Our local Harbor Freight sells 5 packs of colored tape for $2
http://www.harborfreight.com/colored...ack-66462.html
Wrap around (from a prior suggestion) and stick it to itself it NEVER comes off.
And you can color code GREEN + COLOR = Competition Battery RED + COLOR = Practice Battery and keep track that way.
I would go for the multiple colors, create plaids, and say "Oh that's the McGregor battery, why is it plugged into the Clan Stewart charger?" Another way to do crossovers from History to Robotics.
But I have to say the label under the clear tape caught my eye. That seems to be the way to go. Plus when you recycle the battery pigtails when the battery dies, it's a rip and go for the new label.
Good luck!