Evaporust gel on battery contacts.

[rprosperi]

As mentioned by Kiwi, a glass pen works great for heavier oxidation residue. I usually use this first, followed by Vinegar on swabs. One of these pens is the best tool you can buy if you often work on old calc cleanup, they're cheap and easy to use:

https://www.amazon.com/gp/product/B0019V18D2