For old things that should be solderable but have oxidized, a glass fibre brush can be very useful. I have two sizes. The big one uses what certainly appears to be a blackboard chalk holder and the small one uses a drafting pencil lead holder (2 mm? can't remember). They will very quickly remove surface oxides, though they are hard enough they would be unsuitable for something like actual contact areas of connectors. I generally use them with my little home-made lab vacuum cleaner running to suck up any bits of fibre that break off. Eye protection should be used.

I think jewelry makers may use glass fibre brushes like this. If you can find a supplier of tools for professional jewelry makers, I heartily recommend a trip to the store. You will likely find all manner of small tools and supplies useful in electronics. Although many have 3/32" shafts, I've bought all sorts of rotary tool "things" for far less than the usual price for Dremel stuff - and they are generally better made. Treat yourself to some Mizzy heatless wheels.

I also have a cleaner for oxidized component leads, though it is only suitable for axial parts such as resistors. It is essentially a piece of what looks like braided shield from wire about 1 cm in diameter, but made of stainless steel, flattened and held on the inside of a sheet stainless V about 7 cm long. You put the lead in the V, pinch it closed and rotate the body of the part a turn or two or three. It can be used on other things such as wire, but that typically requires a pinch-pull-repeat operation.

On the topic of cleaning things - do investigate melamine sponges.

 

For old things that should be solderable but have oxidized, a glass fibre brush can be very useful. I have two sizes. The big one uses what certainly appears to be a blackboard chalk holder and the small one uses a drafting pencil lead holder (2 mm? can't remember). They will very quickly remove surface oxides, though they are hard enough they would be unsuitable for something like actual contact areas of connectors. I generally use them with my little home-made lab vacuum cleaner running to suck up any bits of fibre that break off. Eye protection should be used.

I think jewelry makers may use glass fibre brushes like this. If you can find a supplier of tools for professional jewelry makers, I heartily recommend a trip to the store. You will likely find all manner of small tools and supplies useful in electronics. Although many have 3/32" shafts, I've bought all sorts of rotary tool "things" for far less than the usual price for Dremel stuff - and they are generally better made. Treat yourself to some Mizzy heatless wheels.

I also have a cleaner for oxidized component leads, though it is only suitable for axial parts such as resistors. It is essentially a piece of what looks like braided shield from wire about 1 cm in diameter, but made of stainless steel, flattened and held on the inside of a sheet stainless V about 7 cm long. You put the lead in the V, pinch it closed and rotate the body of the part a turn or two or three. It can be used on other things such as wire, but that typically requires a pinch-pull-repeat operation.

On the topic of cleaning things - do investigate melamine sponges.

Thanks for mentioning an even better tool for the soldering process that I suggested. AND that tool would indeed be better than the needle I suggested, but the principle is the same:Serious cleaning of the surface.

 

It's nickel. Many such connectors are nickel plated and unbelievably hard to solder. In years long gone by, nickel plated connectors weren't particularly difficult to solder but the stuff out of China is just horrible.

That's an interesting observation. Others also mentioned difficulty soldering nickel, but that has not been my experience. In fact, pure nickel battery tabs are quite east to solder. Any clue of how the process in China is different?