For those who have installed, or are considering installing the ANCEL BM200 battery monitor in their RX350h, I offer the following information.
I have installed one in my 2026 Lexus RX350h recently and became concerned that I often observed "Low Charge" indications, however my car never failed to start although the battery voltage rarely dropped below 12.5 volts.
It's my understanding that the hybrid RX does not have the traditional starter motor as needed in the traditional ICE, and perhaps this means the 12 volt energy required to start the hybrid vehicles is significantly less.
I emailed my concerns to ANCEL and they confirmed that the software employed in the BM200 is not best suited for hybrid vehicles at this time. Therefore, I use it only to observe actual battery voltage and no longer become concerned with the frequent "Low Charge" indications.
For your information, here is the reply shown (within the quotation marks) that I received from ANCEL to my concerns:

"Based on our test data and the details you provided, we found that this situation does appear to be related to the hybrid vehicle working logic you mentioned.
Since your 2026 Lexus RX350h does not rely on the 12V battery in the same way as a traditional gasoline vehicle with a starter motor, the BM200’s current battery status judgment may not always match the actual starting condition of the vehicle.

Your feedback is very helpful. We have already asked our technical team to take this hybrid vehicle scenario into further consideration as soon as possible. As more hybrid vehicles are now being used, we agree that this is an important usage situation to evaluate.

At this stage, the “Low Charge” indication does not necessarily mean your vehicle battery is unable to support starting, especially since your vehicle has never failed to start and the battery has not shown obvious symptoms in actual use.

We sincerely appreciate your patience and the detailed info you provided. We will continue following this with our technical team and do our best to improve the support for this type of vehicle use case."

I hope this information is helpful for Hybrid RX owners.

 

Get a small portable jump start device. Keep it under the passenger seat. Look up how to hook it up as once the 12V fails, it's difficult to get the rear hatch open to access the battery.

Know how to use it (and also have your "other half" there to learn) so when it's dark, raining, or just a sunny day, The stress related to starting the car is diminished.

 

Thank you for that info. We both already have portable battery chargers located in each of our Lexus hybrids, and we’re familiar with getting into the battery compartment if there’s not enough power to open the tailgate from outside the car. We also know how to manually open the driver door with the mechanical key when the battery fails.

 

On my 2025, when the battery died and I was "locked into" the car, I found out that just pulling the door lever "in" two times opened the door.

 

I know that their is dedicated "thread" on the 12v battery issue. Its driving me crazy as i was semi sold on moving into a Lexus RX hybrid for my next vehicle but reading all the feedback makes me wonder is that is the right move. Lexus needs to address and solve this once and for al IMO. The fact that many owners have not had any issues but are concerned that it "could" happen is also worrisome. I have been doing some research and seems that other MFG's have similar 12v battery issues and other don't.. why is that is the question i ask myself. Are the MFg's that are seeing 12v issue using the same technology that Lexus/Toyota has ?

 

Something your sales guy or tech guy should have shown you before you drove off with it on delivery day.

 

you can just as easily hook up your jumper box to the connections under the hood, thus avoiding the gymnastics required to open the rear hatch from the inside.
Some have also reported attaching leads to the battery that are long enough to be reached from the back seat. You can hook the jump box up at that point.

 

I agree wholeheartedly. But, if you show my wife how to hook it up under the hood, don't be surprised if you got a call that says "It doesn't work"....or "Why did the car catch on fire?"

Gymnastics is a far, far better method. She knows +=POS and -=NEG

 

Yup. Shoulda, Woulda, Coulda.

I don't think sales guys are to exceed to show anyone who dropped the coin on a Lexus by saying....."And this is how you get out should your battery go dead."

 

yes indeed that is how to open the digital handle from the inside if no power.

 

Then, after I got off the phone with my dealer to get the car towed back in.....it dawned on me that "what if" I had a grandkid in a car seat in the back? They can't pull the door lever to get out...heck...they can't unbuckle themselves.

Some YouTube searches shows how to unlock the door from the outside. You gotta have the Key located in the Key Fob. For the uninitiated....there is a physical key in those FOB's. And you can unlock the door from the outside with the physical key.

 

The fobs have had key blades in them for many years.

 

True. 99.9% of car drivers who have a FOB know that.

But what Lexus owners may not know is how to enter the rear door (same as the front door) of the vehicle when the 12V battery is dead, it's 98degrees outside, you have a kid in a carseat in the back, and the electronic door function doesn't work.

It's not the Key in the FOB. It's the knowledge of how to use it.

I've always owned vehicles that even when equipped with power locks, their is the manual pull lever\button to unlock the door, and all my previous vehicles had at a minimum one key entry to the drivers door, and most had key entries on both driver and passenger front doors. But if you don't know how to use the key to open a Lexus, and you are in a semi-panic state, it's probably too late to google a video or read the manual. And really.....who thinks of this when you acquire a vehicle?

 

Yes - all these monitors have a hard-coded "lookup table" that converts voltage to state-of-charge - as these monitors cannot directly measure the state-of-charge, they just measure the voltage. However, that lookup table is really not calibrated to the batteries and associated charge in the Lexus or similar hybrids or EVs - Ancel/others probably copied-pasted that lookup table from their ICE car data/knowledge. I have noted this multiple times in the 12 V battery death thread - but I'm glad Ancel directly confirmed this to you. There is a person in the battery death thread who even reprogrammed that state-of-charge vs. Voltage lookup table in their monitor (certain versions allow it) - so as to be more accurate and relevant with its warnings.

Its bad enough that the problem exists in a Lexus - and its worse that these monitors are misleading in their warnings. And lots of misinformation on the internet.