Try doing a parasitic drain test. make sure to let it sit for 20 mins or so, to make sure everything shuts off. anything over 30mah means something is drawing too much power when the car is off.

 

Your off time voltage seems low. Batteries are temperature sensitive. If for example you measured it 4 hrs after the engine cooled and again 24 and 48 hrs, then you can comapare if the ambient temperature is within +/-10° F of original. As the battery temperature decreases so does its voltage. Something like 0.01v for every 25°F below 70°F... Not a huge amount but it also goes up with underhood temps the same amount... What are your current ambient temps??

At a glance your battery is loosing ground pretty rapidly.
If you can find a shop and/or gain access to hand held volt meter with current measuring ability, I would connect it test it all doors closed. Lights off. Hood closed.

Get current draw in mA.
Looking through the fuses and system on my 2IS and knowing the gateway computer on many Lexus cars has been impacted, look through your owners manual and pull the fuses on the circuits listed here to see if the current draw drops to something under 15mA.
MPX, ACC, ECU-IG RH, MULTIPLEX COMMUNICATION.

IMO - A good tech could do current draw tests in less than 20min and not damage anything. Just selectively pull fuses to isolate current draw. The tricky part is door and hood switches need retained in the closed position to get valid data.

Which reminds me, have you confirmed the dome and trunk lights are in fact off when the vehicle is secure?

 

My ambient temperature is around 91F. Will look for a shop to do a current draw test

Dome lights and trunk lights are off.

Thanks!

 

Unless the dealer measured the current draw, there is no way they would know this. I would stay on them to fix this one. Anything left of your warranty?

 

No. Out of warranty 2 years ago

 

I purchased a CPO Lexus IS250 6 months ago. The first thing that went bad was the battery. It wouldn't stay charged. Ended up being the EU.

https://www.clublexus.com/forums/is-...d-threads.html

 

The ECU and battery was changed out.

 

Tier One top quality battery? Higher than normal battery drain when car is parked is something iwrong with ECU or software, IMO. German cars have
some intelligent battery power management system. It's goal is at any circumstances to safe guard minimum battery power to start the car. For an example
if bettery is getting low, system will disable door handle sensors. Touch the door handle, it won't unlock.

 

I have a L Certified 2015 IS250 FSport. And I had my ECU replaced the 1st week I owned the car when the battery died.

 

 

Are all your voltage readings across the battery terminals? If you havent done so try testing between the alternator and the battery while having all your loads on. If the alternator is good you should be at or above 14V

 

 

 

How are your battery cables and connectors? If you have some corrosion on them, it will affect your car. I had this same thing happen to my toyota truck many years ago. Went through some the same issues. Replaced the battery and that helped for a day or 2. I figured it was the starter going bad, so I replaced that myself. After still not getting any improvement, I had my mechanic look at it. He knew I replaced the battery and starter, so he checked out the battery connectors, and they needed to be replaced.

 

You should have well over 14V with the engine running. Around 14.4 is the norm in my days of diagnosing cars. 13.5 is low. The car is so new so I'm not sure if the IS has an internal voltage regulator or external (replaceable). I would look into the alternator (and battery) more on that later..

alternator would never directly prevent your car from starting.

it will prevent the battery from recharging.

I've had auto parts store say a battery tested fine when it was bad. Go get a new battery. you need one anyway regardless of whether or not it's your battery/alternator. 5 years is about the mark where I tell people to replace them, even if your car still does start.

So in short, certainly get a new battery. and i would probably take another look at that alternator.