I slide these rings onto the battery contacts of lipos with JST 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. The tolerances are tight so you might have to increase or decrease the print size a few percent depending on your particular printer and setup. I have made another variant for XT60 connectors than can be found here: http://www.thingiverse.com/thing:700056 And yet another for HXT4mm connectors can be found here: http://www.thingiverse.com/thing:701727