S1: Electric cars produce no exhaust emissions. S2: However, the electricity to charge them may come from fossil fuel sources. The second sentence:

The first sentence makes a claim about electric cars having no exhaust emissions. The second sentence doesn't deny this but adds a limiting condition - that the electricity may come from fossil fuels, which has its own environmental impact. The word 'however' signals that the second sentence modifies or restricts the scope of the first. This is qualification - adding a caveat or condition. Therefore, the second sentence qualifies the assertion of the first.