These are a great idea that someone mentioned on FB (I don’t remember who, so please remind me if it was you, thanks!)
They are tiny battery charge state indicators which you stick to your batteries and can then use them to identify batteries that need recharging. For scale here are the ones I just put on my batteries for my Sound Devices recorders:
Each one comes with double-sided tape already attached, so you just peel the back of them & attach them wherever you want. The slider has a small notch, so when you set it to green or red, it stays there & it would be fairly unlikely that it could slide across accidentally… They work great on my Sound Devices batteries but I was also hoping they would also work for my collection of Canon LP-E6 batteries. Unfortunately there is not enough room to stick them on any part of the battery and still allow the battery to fit inside my 5DmkIII – I tried every permutation and the battery compartment door on the 5D would not close. Probably for most people they don’t chew through enough batteries with their DSLRs that its a problem, but when out shooting timelapse its a different story… plus I have some LED lights & an LCD external field monitor which all use the same Canon batteries, so it would have been super useful.