I slide these rings onto the battery contacts of lipos with XT60 connectors after charging them. Only charged batteries have charge markers. The rings physically block the contacts and have to be removed before the batteries can be used so it is impossible to forget to remove the charge markers. This is a simple system for keeping track of which batteries need to be charged after a trip to the field. I am thinking of printing rings in a different color and use these to mark storage charged batteries. I have not started doing so yet, but it seems like a good idea. The markers are both quick and easy to print. The rings in the images are printed with 0.3 layer height. I have made another variant for JST connectors that can be found here: http://www.thingiverse.com/thing:701473 And yet another for HXT4mm connectors can be found here: http://www.thingiverse.com/thing:701727