Expanders are an essential component of the negative plates of lead-acid batteries. They increase the surface area and stabilize the structure of the negative active material. They can be added to the negative paste mix in a number of ways and each of these has advantages and disadvantages. The individual components can be added to the paste mixer during paste preparation or they can be pre-mixed before addition. This pre-mixing can be done by the battery manufacturer or by companies that specialize in expander manufacture. This paper reviews the different ways in which expander can be added to negative paste mixes and discusses the consequences of each method. It concludes that the simplest, lowest cost and most risk-free option is to purchase pre-blended expander from a recognized supplier. (C) 2004 Elsevier B.V. All rights reserved.