Battery charge marker

I have a lot of batteries, some on my workbench, some in my field bag, and sometime i forget which one is ready to fly and which one needs a charge.

I now apply a small piece of shrink tube on the plug, and it gave me a cheap "cap charge marker".
If needed you can fold a piece of paper/plastic on the tube to get a warning message you need before flying with the battery.

When i found a battery without this reminder cap, i know that it has been plugged and that i need to check its charge.