Moses built an altar. An altar naturally implies a sacrifice, and Moses may well have thought that the signal victory obtained required to be acknowledged, and as it were requited, by offerings. In giving his altar a name, he followed the example of Jacob, who called an altar which he built, El-Elohe-Israel (Genesis 33:20).
Moses' name for his altar, Jehovah-nisi, meant "the Lord is my banner," and was intended to mark his ascription of the entire honour of the victory to Jehovah but had probably no reference to the particular mode in which the victory was gained.