I slide these rings onto the battery contacts of lipos with HXT-4mm connectors after charging them. Only charged batteries have charge markers. The rings block the contacts and are removed before the batteries are 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 XT60 connectors than can be found here: http://www.thingiverse.com/thing:700056
And yet another for JST connectors that can be found here: http://www.thingiverse.com/thing:701473