The UV is used to purify the water inside the bottle (and as a side effect sterilizes the interior walls of the bottle). I manually wash the bottle after a day of use if I've been drinking directly from the bottle, if I've had something other than plain water in the bottle, or if the cap, cap screw threads, or outside of the bottle have been exposed to contaminants.
Based on my experience with multiple bottles over 3 years of ownership, the UV light works on demand. You push a button on the cap and a brief water purification cycle begins. As well, the UV will run on its own periodically if the cap has been on the bottle for awhile. This is a good feature because UV purified water does not stay safe as long as chemically purified water due to differences in the sterilization process.
I also haven't had any problems with charging or the batteries. The bottle is charged using a USB cable that connects to a user-provided charger. I've used both Apple and Amazon Basics chargers without any problems. Batteries can be kind of random, as anybody who owns iOS devices knows, of course but again, the batteries on my bottles have been working as expected over 36+ months of use.
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Now that we're talking about water bottles, I confess that I've accidentally become sort of a collector of bottles and sterilization devices. The place where I live has a de facto ban on commercially bottled water, natural disasters are an ongoing risk in my area, I like international travel, and I used to do a lot of backpacking. So here are some personal opinions:
- When I don't want a bottle affecting the taste of a liquid, I go with either Nalgene or glass.
- If I want to be sure a bottle won't break, I go with Nalgene in most cases. Metal if extremely cold temperatures are possible.
- I like wide mouth bottles best because they are easy to fill and keep clean.
- I try to only buy bottles with a leash for attaching the cap to the bottle so that it is hard to lose or drop the cap. Nalgene has had this feature on most of its bottles for decades.