Maybe it's not the battery could be the the alternator not charging, but replacing the battery is a good starting place if you haven't replaced it in 4yrs. If you only drive short trips, let sit and don't drive for weeks, or have freezing weather sound like a reasonable thing to do. These cars use power while turned off for various remote and proximity features and that can also affect the life of the battery.
Let us know what you figure out...

 

Any update?

Always like to find out what happened on these pop up questions.

 

After installing the dash camera, I noticed it had affected my battery. Although it maybe minor to some, for the battery it is not.

Also had problem starting and the registered voltage was 8 V.

 

well.. I don't know yet. I've retested the battery a few times. It kept testing good.

Added volt meter in the car just out of curiosity. In the following week it did not start on the first try once after sitting all day in the cold. The wife had it so from what I could gather the voltage was at 12v which is kinda low. Also any time I measured the voltage it was always under 12.5 even after long drives but load test and resistance were still good. I also measured the power draw and it was ok. in any case I went and bought a new battery. It's been 2 weeks and so far it did not happen again. But the temperatures were higher. So I really can't tell if it was fixed by a new battery, or a prolonged battery disconnect which reset everything, or it was just a glitch. Or maybe the problem will be back in the upcoming days.

 

On my 2015 RX350F, I had the battery registering 12V. It felt slightly weak to me on startups, though it did start. The car was always garage at home. In the end, I put in a new battery, and it did give me peace of mind. The battery did feel stronger on startup.

I had Lexus change the battery for safety sake and wound up paying a lot for a lesser battery. I did this because I thought all my nav settings and seat seating would remain. Wrong. The car came back with lots of settings wiped. I assume the guy that changed the battery did not bother to piggyback a battery jumper to keep the electrics constant. For what I paid Lexus for a lower end battery, I could have gotten a Duracell.