Seems like a common topic so I thought I'd share my own story. My car is a 2017 GS 450h but with very low mileage (less than 4000 miles).
It appears the previous owner may have allowed the battery to completely discharge on more than one occasion. The factory fitted Panasonic S65D26L
AGM battery does not take kindly to allowing it's voltage to drop much below 12v too many times. Doing this too often permanently damages the battery.
Unfortunately the Lexus warranty appears to specifically exclude the 12v aux battery, at least here in the UK.
Seems like mine has bitten the dust, I was first alerted to this as the car would open up, but would not go into READY mode. You'll also get a myriad of system failure messages on the dash. I already had a CTEK MXS 5.0, a charger specifically designed for AGM batteries so I recharged it. The battery lasted about a week before it was dead again. 😬
I took the battery out of the car. It's actually quite straightforward, all you need is 10mm spanner and undo one nut and two bolts which secure the battery bracket.
First make sure that the car is turned OFF. It's probably wise to leave the driver's door open ajar and keep your key with you, don't leave it in the car (in case the alarm goes off when you reconnect the battery, it didn't happen in my case but the Owner's Manual says it's a possibility.)
You also need to undo the the two large power cables, starting with the negative one FIRST. You also need to remove the water drain tube (just pops out) and unclip the battery temperature sensor. You can then remove the battery, it has a useful carrying handle which you can use to gently remove it from the car (it's heavy-ish, so be prepared!).
With no possibility of parasitic load as it's now off the car, I recharged it again, using the MXS 5.0 in AGM mode. This completed all the way up to Step 7 (which is 100% charged/float stage) but upon disconnecting the charger and allowing the battery to 'rest' (in other words, nothing connected) the battery measured 12.7 volts using my digital volt meter). That is not bad, but still not quite right. The fully charged AGM battery should read just a smidge over 13 volts when fully charged).
I then left it to 'rest' overnight, by all accounts the battery should still read around 13 volts, but, unfortunately not, as you can see in the attached photo the voltage dropped overnight to 10.75 volts. That's a 100% sure sign the battery is toast. I suspect a shorted cell which is generally unrepairable. (There are generally six cells of 2 volts each in a car battery which gets you your 12 volts-ish. A shorted cell accounts for the 2 volt drop).
I know it's possible to fit an aftermarket battery but with my car being such low mileage and original I opted for a replacement S65D26L.
At the moment LexusPartsDirect are doing it on Special Offer for £215.39 inc VAT which is cheaper than my local dealer could do it for, and of course it's delivered right to your door within 72 hours. Lexus GS Phase 4 Auxiliary Battery | 28800-31410 | Lexus Parts Direct
It ships in a cardboard box inside another cardboard box, I advise you open it outside, as one of the plastic bags is full of an absorbent material, best described as wood chippings or cat litter, assumedly to absorb any leakage should the package be damaged, but as it was the packaging is super strong and mine arrived undamaged.
The battery does come about 90% charged, but before fitting I charged it with the MXS 5.0, again using the AGM specific mode. Once that finished, I allowed it to rest overnight and was pleased to see it measured just over 13 volts in the morning.
Fitting was of course reversal of removal, and nothing odd happened during re-connection, a few settings like the auto-fold in mirrors and HUD position were reset to defaults, but everything else was exactly where it was previously.
I will probably take the failed battery to Halfords, as apparently they will take and recycle old car batteries even if you're not making a purchase. Local council tips generally take them as well.
Hopefully someone will find this guide useful, it should apply to all 2012-2018 GS's, and possibly the previous ones too.
Dave.