hasn't been found yet but I doubt that this point that it's software/firmware issue. Are you leasing/financing the vehicle thru Toyota?

 

Edge cases with firmware/software could absolutely affect a tiny minority of owners. One that causes a drain that stresses a battery would cause a slow cascade of issues that would be extremely hard to diagnose, especially if the car doesn’t report the problems. This happens all the time when software is released into production and a bug will only affect some users some of the time. Mine didn’t show any signs of failure until the v2.5 release of the Toyota app, which could be a coincidence or could point to a source of the failure for my case.

As for the car, it’s dead again after only 5 hours of not using it and I caught the failure to start at 2:00 AM on video this time for the benefit of any technicians who want to watch the systems of the car fail one by one as the 12v loses its last bit of power alongside a series of error messages having to do with the parking brake and Park mode.

 

Goodness, hopefully the evidence you collected helps to sway the opinion of the Toyota techs that this isn’t a simple case of user error.

With respect to edge cases and software: while the true frequency of this is unknown, it wouldn’t surprise me if a bug was present in the firmware or software. I’ve been the product owner for two large software projects and the # of bugs triggered by specific conditions is amazing to me. I think that’s why a lot of these 12V battery issues are challenging to diagnose; there’s a lot of variables at play.

Of course, this isn’t to minimize your experience with the 12V battery. Fingers crossed the techs do something more than last time.