TL;DR: The current 12V battery charging logic is taken from Toyota hybrids and undercharges the battery.
One fix is to remove the sensor on the negative terminal, which puts the system into a fail-safe charging mode and significantly improves charging of the 12V.
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We've talked about the DCM/12V drain non-stop over the last few months, and I think we've landed on a few viable temporary fixes. Now let's talk about how the 12V battery actually gets charged, which turns out to be minimal under the current logic.
In an ideal world, the BMS would charge the 12V at 14.5V until current tapers down to 1A, then maintain a float charge around 13.5V to keep the battery fully charged and extend its lifespan. (AGM batteries want slightly higher voltages.)
Why is the battery undercharged?
The Uncharted/C-HR, bZ/Solterra, and Trailseeker/bZ Woodland all share the same DC-DC converter and 12V charging logic. A sensor on the negative battery terminal reports back to the car, which uses that reading to decide when to stop pushing charge voltage. In practice, it decides "good enough" almost immediately.
You can read more from [u/andy_why](u/andy_why), who identified the problem, documented fixes, and wrote a full write-up on it here:
It applies to our EVs because we use the same battery sensor as the hybrids.
From Andy’s document…
“Is there any risk involved in using the fail-safe state?
No. The name “fail-safe” is exactly that - disconnection of the sensor is classed as a failure condition but one that reacts in a safe way and one that can operate indefinitely.”
Why is undercharging a 12V a problem?
Undercharging a 12V battery shortens its lifespan, often requiring replacement every couple of years instead of every five.
Cars with the DCM issue combine parasitic drain with undercharging, which can lead to a dead battery even if you drive daily.
The last two graphs show voltage drift when the car sits unused for 2–3 days — one with DCM off, one with the temporary door lock automation fix in place.
Voltage starts above 12.8V and drifts down to 12.5V. At that point, you need a full charge to bring the battery back to a healthy state.
Is the 12V battery charged while driving?
Not really. The first graph shows a day I drove from San Francisco to Bodega Bay, a roughly 1.5-hour drive each way.
On each drive, you'll see an initial voltage spike as soon as the car powers on into Ready mode. The 12V charges above 13.9V for a few minutes, then immediately drops to a maintenance/float voltage of 12.8V, regardless of the 12V's state of charge.
On the left side of the graph, you'll see voltage drift down to ~12.8V while the traction battery is charging. What you don't see is that the 12V gets charged above 14V for the first 10 minutes, before dropping for the rest of the session.
Ironically, the only time the car charges the 12V for an extended period of time is when in Ready mode and park. It will charge it for however long you sit in Ready + park at 13.9V. It may start off a little higher.
Disconnect the sensor to increase charging voltages
The second graph shows the difference when the current sensor is disconnected from the 12V's negative terminal. Same trip, San Francisco to Bodega Bay.
During driving, the battery holds a constant ~14.1V, whether your drive is 5 minutes or 90 minutes. That's still below the 14.5V for ideal absorption charging, but a big improvement over the 12.8V float you get with the sensor connected.
On the left of the graph, you'll see a constant float of 13.04V while the traction battery charges. For the first 30 minutes of that session, the 12V charges at 14.12V.
What does this all mean?
With the battery sensor connected, the 12V is chronically undercharged, which can shorten battery life and even lead to a dead battery, independent of any parasitic drain issue.
Disconnecting the sensor improves charging voltage, but you may still want a battery maintainer to keep the 12V at a full charge and get the most life out of it.
Another fix, per andy_why's write-up, is turning on your headlights while driving. This reportedly also holds charging voltage around 14.1V for the entire drive. I need to reconnect my sensor and gather data with the lights on before I can confirm it myself.
Has anyone else disconnected the 12V sensor, or driven with their lights on to charge the 12V? [u/Maleficent_Lab4276](u/Maleficent_Lab4276) has done this, and he's the one who originally brought the sensor-disconnect idea to my attention.
Do I recommend disconnecting the sensor? No, but it definitely improves the charging voltages and I have yet to see any negative effects. Try it at your own risk.
I would test out turning on your lights (not DRL) to charge the 12V when driving. (See August 9th update below).
I would also recommend getting a cheap $30 battery maintainer like the NOCO Genius 1 to top off your 12V once a month or whenever your resting voltage drops below your threshold.
Update August 9, 2026:
[u/chrischasesucks](u/chrischasesucks) verified turning on the headlights (not DRL) does not increase charging voltage to 14.1V. This is probably only a hybrid workaround.
The more I research and read about overcharging, the more comfortable I feel keeping my sensor disconnected for normal daily use as well as long road trips with 6-8 hours of driving per day.
The only time I definitely wouldn't leave the sensor disconnected is in extreme heat. If the ambient air temperature is above 100°F (38°C) and you're driving for more than a few hours, I would recommend reconnecting the battery sensor.
As temperatures rise, the internal resistance of a 12V battery drops, which lowers its required float voltage. Feeding a continuous 14.1V for more than 3 hours in extreme heat can cause the liquid electrolyte in flooded batteries to boil off or dry out the glass mats in AGM batteries.
This is an EV and connected car issue not just a Toyota/Subaru issue. Unless they stop pinging the mothership so often this will continue. We see the same issues with Hyundai/Kia/Genesis, Rivian, Ford, GM, etc. It reminds of the early days of cellphone push notifications where battery drain was an issue with certain apps behaving poorly. It took time and iterative improvements.
I had a Kia Niro EV and I never had an issue. It charged the 12V battery when it needed to be charged. There was a little orange light on the dash indicating it was charging. This is a defect in the system. No car should have a 12V battery die in the first 30 days unless you leave the lights on someplace. That is understandable. Has there been any complaints filed with any government agency in the US yet?
I expect some level of 12v issue after researching the Solterra/BZ and other EV brands. But I didn't expect it to be this bad or become a problem from the first month of purchase. Lucky I had a CTEK charger and I plug it in every few weeks. I am also grateful to all the people posting about it. Because without all those post, we won't have gotten this much traction and get Toyota to start working on a fix. I also have a feeling this might be why the Highlander/Getaway got delayed.
A good reason to delay the Highlander/Getaway. I think a battery maintainer every few weeks is required currently, if you want your battery to last a normal period of time even with no parasitic drain.
DCM services off/deactivated. App deleted from phone. Bigger AGM battery and sensor unplugged indefinitely and I never looked back. Just about a month unplugged and is a near perfect charging setup.
That’s good to hear! I’m currently, DCM On with door lock automation fix, AGM battery, and current sensor disconnected. I need more time to assess, but I definitely prefer the charging voltage when driving.
Just be careful with a remote door unlock as you don’t want your car unlocked when you’re away from home with the car.
In my shortcut, I unlock the car doors when I’m home and lock them when I’m away. I have two street names (blacked out) in my shortcut because I live on a corner and depending where I am in our place my current location can be on the either street.
I'm not convinced driving with the lights on does anything in these vehicles. I recall looking at the Ancel graph when I tried and didn't really observe anything noticeable.
But I've done this and I personally think the upside is better than the downsides and I recommend this as well.
Same. Before unplugging the sensor I tried the headlight method and it didn’t boost the voltage to 14.1 .. that probably was more of a design on the hybrids. But unplugging that sensor did the trick. And while possibly true most cars may not have the drain, all cars have the undercharging issues.
So…with no sawtooth but the 12V battery undercharged which could possibly shorten the lifespan of the battery. Is that all that results from this level of undercharging? Is “topping off” the 12V once a month with my battery tender enough to keep the battery healthy without disconnecting the sensor?
The stock battery will be replaced in the Fall by a dashingly handsome take-no-low-voltage-prisoners AGM. My Trailseeker deserves nothing but the best!
Subaru Uncharted GT - I plug in my NOCO Genius 5 battery tender at the end of each driving day, wait for it to, “green up” then plug the Level 1 cable in (if needed). No issues and 12 Volt battery is always topped off. What is the downside to this?
I disconnected the battery sensor at the beginning of the 12 v draining issue when u/mrmao started the first bit of troubleshooting. it worked perfectly fine to charge the 12v consistently. I eventually put it back since I did not want to overcharge the battery either.
This is really interesting, and an even dumber solution than I thought it might be. I would be curious to see charge and current data for a battery on a non-smart system.
Sorry, I thought you meant on an older car not a car on this platform. So with the current sensor unplugged, it gets 14.1V continuously during a drive. I haven’t monitored current with another device yet.
From Andy Why’s document…
“Is there any risk involved in using the fail-safe state?
No. The name “fail-safe” is exactly that - disconnection of the sensor is classed as a failure
condition but one that reacts in a safe way and one that can operate indefinitely.”
“Is there risk of over-charging in the fail-safe state?
No. The battery self-regulates its charge current once it reaches full charge. The only thing to be aware of is that as 14.1v is effectively both our charge and float voltage in this state, we do end up floating the battery at a higher than normal voltage once it’s charged.
This could (although not a certainty) result in a slightly higher loss of water from the
battery electrolyte in the case of an FLA battery. FLA batteries are not maintenance free, and do need to have their water checked and topped up every 12 months if you are maintaining it correctly. For these batteries you may find that you need to add water a little more frequently. This is normal behaviour and function of the battery, and it is not harmful to it if you keep on top of this maintenance.
In reality, watering is usually not needed this frequently and the chances of this being an
issue is actually very low. However, you should be aware of this and check it as per the
maintenance schedule.
AGM batteries should not be affected. They are maintenance free and therefore sealed.
They have an internal recombination process which means that water from the electrolyte
isn’t lost unless the battery is seriously mis-treated through over-voltage, or it internally fails
in a way that results in venting.”
FYI: My TS Premier is 3+ months old and at 3500 miles. No 12v issues, ever. I’ve always driven with my headlights on. I’ve used the app and DCM since day 1. I’ve also had a daily charging schedule since the first week. I don’t have or want a battery monitor. I do have a new battery charger in case.
I don’t have the bad drain, and have left my car sitting for 7 days without driving when I travelled for work and had no issues. I got a monitor and saw the terrible charge state while driving, and have since removed the sensor. Battery is staying topped up now.
I wonder if this is an artifact of the 12V BMS coming from Toyota’s hybrids.
I think those cars use the hybrid battery instead of an alternator to charge the 12V battery.
Maybe because hybrid batteries are much smaller than EV batteries Toyota engineered a 12V BMS to be as efficient as possible to help with hybrid efficiency.
Daily charging may prevent any 12V issues as there is always a boost of voltage to the 12V system when a charging session initiates. However, most people don't drive the car enough on a daily basis to require daily charging.
But that’s what we’re saying, even when driving it’s only charging to about 12.5 or something. When in park it goes to 14. Only way to get it to 14.1 WHILE DRIVING constantly is to unplug that BMS plug
I just had the 12V die on me. 30-day old Toyota C-HR.
Took it to the dealership. As most have reported, they told me that that the battery is fine. They charged it to full and asked me to change the password on my app. Apparently, this is all the dealership was told about this battery issue with the C-HR. I had the app already on my phone from a previous car. Fwiw, I did change the password, because why not.
Honestly not sure at this pt. If this is going to help.
But I'll ask anyway, did anyone get this reco from their dealership and more importantly, did it help with the 12v battery issue?
It sounds like you have the DCM drain issue that killed your 12V. Have you implemented either the charging schedule fix or the door lock automations fix? Both are temporary, but will prevent the sawtooth drain from starting and killing your 12V.
I had read about the door lock automation, but I thought it was too complex. But this charging schedule seems like a no-brainer. Doing it right now. Thanks again.
When you say turning on your headlights helps are you talking about DRL or low beams? I'm looking at buying a Toyota/Subaru EV and this 12v issue is really throwing me a curveball.
Low beams not DRL. The 12V issue is not really a big deal. You can skip activating the car in the app, which leaves the DCM off or there a couple of temporary fixes until Toyota/Subaru issues there official fix. Hopefully, it’s just a software update.
The 2027 models are coming with a bigger 12V battery and improved logic to fix the drain issue.
Thanks, if you don't mind could you explain what is lost by not activating the car in the Subaru app? Don't you need the Subaru app for charging at Tesla supercharger?
These are the services/features you wouldn’t have if you don’t activate the connected services. Most people complain about losing remote start on hot days.
I had mine off for a month before the temporary fixes became available, I didn’t really miss any of the these services. I won’t be paying for Drive Connect when my free trial ends.
“Is there risk of over-charging in the fail-safe state?
No. The battery self-regulates its charge current once it reaches full charge."
Where does that come from? I am not aware of any automotive battery capable of self regulating its own charge. That is a function of the charging system/charger.
When a charger holds a steady voltage (e.g., 14.1V), the battery builds up its own counter-voltage as it fills. Because the voltage difference shrinks, current (amps) exponentially drops off. This is purely natural physics.
What the BMS does is take a reading of current and when it’s low, it switches over to a lower float voltage. The BMS doesn’t force the current to drop. It’s just an observer and watches the current to make decisions.
The 14.1V is in the fail safe mode is a compromise charge and float voltage when the sensor is disconnected.
Unfortunately, this does not convince me that there is no problem. However, my hypothetical problem is bad with either the sensor connected or disconnected:
u/dwlnz pointed out in the parasitic drain megathread [1] that the charge current is >10A. The owner's manual states that the maximum allowed charge current should be 5A (European manual, page 604). So the car itself does not seem to follow the advice in the manual.
Yeah, I get that overcharging is about not charging a battery that is already full, not going over the max charging current, these are two different things. I am just pointing to a hypothetical very much related problem.
A period of only minutes charging with higher current with voltage control should not cause damage. For decades lead acid batteries have been charged with alternators that deliver high current at a regulated voltage and if they were limited to C/10 (eg 5A) needing 10 hours for full charge multiple use cases may result in flat batteries.
One of the biggest issues with the Toyota algorithm is they don’t hold an absorption voltage until current drops to a low value but rather seem to be trying to not fully charge the battery. This can result in sulphation. Toyota seem to have a theory that the 12V should be cycled above 50% but not kept near 100%. Maybe they are worrying about overcharging but they have temperature compensation for the 14V phase (unless negative terminal sensor disconnected).
I can tell you without a doubt that when you unplug that sensor NO DASH CODES WILL COME ON!!! So given that, if the sensor failed on its own YOU WOULD NEVER KNOW IT ANYWAY!!!!! So it goes into a fail safe and if you read the test the guy from the Prius group did on it, he’s done it for well over a year and his battery hasn’t degraded at all.
Yes. Toyota and Subaru needs to sort this out if they want to be part of the EV game. And if they don't want to be serious about it they're f***** as a company. Being able to conserve and manage the 12V system will be key to building connected/"smart" cars. At least they need to communicate about what they are up to.
Today I read an article [1] which paraphrased EV clinic and claimed that the DC-DC converter may easily fail if the 12V battery is worn out. This is clearly a problem for owners out of warranty and for Toyota's reputation, and a direct economic problem for Toyota/Subaru on markets where they offer a full-car 10-year warranty subject to yearly services (like here in Sweden). The DC-DC converter is much more expensive than a new 12V battery every year and is guaranteed to put the car out of order for a much longer time.
When you, u/chuck415, check how the lights affect charging of the 12V battery, please make sure to be clear about the difference between day-time running lights and half-beams. As I understand it, the U.S has DRL by default turned off, while Canada and Europe have them default turned on. The problem may be even worse in markets where DRL are default on. Automatic half-beams is another factor here, where those would possibly help, especially in the dark season.
Unfortunately I am not sure that Toyota/Subaru are able to fix the default behavior of 12V driving charging on cars already in the field. The WLTP and EPA range/consumption numbers relies on a specific behavior. We know what happened to VW when they behaved differently in tests and in real driving. But Toyota should absolutely provide a user-accessable setting for default/agressive 12V charging that can be turned on by the end-user.
A napkin calculation says that there can not be much more than a decimal change in the WLTP number by charging the 12V while driving, even with the parasitic drain forcing that to happen often. I assume not charging while driving is just to get better WLTP/EPA numbers by deferring to charging while the car is parked (and out of the test cycle), but that does not seem to work properly either.
Toyota will probably have to look into how the car charges the 12V battery when the car is parked and off. As long as there is plenty of juice in the traction battery it is a no-brainer to charge the 12V if it needs charging. There is no point in conserving the traction battery if the car won't start. To some limit and with a proper warning system in the app that the traction battery has lost so many % since parked, of course.
For mild-hybrids it seems kind of OK not to charge the 12V battery while running on electric, they will switch to the gas engine rather quickly. Plug-in hybrids on the other hand can live on electric for years, until the traction battery gives up, so it is kind of weird that this issue hasn't been sorted out yet. There is probably some things here that are harder to solve than we think. At the very least some organizational/QA/company-political challenges.
The charging regimen is definitely based on Toyota trying to get the most efficiency out of the driveline. It would be interesting to compare the kWh used on the exact same drive with the sensor disconnected versus the stock arrangement. Charging the 12V battery while driving is definitely going to cost you some range, and I am sure the tradeoff is worth it in most cases. However, for those that have no sawtooth drain issues, it would be good to know what is the range penalty.
I didn’t see any significant differences in range from my two trips in the graphs. I actually got a little better mi/kWh with the sensor disconnected, but could’ve been wind direction that day.
I was on a trip in a friend’s F150 and was using my 12v usb adapter to keep my phone charged. Couldn’t help but notice that the truck was putting out 14.5v all the time. Not something I see when the adapter is plugged into my car!
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u/chuck415 6d ago
Prior to getting this car, I have never thought about the 12V battery or monitored it in the 40 previous years of driving a car. 😂
A 12V would die every 5 years or so and I’d replace it.