NO! The use of compatible or aftermarket battery packs will not void your laptop warranty in any way.
Actually, consumers are protected by the following legislation:
MAGNUSON-MOSS WARRANTY IMPROVEMENT ACT
Magnuson-Moss Warranty Improvement Act United States Code Annotated Title 15 Commerce and Trade Chapter 50 Consumer Product Warranties 15 Section 2302 states that:
"No warrantor of a consumer product may condition his written or implied warranty of such product on the consumer's using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade or corporate name; except that the prohibition of this subsection may be waived by the commission if:
1. The warrantor satisfies the Commission that the warranted product will function properly only if the article or service so identified is used in connection with the warranted product, and
2. The Commission finds that such a waiver is in the public interest."