It's also worth noting that in the 2013-2014 model years, there were a LOT of ****ty batteries shipped in various Lexus cars. My moms 13 ES had a battery that was basically garbage. The cells were flakey internally. She'd leave the car for a few days and it wouldn't start. That said, if you managed to get 5 years out of it, that's a REALLY long time for a battery in my book.

Back when I had my 2010 Camry I basically replaced the battery every 2 years or so, but I drive 25-30K/yr and invariably my battery would also get some abuse here and there as well, including lots of sub 20F starts during the winter, etc.

-Mike

 

I am not sure that it is correct that the charging voltage drops as under hood temperature rises. It is tied to the batteries charging needs. When you first start the car the battery voltage drops and when the alternator senses this it increases the charge voltage to provide a higher amperage charge to the battery. Once the battery voltage comes back up it then drops the charge voltage to deliver less amperage so as not to over charge the battery (that's a bad thing too). There may be some subtle changes to the voltage that are related to temperature but it's mostly due to charging needs. Remember that a higher differential voltage between the battery and alternator will provide more potential for a higher amperage charge. Once the voltages move closer together the potential decreases and unless the alternator runs at a higher voltage it cannot produce the same charge.

 

As you can see from the graph chart (below) alternator voltage output is [purposely] biased by ambient/underhood temperature.

 

I tested mine yesterday 08 gs350 at idle it was 12.8 and he turned on everything to see where it will drop A/C headlight, radio, the mechanic told your alternator is fine and nothing wrong with it, but when I go to autozone for a test it shows Volt regulator “FAILED” 🤔 I already bought remanufactured alt denso and I’m not sure my alternator is good or not ????

 

What brand AGM battery you got ?