Also known as a fibreglass pen - which is what the brush part is made of. Watch out for the fibres snapping and leaving masses of little bits which will get into your fingers. Best is to work over something like kitchen paper that has a fibrous texture that will catch the fibreglass shards then dispose of it afterwards.
I've used them on gold contacts without problem. It's very effective but gentle. You can also get versions with brass or steel brushes. Not sure I'd use the steel one but the brass one is probably better for battery contacts though I wouldn't use it on circuit boards or switch contacts.