I am not an engineer [IANAE?], but I have been impressed by the advice of Steve Gibson, www.grc.com, a security researcher who has produced more than 300 consecutive weekly shows for his podcast, "Security Now."
His advice is charge lithium ion batteries early and often.
Two weeks ago he stated it thusly:
STEVE: Yes. Now, the universal agreement is that, unlike prior technologies, prior chemistries, both nickel-cadmium and then nickel-metal hydride, which was actually the same fundamental electrochemistry, those had a memory effect. So if you only discharged them a little bit and recharged them often, they would forget that there was, like, a lot more room down below the point that you were normally starting the recharge. So the logic there was run them all the way down to the ground before you recharge them. And if you can't, then you could reset their memory by deliberately doing some deep cycles.
Lithium-ion, completely different story. It has no memory. And it actually is better for the chemistry not to be running it all the way down. So there the logic is charge often. And if you are in a situation where, for example, you have access to a plug, and you've got your adapter, always use the adapter when you can.
I took this quotation from the transcript of episode 334, available here, on grc.com.
I hope he's right.