You need a powerful iron, 40w min, 60w better. If you cannot make a clean joint in 2-3 seconds, you either havn't got a powerful enough iron or your technique is wrong. If you leave the iron in contact with the cell too long you get too much heat transfer and risk damaging your cells. You will always find the -ve side is harder to solder. Solder isn't a particularly good conductor of electric (comparatively) so "less is more" is a good policy.
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