You can use for contact cleaning some solution. I prefer (it is avaiable from local store) Cleaner KONTAKT 60 by KONTAKT CHEMIE. You can add some acid to it if it is a strong rust vinegar or lemon juice. Don’t forget to wash out acid. For washing electronics I use isopropyl spirit rather then ethanol. As for rubbers I prefer not to use any because you can easly scratch electroplated layer of nickel, and then oxydation occurs much faster. BTW, I don’t know about your camera, but my canon accepts batteries with some special chip only. Otherwise it can’t show level correct.