I agree i get a couple of minutes warning a bleep then they shutdown, no notice. I've just spent 3 days moving and used my Slim during flights and to talk with Uber driver in noisy airport. The Slim do a good noise isolation job, so good for flights. I carry them plus some corded earbuds so I have another earbud while charging.
What I do is just plug in the Slim when not using, because I don't know their charge, e.g. last evening I had time in airport, then flight, so I used the buds in the airport for about 3 hours, then during the 15mins or so time of walking to gate and boarding, I plugged in the Slim into the Powercore+ mini and got some charge, then used the buds for about 3 hours then during landing I plugged them back into power for the 20 min or so of the landing period then used the buds again through the Uber process. i.e. took opportunistic short times to recharge. If I had not taken those two combined 35mins or so of recharging, I might not have lasted the total of 6+ hours of use.
If I knew the charge level I might not need to do such preventing proactive top-up charging.