Does hot weather causes Lithium-ion batteries in Apple devices to drain fast? I live in a tropical region which sees very hot summers, which we are experiencing currently. I have been observing for some time that the battery on my Apple devices are draining way quicker than they use to.

My recently bought, fully charged iPhones goes from 95% to around 25% charge even if kept unused, put on low power mode and running the latest version of iOS (both iPhone 7 and 1st generation iPhone SE bought under a year ago). I am observing the same with my iPad mini 4 (relatively old) and a recently bought Apple Watch Series 5.

Is there any technical reason and evidence or explanation thereof that would support this theory? Or could there be any other factors at play here?

I have not observed such battery drain pattern just a month or so ago when it didn't used to be as hot. Also, I do not use any kind of air conditioning, so the room temperature is usually higher than normal further aiding with device heating.
we are experiencing hot summers....the battery on my Apple devices are draining way quicker than they use to from before.  Is there any technical reason and evidence or explanation thereof that would support this theory?


Yes.

Temperature extremes (cold and hot) are bad for batteries in general, but they have different charging and discharging characteristics depending on temperature.  Batteries are much more "sensitive" to charging than they are discharging.

Basically, the battery should be brought back to a "normal" temperature when charging to get the best performance.  



Source:  Battery University - Charging at High and Low Temperatures

A Li-ion battery has a charge temperature range from about 0°C to about 45°C.  In a tropical climate, freezing temperatures aren't really a problem, but past 45°, you can get reduced charge.


  Batteries operate over a wide temperature range, but this does not give permission to also charge them at these conditions. The charging process is more delicate than discharging and special care must be taken. Extreme cold and high heat reduce charge acceptance, so the battery must be brought to a moderate temperature before charging.