Dead 12v battery Incident (merged threads)
Intermediate
Joined: Apr 2017
Posts: 298
Likes: 124
From: CA - California
This is 100% a charging issue based on the graphs presented earlier in the discussion. There is no chance that you got 4 faulty batteries in 3 years, and if it's not a battery issue that only leaves one thing...
Driver
Joined: Jun 2024
Posts: 71
Likes: 44
From: DE
I have heard a lot of discussions on dead 12V. I am on my 4th battery in 3 years. Lexus had performed a load test to see if there is an excessive draw and said everything is working fine. They did however find that the battery failed and they replaced it under warranty. They claim I do not drive my RX enough to charge the battery. That I need to go for a 20+ min. drive once a week. So after my 3rd battery, I started tracking my voltage daily. I even drove 250 miles one day and noticed the battery voltage after a few minutes got up to 14.2V which I thought now its charging but that only lasted for 10 min. The rest of the way it measured 12.6V. The next morning, I measured the resting voltage and it was at 12.2V. This is a brand new battery that you would think would be fully charged. I have actually had longer charge times with the RX parked in Ready mode then driving longer distances. I wonder if this is more of a charging issue then a battery issue. Could it be a software issue that limits the voltage to prevent overcharging? I questioned the dealership and provided them with the data I took. They said they would pass it along to Lexus. As a good will, they installed a battery tender to my battery so I can plug it in once a week. Anyone have any thought on how to checking out the charging system?
- Charging algorithm under charges to save wear and tear on the DC to DC converter; and possibly to claim a higher mileage rating (i.e. ICE would run longer to charge the 12V if programed to charge at 14+ Volts for longer periods, decreasing overall mileage).
- Plain old bad batteries (many who had their battery replaced seem to do ok after replacement). Also too low Amp rated batteries installed which cannot handle the high draw of the Lexus electronics. Some electronic draws can be turned off but I don't see that as the bigger problem.
NOTE: I have a Pruis that sits in the garage for a couple months while away on vacation and it still opens up the system to start the car when I come home. No need to drive daily or charge the battery every week. The Lexus electronics and DC charging are mismatched.
Pit Crew
Joined: May 2025
Posts: 197
Likes: 74
Toyota/Lexus, with all their PhDs in software engineering, electrical engineering and other disciplines, should have now given us solutions in the dead battery issues, because Consumer Reports as well as other publications, have been (since 2022-2025) reporting these issues based on owners complaints throughout such years. It seems crazy that we are using this forum to speculate about what has gone wrong and what can be done to definitively resolve these issues. But what else can we do (besides implementing these band-aid fixes) for the time being??
Pit Crew
Joined: May 2025
Posts: 197
Likes: 74
The following is a cut and paste from a recent AI search:
"While Lexus and Toyota have issued several technical service bulletins (TSBs) and software updates, many owners report that these haven't fully resolved the recurring dead 12V battery issues. The persistence of these problems is primarily attributed to complex interactions between modern vehicle technology and traditional lead-acid battery limitations:
"While Lexus and Toyota have issued several technical service bulletins (TSBs) and software updates, many owners report that these haven't fully resolved the recurring dead 12V battery issues. The persistence of these problems is primarily attributed to complex interactions between modern vehicle technology and traditional lead-acid battery limitations:
- Parasitic "Always-On" Systems: Modern
models use Data Communication Modules (DCM) that stay active to communicate with smartphone apps. Software glitches in these systems can cause excessive "phantom" drain that depletes the 12V battery overnight.
- Inefficient Charging Logic: Some experts suggest the "smart" charging algorithms are flawed, sometimes stopping the charging process after 90 minutes of driving or maintaining the battery at only 80% capacity to leave room for regenerative braking energy. This can lead to sulfation, which permanently damages the battery’s ability to hold a charge.
- Undersized 12V Batteries: In many hybrid and EV models, the 12V battery is primarily used to engage the high-voltage system and power accessories rather than crank a heavy engine. Lawsuits allege these batteries may be 15% to 20% undersized for the high electrical load of modern electronics.
- Reliance on Lead-Acid Tech: While Tesla has moved toward lithium-ion for its low-voltage systems, Toyota largely continues to use traditional lead-acid batteries, which are less resilient to the deep discharge cycles common in modern connected vehicles.
- Class Action Lawsuits: A major class action lawsuit was filed covering 2023–2025 models like the Toyota bZ4X and Lexus RX hybrids, alleging the manufacturers knew of the defects but provided inadequate fixes.
- Dealer Interventions: Dealers often perform a DCM reset or firmware update to slow down app pings, but many owners find the battery needs full replacement after it has been deeply discharged even once."
Pit Crew
Joined: Jan 2022
Posts: 243
Likes: 111
From: Canadian Prairies
The following is a cut and paste from a recent AI search:
"While Lexus and Toyota have issued several technical service bulletins (TSBs) and software updates, many owners report that these haven't fully resolved the recurring dead 12V battery issues. The persistence of these problems is primarily attributed to complex interactions between modern vehicle technology and traditional lead-acid battery limitations:
"While Lexus and Toyota have issued several technical service bulletins (TSBs) and software updates, many owners report that these haven't fully resolved the recurring dead 12V battery issues. The persistence of these problems is primarily attributed to complex interactions between modern vehicle technology and traditional lead-acid battery limitations:
- Parasitic "Always-On" Systems: Modern Toyota and Lexus models use Data Communication Modules (DCM) that stay active to communicate with smartphone apps. Software glitches in these systems can cause excessive "phantom" drain that depletes the 12V battery overnight.
- Inefficient Charging Logic: Some experts suggest the "smart" charging algorithms are flawed, sometimes stopping the charging process after 90 minutes of driving or maintaining the battery at only 80% capacity to leave room for regenerative braking energy. This can lead to sulfation, which permanently damages the battery’s ability to hold a charge.
- Undersized 12V Batteries: In many hybrid and EV models, the 12V battery is primarily used to engage the high-voltage system and power accessories rather than crank a heavy engine. Lawsuits allege these batteries may be 15% to 20% undersized for the high electrical load of modern electronics.
- Reliance on Lead-Acid Tech: While Tesla has moved toward lithium-ion for its low-voltage systems, Toyota largely continues to use traditional lead-acid batteries, which are less resilient to the deep discharge cycles common in modern connected vehicles.
- Class Action Lawsuits: A major class action lawsuit was filed covering 2023–2025 models like the Toyota bZ4X and Lexus RX hybrids, alleging the manufacturers knew of the defects but provided inadequate fixes.
- Dealer Interventions: Dealers often perform a DCM reset or firmware update to slow down app pings, but many owners find the battery needs full replacement after it has been deeply discharged even once."
Toyota and Subaru’s first-ever all-electric SUVs are now under legal fire. A new class action lawsuit alleges that the 2023–2025 Toyota bZ4X and 2023–2025 Subaru Solterra suffer from a critical defect in their 12-volt battery systems, leading to repeated failures, stranded drivers, and out-of-pocket towing costs.
Lexus Fanatic
Joined: Sep 2010
Posts: 26,049
Likes: 4,327
From: Alberta
At this point I'm going to assume the NHTSA doesn't consider this a critical safety issues which means there will never be a fix. On disconnecting the battery temperature sensor does this trigger any codes? If not then seems like the perfect solution.
I completely reject the theory that Toyota did this to reduce strain on electronics, Toyota has been making reliable hybrids for 3 decades that properly charge the 12 volt battery.
I completely reject the theory that Toyota did this to reduce strain on electronics, Toyota has been making reliable hybrids for 3 decades that properly charge the 12 volt battery.
Instructor
Joined: Nov 2023
Posts: 979
Likes: 646
From: California
At this point I'm going to assume the NHTSA doesn't consider this a critical safety issues which means there will never be a fix. On disconnecting the battery temperature sensor does this trigger any codes? If not then seems like the perfect solution.
I completely reject the theory that Toyota did this to reduce strain on electronics, Toyota has been making reliable hybrids for 3 decades that properly charge the 12 volt battery.
I completely reject the theory that Toyota did this to reduce strain on electronics, Toyota has been making reliable hybrids for 3 decades that properly charge the 12 volt battery.
Instructor
Joined: Nov 2023
Posts: 979
Likes: 646
From: California
A On disconnecting the battery temperature sensor does this trigger any codes? If not then seems like the perfect solution.
I completely reject the theory that Toyota did this to reduce strain on electronics, Toyota has been making reliable hybrids for 3 decades that properly charge the 12 volt battery.
I completely reject the theory that Toyota did this to reduce strain on electronics, Toyota has been making reliable hybrids for 3 decades that properly charge the 12 volt battery.
Last edited by wrinkle; Feb 10, 2026 at 05:50 PM.
Racer
Joined: Oct 2023
Posts: 1,292
Likes: 1,077
From: CA
Voltage sensors and batteries not being fully charged by the charging system, by design, have been around for a while now. I first ran in to both with my 2017 Ford Focus RS, which intentionally kept the battery at a reduced state if charge, much like our RX's.
My 2025 GR Corolla has a voltage sensor, but also has a traditional charging profile, always putting out >14v, as well as using the same exact make and model of battery as our RX450h+.
My 2025 GR Corolla has a voltage sensor, but also has a traditional charging profile, always putting out >14v, as well as using the same exact make and model of battery as our RX450h+.
Instructor
Joined: Nov 2023
Posts: 979
Likes: 646
From: California
Voltage sensors and batteries not being fully charged by the charging system, by design, have been around for a while now. I first ran in to both with my 2017 Ford Focus RS, which intentionally kept the battery at a reduced state if charge, much like our RX's.
My 2025 GR Corolla has a voltage sensor, but also has a traditional charging profile, always putting out >14v, as well as using the same exact make and model of battery as our RX450h+.
My 2025 GR Corolla has a voltage sensor, but also has a traditional charging profile, always putting out >14v, as well as using the same exact make and model of battery as our RX450h+.
Why put these in every car, as if the intent is that the user would take the sensor off after purchasing the car ?
Instructor
Joined: Nov 2023
Posts: 979
Likes: 646
From: California
At least the PHEV - which is the heaviest of the bunch, gives 2.5 miles for every 1000 Watts-hours (1 kW) of HV battery power. I'm getting 2.9 miles/kWh.
Do you really believe that charging a puny 60 A-h 12 V battery, sipping 5 Amps, if that, while driving, if continuously fed with 14.5 V - would make any meaningful difference to the 2.5+ miles/kWh or equivalent 35 mpg fuel economy ? Does that math make any sense to you, WellsB ?
Do you really believe that charging a puny 60 A-h 12 V battery, sipping 5 Amps, if that, while driving, if continuously fed with 14.5 V - would make any meaningful difference to the 2.5+ miles/kWh or equivalent 35 mpg fuel economy ? Does that math make any sense to you, WellsB ?
Lexus Fanatic
Joined: Sep 2010
Posts: 26,049
Likes: 4,327
From: Alberta
At least the PHEV - which is the heaviest of the bunch, gives 2.5 miles for every 1000 Watts-hours (1 kW) of HV battery power. I'm getting 2.9 miles/kWh.
Do you really believe that charging a puny 60 A-h 12 V battery, sipping 5 Amps, if that, while driving, if continuously fed with 14.5 V - would make any meaningful difference to the 2.5+ miles/kWh or equivalent 35 mpg fuel economy ? Does that math make any sense to you, WellsB ?
Do you really believe that charging a puny 60 A-h 12 V battery, sipping 5 Amps, if that, while driving, if continuously fed with 14.5 V - would make any meaningful difference to the 2.5+ miles/kWh or equivalent 35 mpg fuel economy ? Does that math make any sense to you, WellsB ?