Sticky backing on wii battery pack

[solidute]

Hi all.

First post so bear with me.

We have just bought a second hand wii for the kids and it came with 2 rechargeable battery packs and charging dock. The only problem is that the back of the battery packs are sticky as if something was stuck on and peeled off leaving a residue that I can't seem to shift. Is this a common problem with these and what should I do about it?
Thanks for helping a noob