Battery Lug Terminals

Good luck finding non-acid solder, or even leaded solder. If you can't find anything else, look for some with NO flux internally and use added flux. You can get by with no-lead with some extra flux added. DO NOT use acid flux or acid fluxed solder.

Find some good Rosin based flux. I see SRO is advertised on egag in small containers.
This is what I use:
Amazon product ASIN B0049RNTW6