A123 Systems Inc., a Waltham company that makes lithium ion phosphate batteries, announced an agreement Thursday with BAE Systems to supply the company with additional battery packs for a propulsion system designed for city bus systems around the world.

A123 has been supplying BAE with battery packs for about five years. Currently, about 3,000 buses using BAE’s propulsion system are in operation in various cities around the world, including London and New York.

The agreement announced Thursday calls for A123 to provide BAE with additional battery packs. But the number of those packs and the dollar value of the agreement to A123 are not being disclosed at this time. (To read the company’s press release, please click here.)

Headquartered in London, BAE Systems plc is a global defense and security company with about 100,000 employees worldwide and 2010 sales of $34.6 billion. The company delivers a full range of products and services for air, land, and naval forces, as well as advanced electronics, security, information technology solutions, and support services. It also has some commercial operations, including the HybriDrive propulsion program for city buses. BAE has operations in Lexington and Nashau, N.H.

A123 has recently had its share of ups and downs. A123 said in a recent regulatory filing that it could run out of cash in four to five months unless it can raise more money. The company may have bet more heavily on electric cars that it should have, and is now looking for new markets for its batteries. The aviation industry is one industry that offers promise, the company recently indicated.