neither car will use the 12V battery to start the engine. It just powers the 12V electronics and computers.

 

When I say "startup power requirements", I'm talking about the requirements to start the hybrid system and electronics, not the ICE.

 

As informative as this guy is (seen his content over the years). It does absolutely nothing in reality with charging the vehicle. I've never had a battery die on me...EVER! I've owned a lot of different vehicles over the years and it wasn't until this hybrid that issues starting coming up.

 

Well, its not this guy’s job to offer us a solution - he’s merely describing how it’s supposed to work.

That is/was the job of Lexus, and they probably loaded a Prius’ charging algorithm and same converter on to RX/NX with much higher current loading. If we ran the same diagnostics as Kelly on a RX or NX hybrid, they might fail in showing that there is any current going into the 12 battery when all of the other loading is present (even in Ready mode). IF the total current loading capacity of the DC to DC converter is the same as its in the Prius to save Lexus cost, then that converter’s loading is maxed out in the RX or NX hybrids. That would certainly explain why even modifying the algorithm may not be enough, if the mis-spec’d converter’s reliability in the RX/NX is going to be compromised by the overall needed loading by all the sensors and lights and hvac in the RX/NX hybrids.

 

I appreciate your quick response. I was looking at the whole merged thread, and it took several days to get through the 900+ posts☹️

 

I just had my battery replaced yesterday. I was sitting in the car in a pkg lot early in the morning answering some emails when the electrics went totally out!

After getting Roadside Assistance to jump me and bringing it to Lexus dealer they determined the battery was failing. The car recently surpassed 8K miles. This is the battery they replaced the original with.


2025 Lexus NX 350h battery dealer replacement

Again, 540 CCA/RC100. Now they said it had a 7 year warranty, but total replacement only for first 2 years, 50% cost for next 2 years, and pro-rated for following 3 years.

Not thrilled with this solution, nor continually going back to the dealer... Thoughts?

 

ALWAYS ALWAYS ALWAYS keep the hybrid ignition ON (READY) when when you sit in the car. The ICE engine will run once in a while but the battery will last.

Otherwise, YOU will cause the battery to go flat within an hour. <<---- this was emphasized to us when we first purchased our RX

 

After all the battery talk began, I thought it would be prudent to get a voltmeter. I've been monitoring for months and only lately have I observed a change in the charging patterns.

In warmer weather it would read 14.1 for less than 2 blocks, then drop to 12.7, and stay there. typical drive would be between 6 to 10 miles. If there were stops in the middle that pattern would repeat.

Now, in colder weather, it starts at 14.1 and holds that for several miles, before dropping to 12.7.

Don't know if others have reported similar behavior, but I'm satisfied that the charging system seems to be operating in a sensible way.

 

Yes, we're well aware of the charging system only charging (providing >14v) for a short time. Many of us do not consider that to be sensible behavior.

My GR Corolla, which has the same exact make and model of battery as our RX450h+, charges constantly, never dropping below 14v. the battery maintains a consistently higher and healthier state of charge as a result. Even with that constant charging and most of my drives lasting 30 minutes or more, the battery is not kept fully charged,(though it's close) and a 4-amp charger will spend an hour or more topping it off.

The available solutions(as I see them);

Disconnect the voltage sensor. This puts the charging system in "fail-safe" mode and causes it to constantly provide >14 volts, like a traditional alternator system. This has an unknown effect on the vehicle systems and some have warned that this could cause the DC-DC converter to fail on your own dime. I also have concerns about constantly charging the battery, but traditional alternator systems have been doing that for decades and continue to do that today. It's the only thing that directly addresses the underlying issue, but could cause costly other issues.

Install a Bluetooth battery monitor. This does not address the underlying issue, but does give warning when the battery is dangerouslly low on charge.

Charge the battery with a standalone charger every 2 weeks. This prevents the battery from dropping below 12.4v, which is where sulfation starts. Sulfation shortens the lifespan of a lead-acid battery. I believe this will prevent most dead batteries and premature battery failures. The timing can be tweaked using a Bluetooth battery monitor.

Carry a jump starter such as the Schumacher SL1648, which has leads that will reach the Lexus approved jump points. Familiarize yourself with manually unlocking the door and with jump startimg the vehicle. This won't solve the underlying issue, but will prevent you from having to call roadside assistance with the attendant inconvenience and delay.

Install a Ionic Emergency Start which is a Bluetooth enabled permanently mounted jump starter. This won't solve the underlying issue, but will prevent you from having to manually unlock the door and hook up a jump starter if the battery dies.

 

Could it be the DC/DC converter hardware design that cannot be corrected with firmware and deemed too costly for a replacement recall ? Why else would Lexus delay a simple algorithm change that affects so many.

 

Even if a firmware update could not fix it, I would expect to see it changed in new vehicles, if Lexus / Toyota saw it as a flaw.

 

Wells, can you install both a Bluetooth mom AND the Ionic?

 

Yes, I have the Bluetooth battery monitor and an Ionic Emergency Start installed. I also charge the battery when it reaches 12.4v resting.

I recently tested the 2-year old battery and it is showing almost identical cranking amps (629 CA) as the 1-year old battery in my GR Corolla (673 CA) The batteries are rated at 345 CCA.

 

Insightful question. You have to consider that Lexus would have to first own up the problem and replace/change the DC-DC converters and charging algorithm in EVERY NX and RX hybrids (i.e. NX/RX 350h/450h+) ever sold, for proper legality - they can't simply pretend the problem is for specific VINs. At the very least they have to make every single such car ever sold be eligible at the dealers to get the fix, if a user brought the car to the dealer and asked for the fix. This is similar to the 500H transmission problem - whenever an owner brings up the problem at the dealer, they are either fixing with ECM code update or hardware changes if necessary. And also be ready to buyback the 500H if the owner was irate, thus avoiding a lemon-law lawsuit from the owner. So yes, it would be definitely more costly to replace the converter+algorithm, and hence the more credible reason behind lack of a fix by Lexus, if they also need to replace the DC-DC converter with the charging algorithm to make the problem truly be fixed. And DC-DC converters not being reliable with long, high current draws is a known reliability problem (due to fundamental reliability physics) in the entire EV/hybrid industry, not just for Lexus - which is why the voltage sensor and algorithm is necessary to be there in the first place - otherwise carmakers could've just treated charging the same as an ICE alternator, always charging the 12 V battery at constant 14 V from the DC-DC converter, without any voltage sensor or algorithm. So I definitely don't consider removing the voltage sensor a "fix" by any means, and won't be doing it in my car at all. Its very poor advise from a stranger on the internet, as far as I'm concerned.

On the other hand, Lexus can just look the other way and pretend its not a problem. After all, they will never be forced by any govt. authorities to fix it, because the problem doesn't happen when the car is moving - and therefore not legally deemed by the authorities to not be a safety problem (irrespective of the fact we think it can be, because an a child/aged person can get locked in without them/owner knowing that there is a mechanical/manual override to open the door, etc).

You can see the 25LA06 recall for rear-view camera firmware they are doing - its for specific VINs and hardly affecting people in the real-world - how many times does a typical owner actually experience that problem in the real world ? I did not see anyone in this forum or online complain about it, because Lexus is still doing it as a safety recall, because it (even if very rarely, if at all) happens when the car is moving - and therefore comes under the purview of NHTSA and such govt authorities, forcing Lexus to take ownership.

Hence, I think its a purely financial decision by Lexus to not fix it, as you said - but removing the voltage sensor is not the hack I choose to adopt, as I understand the system design.

 

Probably a stupid question but is it ok to leave a window open for a couple of weeks? I am going on vacation and want to ensure I can easily open the door. It will be in a garage.