I dont know if the problem is due to the hot weather.
At the moment my car will switch into petrol after 25 mph, in the past it use to switch 30mph or even 35.
even if i press the throttle a little hard when in ev it goes into petrol, in the past it felt like i can use more torque in ev mode.
I have no warning the car is still doing 38 mpg to 40 mpg.

ive used dr hybrid, i know i can pay to do a battery health check but am told not reliable.
above is a screen shot while idle, cell #11 12 13, seems low or is that normal?

 

I'm guessing your IS300h is a 2013 with about 80,000 miles on it...or around there.
Unfortunately we don't get the IS300h here in North America, which is the vast majority of the forum members.
There are a few folks from your side of the world on here with the same car.

Just a general comment about the hybrids...at this age (7+ years old) the HV battery is getting older, so likely not holding a charge as well as it used to.
The hot temperatures will definitely have an effect on it, and the low battery cells are likely a sign the battery is getting close to the end of it's life.

 

Thanks agian for your advice, am pretty sure you've replied to all my threads and always have the right information, done 110k car is still smooth and runs better then most cars after half the mileage, you a Toyota Toyota Lexus technician by any chance?

 

My IS hybrid battery died at 90,000km. Drove around for a month while waiting for the replacement. Big jump in economy after replacement. Earliest symptom I recall was how it won’t stay in EV mode before battery replacement. Actually I know a surprising number of people who had to replace their Hybrid battery in IS, NX and GS models. The GS apparently also could suffer from inverter issues.

Unsubstantiated info is that the IS 300h was not offered in the US because of safety concerns in rear crash testing because of its battery placement, and that it would be uncompetitive with the Camry hybrid in performance, as well as the German rivals. (Well this part doesn’t need substantiation).

 

Did you have replacement from Lexus and how much did they charge?
my one doesn't always want to stay in ev. But still getting around 40mpg, some did say try changeing the 12v battery first as the ecu for the hybrid needs a good 12v battery.

 

It was recommended by Lexus to change under warranty. Not sure about the 12v battery but if that’s no good it could affect other systems. Not sure about 40mpg but my average range per tank is between 900-1000km.

 

There is nothing wrong with your battery pack. You would get check hybrid if there is.

 

once your car starts how good or bad your 12v is doesn’t matter as power is constantly supplied by the inverter

 

Not getting check hybrid. But after watching YouTube and speaking via comments, my symptoms are the same, many of them are Toyota Prius but i know they have the similar systems.

 

I would agree and disagree with this...there is nothing wrong enough to activate a warning light due to a trouble code, but you could have degrading performance that hasn't caused an all out issue.
The OP's original post is more of degrading performance rather than an actual fault.

But you still need the 12V battery to run things like the computers, so the performance of items like that will be impacted to an extent.

 

i had my car serviced and mot 3 weeks ago by my own garage, as you know the hybrid system has been playing up with out any error code. But today i had this and stays on, sometimes it say pop hood failed and check brake error. Some the prius boys said they had the same before needing a battery replacement, i dont know for sure but it goes into lexus tomorrow, i thought id keep you all up to date.

 

Kinda expected this to happen sooner than later.
The reality is the vehicle is getting up there in mileage in terms of the HV battery charging cycles, so no surprise it may need a battery.

 

2014 ES300h owner here (same engine). 116k on the odometer. All the cell voltages are within 0.10V of each other. Life expectancy test reports 67% capacity left.

My wife has 2017 Rav4 hybrid (same engine as well) with 39k miles. All the cells are within 0.10V and 72% capacity left.

Although I am not convinced the "capacity" test numbers as calculated by Dr. Hybrid are trustworthy.