Okay. Who HASN'T had a 12V battery issue with their 2023-24 Prime?

Yes, the true solution is to figure out why the batteries are dying in the first place. However, that still doesn't address the laundry list of user "don'ts" that Gokhan outlined. The only way to attempt to cover all those is to ensure the 12V system stays topped off whenever the car is plugged in (again, whether or not the traction battery is actively charging).

As for why they didn't choose to design around this problem, I suspect they figured it wouldn't bite them. Someone has chosen poorly....

 

they're sellingmore than they can make. so obviously...

 

SIGH...I know. The bean counters win again.

 

Most of the things on that list, ex. "leave lights off", are things for any car. The people having issues seem to have to made sure that wasn't the case.
The warning about the drain on the 12V while plugged in, where is it in the manual? It has been quoted here, but I didn't see it in the after charging section. They do warn not to leave it plugged in for a long time to avoid dirt, dust, and water building up around the plug. If the 12V going flat in such a time was a problem, shouldn't it be mentioned there?

 

Is this from the wiring diagram, or what? So if it is fully drivable why not just take the fuse out? What aux items are turned off? They said in the bulletin there is a way to recalibrate the intelligent sonar by driving. I would like to learn that if it is in the service manual.
Plus how to test the 12 volt battery sensor on the neg terminal.
Would be good info for the site.

 

You could add a function to keep the 12V battery topped off whenever plugged in. But that would only help plug-in owners when the car happens to be plugged in. It would be an auxiliary function benefiting a subset of people, and not necessarily the people who need it.

(My personal nice-to-have in the same area of leveraging the plug-in would be powering the engine block heater from the plug-in socket, so I didn't have to connect a second connector. They could have done that too.)

Adding a "top up the battery from the mains" function is a thing you could do to save the battery, but it's niche, targetting a supposed potential "flat battery while plugged in" problem. Topping it up from the traction battery or having some automatic "do the equivalent of DC cut" relay triggered by low voltage.would be more general.

Yes, it's in the wiring diagrams, obviously. And it's described in the bulletin. Basically it cuts most of the power supplies for OFF or ACC, leaving READY supplies unaffected.

There was a post here where someone had blown ECU-DCC No. 2, which is downstream of DC Cut - he lists all his symptoms. DC Cut would have killed all this and a bit more.

(His radio and multimedia and wireless charger were also always dead, but that could have been due to a blown Radio fuse. But that might also be covered by DC Cut).

If you want to try to make your battery last longer when you're not using it, the DC Cut fuse is less trouble than fully disconnecting - it's specifically designed with that function. The only "why not" would be that a buggy power drain could conceivably be coming via another route? Probably not though - this basically kills all the smart computery bits, and it seems that whatever problem there is is likely an "ECC active when it shouldn't be" problem.

The sonar reinitialisation stuff was for G4. In G5 the only listed items to recalibrate are the driver powered seat, and the powered back door if it was open when the fuse was removed.

 

It says there is a way to recalibrate the sonar by driving method. Just for info, mine seems to work fine. I was also hoping to find the ohm value or whatever is required to test the battery sensor on the negative battery cable. I could spend $25 but if someone already has access to the service manuals, it would be good to share the testing procedure.
All this still leads to no one is removing the fuse every time they park. The cars still have a 12 v battery discharge problem, just as many other makes besides Toyota have. Like I wrote before, my 1998 Chrysler warned about leaving the vehicle over 21 days. I found that to be about right too. The difference is the alternator charges fast, but this Prius does not, it’s slow and inconsistent.
That's why I want to test the battery sensor. Maybe there was a bad batch of those, or even just a few.

 

Can you share where the closer access point is - thank you!

 

From my observations with my battery monitor, the main culprit is the battery-sensor-equipped battery-management system, which doesn’t like to charge the 12-V battery frequently. If you install an AGM battery, it greatly helps because it charges much faster than a flooded-cell battery (due to a much lower internal resistance and a much higher charging current), which helps avoid sulfation due to infrequent charging, which is probably what happened to your flooded-cell battery.

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

On another note, apparently, Gen 5 Prius Prime charges the 12-V battery during traction-battery charging, which is an improvement over Gen 4, which doesn’t charge it.

 

No issues with our 12V battery after 6 mos of ownership. We do recharge the traction battery nearly every day and if I understand this thread correctly that charges our 12V battery as well thus keeping it healthy.

KH

 

When the car's charging system is pulling current, whether to charge the battery, precondition the cabin, or heat the battery, power is supplied to the 12V system. So the battery is getting a charge then. When current isn't being pulled, the 12V system is powered by the starter battery.

Another draw is added to the other battery drains while the car is off from being plugged in, as charge system monitoring is going on. The manual warns against leaving the car plugged in when not charging. I believe this to be for CYA reasons. Maybe a concern if the car is going to be parked for a week or more, but the extra draw shouldn't be an issue for daily use.

The issues people are seeming to come down to two things. A batch of bad 12V batteries, and/or a software glitch that keeps something from going to sleep when the car is shut down.

 

Why are you insisting on speculating and spreading false information? You don’t have any battery-monitor data. You can't even have any battery-monitor data because you don’t even own a Prius.

Your speculations are mostly wrong.

Gen 4 does not charge the 12-V during traction-battery charging. The voltage stays at as little as 12.83 V. You should realize that the DC–DC-converter voltage is not a constant but controlled by the battery-management system. However, apparently, Gen 5 does according to data from another owner.

No, the problem was not a bad batch of batteries. It was sulfation that resulted from infrequent charging by the battery-management system, which kept the 12-V battery at a low SOC.

Again, it is not the parasitic drain, a software glitch, or a bad batch of batteries that is the problem. The charging algorithm of the battery-management system is extremely complicated, and sometimes, when the stars are not properly aligned, the battery isn’t charged when it needs to be—no matter how the car is driven—which results in extended periods of a low SOC, which in turn results in battery degradation due to sulfation.

 

So the 12V battery can drain while the DC-DC converter is supplying energy to the 12V grid of the car?

 

Aren't you also speculating using Gen 4 data on a Gen 5 car?

You speak as if you know the definite cause of the Gen 5 issue without even owning one.

 

That’s not true. @Roy Peterson has also been posting his data for his Gen 5 Prius Prime in my AGM-battery thread. Gen 4 and Gen 5 battery-management systems behave in an identical manner as much as we can tell, down to small details. The only difference we have discovered so far is that Gen 5 Prius Prime charges the 12-V battery during traction-battery charging, but Gen 4 Prius Prime does not.

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

 

Yes, it can, as @Roy Peterson has current (amperes (A)) data as well thanks to his sophisticated diagnostic tool:

AGM battery for Gen 4/Gen 5 Prius/Prius Prime and observations on the 12-V charging system | Page 2 | PriusChat

The DC–DC-converter voltage is controlled by the battery-management system as well as other parameters. It typically varies between 12.86–14.20 V during driving. Sometimes it could briefly fall below that range. The float voltage is typically between 12.86–12.92 V, at which there is minimal battery current (charging or discharging). However, I also saw that it sometimes increases to around 13.05 V or even slightly higher. Aggressive charging happens at about 14.00–14.20 V. It never goes over 14.20 V as far as I’ve seen. Less aggressive charging also happens at 13.53 V, such as when the gear shift is in park.

The battery-management system is pretty smart. It determines the SOC from both the voltage and current, and it tries to keep it in a target range, which seems to be about 85–90%, or perhaps a little lower. Every few weeks, it will bring it to 100% SOC or higher to prevent desulfation.

I think the battery-management system works well for the most part. With my OEM flooded-cell battery, I would probably get eight years of life, but I made the mistake of not connecting a battery maintainer when the car wasn't driven for a month or longer in two occasions. The parasitic drain is several percent SOC everyday (even without plugging in), and under no circumstances, you want to leave the car undriven without a battery maintainer for longer than a week.

I think the 12-V-battery problems happen because the battery-management system is so complex in trying to balance fuel efficiency etc., and occasionally, it will fail to charge the battery properly (my stars not aligned comment). This could create a domino effect of decreasing SOC everyday, which causes sulfation, which in turn tricks the battery-management system to think that the battery SOC is high as sulfation artificially raises the battery voltage, which quickly degrades the battery in a matter of a week or two. There were posters here who were driving their Gen 5 Priuses everyday for long distances but their batteries still failed. That’s because you can drive the car all day, but the DC–DC-converter voltage could stay at 12.89 V all day long since the battery-management system thinks that’s the correct voltage to maintain the SOC.

Last but not least, both @Roy Peterson and I agree that the AGM battery is charging better than the OEM flooded-cell battery. That would be thanks to the much lower internal resistance of the AGM battery, resulting in a much higher charging current. @Roy Peterson's SOC was staying pretty low with his OEM flooded-cell battery, but it’s been staying around 90% with his AGM battery, and so has been mine with my AGM battery.

Again, for most of the time, you shouldn’t experience any problem. However, because of the complexity of the battery-management system, there could be insufficient daily charging in some occasions, which would lead to battery degradation, especially with a flooded-cell battery. You would think that driving the car daily for long distances would recharge the 12-V battery, but this isn’t necessarily so with today’s modern complex battery-management systems.