S1: The new laptop model is very desirable. S2: The battery life of the laptop could be better. The second sentence:

The first sentence makes a positive claim about the laptop being desirable. The second sentence doesn't deny its desirability but points out a specific shortcoming - the battery life. This adds a limiting condition to the overall positive assertion, refining it rather than contradicting it. Therefore, the second sentence qualifies the assertion of the first.