The only other discernible feature on the Energi 10K is a little square button on the left side, flanked by a green LED whenever the battery pack is on. Press it to reveal five LED dots running along the side. Those give you an estimate of how much power you're still packing; when the lights go out, you're out of juice. A little green LED will remain illuminated whenever the battery pack is on. Press and hold the button to shut it off off -- it'll also turn itself off after 10 minutes.
Power to spare
In my tests, the beefy 10,400mAh battery was more than enough to service a smartphone (like my Nexus 5) several times over, and could top off three phones at once with a little juice to spare. But my 3rd-generation iPad's (relatively) gargantuan 11,560mAh battery took the Energi 10K from somewhere near full capacity to flat in about four hours, while taking the iPad's battery capacity from 8% to 69%.
That's actually pretty good, but highlights how that LED readout can get a bit risky -- the five LEDs only give you an approximation of how much capacity is left, so it's tough to gauge how long the thing is going to last. That said, you'll run into this caveat with just about any battery pack on the market. Maybe someday battery packs will spit out their estimated remaining capacity with actual numbers. Until then, you'll just have to get a feel for your capacity by getting out there and using it.
But here's my favorite feature: the Tylt Energi 10k supports pass-through charging, which means it's capable of charging other devices while it's being charged. That's something I don't actually see on too many batteries. It's only available once the battery pack has been charged to at least at 50% capacity, but that'll still be incredibly handy if you're low on outlets and have several devices you want topped off. It'll also be useful because the battery takes a whopping six hours to reach full capacity. If you're planning to take a long trip or just suspect you'll need a bit of extra juice, you'd better top up the night before.