Lithium batteries, even when they don't provide power, slowly lose power.
They can also "degrade" when they're not in designated voltage range. This is one reason why most batteries are soldered, because they need to be regulated at all times.
So no, you cannot "turn off" a battery. A battery should either provide power, or be plugged to a charger.
According to the table in the following Article, lithium batts have the lowest self discharge rate. When you dont have credentials to present please present citations.
Well, thats because I am a theorist who lately had to look at industry technical engineering websites about matters that academics dont care about and I find them to be mostly hearsay and heuristic garbage. I need to know whether your “credentialed” experiences can be written off.
Citation: search for the terms “self-discharge”. I dare you to find something less kosher than the wikipedia page. I already find the wiki page less than kosher.
There's also the safety issue. In the event of an internal short, the battery can actually catch fire. Would be nice to have a hardware power button that physically disconnects the battery from that load.
Sure, the battery degrades once its voltage drops to low or rises too high. But it also degrades from charge/discharge cycles. And without the ability to turn the device off the battery loses power even more quickly, either causing more charge/discharge cycles or bringing the battery quicker into the unhealthy zone.
They can also "degrade" when they're not in designated voltage range. This is one reason why most batteries are soldered, because they need to be regulated at all times.
So no, you cannot "turn off" a battery. A battery should either provide power, or be plugged to a charger.