It's better maintain 14 volts instead of having the battery fall to partial charge. Battery tenders typically will go to about 14 volts then settle down to ~13 at least the ones I've seen. What kills any battery is a deep discharge/charge cycle, always charging it not so much. Low charge causes sulfation which is what is killing the batteries in this RX.

 

I did some more reading and think I grasp things a bit better. Apologies as I'm not too versed on the topic.

So basically, the Battery Tender that I have (1.25A variant) is modern and perfectly safe for the AGM lead acid? type batteries that these vehicles use.

Meaning, I can either use it for a few hours to add some AH to the 12V or leave it on for an extended period, where the battery will go into excess 'surface charge' so the voltage reading will temporarily be higher but will settle down as the excess free ions naturally discharge (~24-48 hours).

I pulled the charger off just now and got a reading of 13.3V.

If I retest after some period I will get a more accurate voltage measurement and it should read at/around 12.6V assuming low drain has occurred since the Battery Tender topped it up. In general, these batteries are designed for this so there's no harm or depletion of its longevity to let the Battery Tender go into its maintenance state?

And effectively, any charge at/above 12.6V is fully charged regardless if the Battery Tender doesn't see it that way?

But basically, if the vehicle was previously at 12.25V, that is at a discharged state but I'm still confused as to if that's within the normal range. What does Lexus try to keep the battery charged at? And what does it mean that the battery won't last long in this state? Does that mean it's lifespan or just that if I don't drive a lot or give it a few 'Ready' sessions in park that the battery will go low enough to not start?

Just trying to better understand and hopefully bring more attention so that some kind of OTA software update will come out this Summer to address this.

Edit: It seems that if I just charge to 12.6V with a custom cutoff, it won't be a 100% charge. You would need to go into the 14V+ range to eventually get to the 100% when the surface charge dissipates. Is that more correct of an assumption?

From what I read, the Genius 10 is a pretty good one and will work a lot faster than my 1.25A Battery Tender but is it still just as 'safe'?

 

0.1C or 10% of the amp-hour capacity of the battery is the maximum healthy charge rate. 6 amps for a 60 A-H battery as an example. A slower rate of charge doesn't hurt anything. A quality battery tender can be left on forever.

The following screenshot shows how a BatteryMinder 128CEC, which is one of the best chargers available, charges at 4 amps. It started at a resting voltage of 12.28v, rapidly climbed to 13.29, then gradually climbed to 14.18, whereupon it switched to a float charge of 13.48v, which it would keep a long as it is attached to the battery varying with temperature. A battery will last a very long time when kept at the proper float charge, 10 years not being unheard of. Most chargers are not temperature compensated though and it really only matters at the temperature extremes for automotive batteries. I typically recommend the BatteryMinder 1510 as it is much more affordable than the 128CEC and meets most people's needs, being a 2 amp charger with temperature compensation.



I find that the RX450h+ allows the battery resting voltage to fall below 12.3v. I typically charge the battery when it falls below 12.4, which is the voltage at which sulfation occurs. The charging system will occasionally give the battery a higher state of charge, I believe to prevent sulfation, but usually it does not fully charge the battery. The only time I see the charging system act like a normal charging system consistently is when the vehicle is on and in Park.

By way of comparison, the charging system in my 2025 GR Corolla always keeps the battery much closer to fully charged and will raise it to fully charged if driven regularly, even after sitting for a while. In the following screenshot, it starts at 12.51 and is at 12.81 within 2 days of driving at the start of the 3rd day.


This is the RX450h+ over the course of a 4 day 1,000 mile road-trip, starting at 12.55v, and winding up at 12.46 at the start of the day after the conclusion of the trip.

 

Interesting. Thank you for the reply. Do you have an opinion on the Noco Genius 10 and if it would be as safe or is it beyond what should be used? Apparently, they claim it’s safe but I wanted to know why or how since it’s a 10A charger.

 

I only have experience with one NOCO Genius charger and don't remember the model. It's been over 10 years. I found that it didn't have a float mode, but that may have changed. I also didn't like their proprietary quick connector, which stopped working after a few years. The NOCO Genius 5 would be more appropriate for the RX battery. The 10 will supply too much amperage.

 

Did some further research and it looks like NOCO Genius still does not have a float mode. It goes into a "maintenance mode" where the charger stops charging and monitors voltage. If the voltage falls below a certain point, it starts charging again. That's much better than nothing, but I prefer a charger with a float mode.

https://no.co/support/gx-understanding-charging-steps

 

Just picked up my RX450h+ 10 days ago, I have put 288 miles on it since then. I will be going away for 5 days so I decided to hookup my NOCO Genius 5.

Before doing so I tested the battery and it was 12.2v. Not good?

How low can it go before the car will not start?

 

12.2v is not surprising. Keep in mind that, if you are measuring with a door open or even with the the key fob near, that will pull the voltage down. I used to leave a charging pigtail hanging out of the rear to measure resting voltage, before I got a Bluetooth monitor. I would wait an hour before measuring and make sure to not bring the fob near.

I don't know what the minimum voltage is, but I'd start getting worried about it not starting around 12.0v resting voltage.

 

Thanks for the quick reply.

I was wondering about the fact that the hatch was open and I had my fob in my pocket. Also, will the voltage go back up after shutting the hatch and hiding the key or when it goes down it is down?

I think I am going to have to invest in a bluetooth battery monitor. Is this the exact one you guys recommend?

 

That's the battery monitor i have and like.

I always give it about an hour to recover from being open.

 

I will definitely pick one up when I get back from Sonoma.

Thanks again!

 

@WellsB Since I have your attention....

What voltage level do you set your BM300 to alert you?

 

I bought an Ancel BM200. Would there be any difference using the BM300 or BM300 Pro instead? I liked the smaller form factor and we don't need weather proofing for the hatchback area.

 

I see conflicting information about the BM200 vs 300 vs 300 Pro. I went with the pro because it could monitor multiple vehicles from the same app, but it looks like the BM200 can do that too. It looks like the BM200 was recently updated though so might have more features than before.

30% and 50%

 

Based on Ancel's specs, https://www.ancel.com/products/ancel...0284ac40&_ss=r it looks like the BM200 is the way to go. It is $33 CAD on Amazon.