If the ring terminals are plated like the ones in the picture (tin or nickel?) then they should not need much more than a quick wipe with alcohol. If they are bare copper then they probably do need cleaning. Clean copper looks like a bright new penny; oxidized looked like a penny that has been used for a few years.
If you have or can find a good electronics contact cleaner, try that. If it needs scrubbing, then use a woven fabricc scrubber (ScotchBrite, for example), or a copper kitchen scouring ball.) You want it clean, with minimal scratches, and no foreign metals in it (like bits of a steel scouring pad. Wipe with a soft cloth, and the try not to touch the copper with bare fingers.
If corrosion is Really bad, you could use fine grit garner sandpaper as a first step.
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